OSPF- Multi area

Ch.6 – OSPF
Part 2 of 2: Multi-Area OSPF
CCNP version 3.0
Rick Graziani
Cabrillo College

Note
• Optional: This lab coincides with my Multi-area OSPF Lab
handout, Rick's OSPF Scenarios (Word doc)
• Most of the slides in this presentation do not come directly
from CCNP 1 version 3.0, OSPF, but slides which contain
a little more detail and explanation of OSPF.
Rick Graziani graziani@cabrillo.edu 2

Optional: Rick’s OSPF Scenarios
Optional: We will be using the following handout for this presentation:
Rick’s OSPF Handout:
1. OSPF Multi-Area - All Normal Areas
2. OSPF Multi-Area - Stub Area
3. OSPF Multi-Area - Totally Stubby Area
This handouts can be downloaded from (Word doc):
• http://www.cabrillo.cc.ca.us/ciscoacad/curriculum/presentations/semes
ter5/OSPF_Scenario_Handout.doc

Part I - LSAs using all normal areas
Multi Area OSPF
Normal Areas
11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Suggestion to Instructors:
Draw this network on the white-board
as it will be used for
discussion throughout these
slides.
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

Topics
• Areas
• LSAs
• show ip ospf database (summary of link state database)
• show ip route
• Stub Areas
• Totally Stubby Areas
• E1 and E2 routes
• Default Routes
• Route Summarization
• NSSA (Not So Stubby Areas)
• Multiple ABR Scenario
• Multiple ASBR Scenario
• Virtual Links
• Load Balancing
• show commands

Issues with large OSPF nets
• Large link-state table
– Each router maintains a LSDB for all links in the area
– The LSDB requires the use of memory
• Frequent SPF calculations
– A topology change in an area causes each router to re-run SPF to
rebuild the SPF tree and the routing table.
– A flapping link will affect an entire area.
– SPF re-calculations are done only for changes within that area.
• Large routing table
– Typically, the larger the area the larger the routing table.
– A larger routing table requires more memory and takes more time
to perform the route look-ups.
Solution: Divide the network into multiple areas

OSPF uses “Areas”
• Hierarchical routing enables you to separate large internetworks (autonomous
systems) into smaller internetworks that are called areas.
• With this technique, routing still occurs between the areas (called inter-area
routing), but many of the smaller internal routing operations, such as
recalculating the database – re-running the SPF algorithm, are restricted within
an area.

An advantage of Multiple Areas
• Question: I understand the routing table is recalculated every time the router
receives an new version of an LSA. Does OSPF recalculate its routing table
when their is a topology change in another area? show ip ospf displays no
change in SPF execution, but show ip ospf database shows a change in the
topology?
• Answer: Good question! OSPF areas are designed to keep issues like flapping
links within an area. SPF is not recalculated if the topology change is in
another area. The interesting thing is that OSPF distributes inter-area (between
areas) topology information using a distance-vector method. OSPF uses link-state
principles only within an area. ABRs do not announce topological
information between areas, instead, only routing information is injected into
other areas. ABRs relay routing information between areas via distance vector
technique similar to RIP or IGRP. This is why show ip ospf does not show a
change in the number of times SPF has been executed when the topology
change is in another area.
• Note: It is still a good idea to perform route summarization between areas,
announcing multiple routes as a single inter-area route. This will hide any
changes in one area from affecting routing tables in other areas.

OSPF uses “Areas”

OSPF Router Types

OSPF Router Types
IInntteerrnnaall: Routers with all their interfaces within the same area
BBaacckkbboonnee: Routers with at least one interface connected to area 0
AASSBBRR: (Autonomous System Boundary Router): Routers that have at least
one interface connected to an external internetwork (another
autonomous system)
AABBRR: (Area Border Router): Routers with interfaces attached to multiple
areas.

OSPF Packet Types
• Last week we discussed various OSPF packets, used for:
– Means for dynamic neighbor discovery
– Detect unreachable neighbors within a finite period of time
– Ensure two-way communications between neighbors
– Ensure correctness of basic interace parameters between neighbors
– Provide necessary information for the election of the Designated and
Backup Designated routers on a LAN segement
– Request link state information from another router
– Sharing data base summary and detailed information
– Acknowledge the receipt of an OSPF packet
OSPF packet types

OSPF Type 4 - Link State Advertisements
• This week we will look at OSPF Type 4 packets more closely
OSPF packet types

OSPF packet types
OSPF Type-4 packets have 7 LSA packets (later)

LSA Types
LSAs used for discovering routes and reaching Full State, along with
Maintain Routes

LSA Types
LSA Types 1 through 5
• We will look at these in detail as we discuss areas in this chapter.
LSA Type 6 MOSPF (Multicast OSPF)
• Not supported by Cisco.
• MOSPF enhances OSPF by letting routers use their link-state
databases to build multicast distribution trees for the forwarding of
multicast traffic.
LSA Type 7 NSSA External Link Entry
• Originated by an ASBR connected to an NSSA.
• Type 7 messages can be flooded throughout NSSAs and translated
into LSA Type 5 messages by ABRs.
• Routes learned via Type-7 LSAs are denoted by either a “N1” or and
“N2” in the routing table. (Compare to E1 and E2).
• We will discuss this more later when we look at NSSA areas.

Area Types
• Standard or Normal Areas
– Backbone
– Non-Backbone
• Stub
– Stub Area
– Totally Stubby Area (TSA)
– Not-so-stubby-area (NSSA)

Area Types

Multi Area OSPF
Normal Areas
11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

Routes Received on all OSPF Routers
Overview of Normal Areas – This will all be explained!
Receives all routes from within A.S.:
• Within the local area – LSA 1 and LSA 2
• From other areas (Inter-Area) – LSA 3, LSA 4, LSA 5
Receives all routes from External A.S.’s (External AS means routes not from this
OSPF routing domain):
• From external AS’s – LSA 5
• As long as routes are being redistributed by the ASBR (more later)
Default Routes
• Received only if default-information-originate command was used (later)
• If default-information-originate command is not used, then the default
route is not received

• Your Turn - In groups, examine running-configs
• Look at the running-configs for “1. OSPF Multi-Areas
- All Normal Areas”
• Look at the OSPF network statements!

1. OSPF Multi-Areas - All Normal Areas
ASBR
router ospf 1
redistribute static
network 172.16.1.0 0.0.0.255 area 0
!
ip classless
ip route 11.0.0.0 255.0.0.0 Null0
ip route 12.0.0.0 255.0.0.0 Null0
ip route 13.0.0.0 255.0.0.0 Null0
ABR-1
interface FastEthernet0/0
ip address 172.16.1.2 255.255.255.0
ip ospf priority 200
router ospf 1
network 172.16.1.0 0.0.0.255 area 0
network 172.16.51.0 0.0.0.255 area 51
ABR-2
interface FastEthernet0
ip address 172.16.1.3 255.255.255.0
ip ospf priority 100
!
router ospf 1
network 172.16.1.0 0.0.0.255 area 0
network 172.16.10.4 0.0.0.3 area 1
Internal
router ospf 1
network 172.16.0.0 0.0.255.255 area 1
ABR contains network
statements for each area
it belongs to, using the
proper area value.

Understanding LSAs
• show ip ospf database
– This is not the link state database, only a summary.
– It is a tool to help determine what routes are included in the routing table.
– We will look at this output to learn the tool as well as become familiar with
the different types of LSAs.
– To view the link state database use: show ip ospf database [router|
network|…]
LSA Header
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | LS type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LSA 1 - Router Link States
• LSA 1 – Router LSA
• Generated by each router for each area it belongs to.
• Describes the states of the links in the area to which this router belongs.
15
2
B
A C
D
5
Router A’s LSA 1s
which are flooded to all
other routers in this
area.
“Leaf” network
• Tells the other routers in the area about itself and its links to adjacent OSPF
routers, and “leaf networks.”
• Flooded only within the area. On multi-access networks, sent to the DR.
• Denoted by just an “O” in the routing table or “C” if the network is directly
connected.
• ABR will include a set of LSA 1’s for each area it belongs to.

• When a new LSA 1 is received and installed in the LSDB, the router
forwards that LSA, using hop-by-hop or asynchronous flooding.
• The LSA is sent out all OSPF interfaces that are in the Exchange State
or a higher state.
• For interfaces in Exstart or lesser state, the router will wait until it is out
of Exstart.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |V|E|B| 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # TOS | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | 0 | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
LSA 1 .1
LSA 1 LSA 1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
LSA 1’s being sent
within Area 0
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
within other areas
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
LSA 1
.1
172.16.51.0/24
.5
LSA 1
172.16.10.4/30
.6
LSA 1
Area 51 Internal
LSA 1
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
LSA 1’s are flooded out
other interfaces within
the same area.
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
LSA 1
Originated
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
LSA 1
flooded

For Router Links:
• The Link State ID is always the same as the Advertising Router
• Advertising Router is the Router ID of the router that created this LSA
1
Internal#show ip ospf data
OSPF Router with ID (192.168.4.1) (Process ID 1)
Router Link States (Area 1) <- Note the Area!
(LSA 1 - Links in the area to which this router belongs.)
Link ID ADV Router Age Seq# Checksum Link count
192.168.3.1 192.168.3.1 898 0x80000003 0xCE56 2
192.168.4.1 192.168.4.1 937 0x80000003 0xFD44 3
• Bottom line: Router Link States (LSA1’s) should display all the
RouterIDs of routers in that area, including its own.
• Rick’s reminder: LSA 1 -> “my one area”

ABR-2#show ip ospf data
Router Link States (Area 1) <- Note the Area!
(LSA 1 - Links in the area to which this router belongs.)
192.168.3.1 192.168.3.1 786 0x80000003 0xCE56 2
192.168.4.1 192.168.4.1 828 0x80000003 0xFD44 3
• Bottom line: Router Link States (LSA1’s) should display all the
RouterIDs of routers in that area, including its own.
• Rick’s reminder: LSA 1 -> “my one area”

ABR-2 - show ip route
172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:11:44, FastEthernet0
O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:12:29, Serial0
C 172.16.10.4/30 is directly connected, Serial0
C 172.16.1.0/24 is directly connected, FastEthernet0
O E2 11.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0
192.168.3.0/32 is subnetted, 1 subnets
C 192.168.3.1 is directly connected, Loopback1
• Denoted by just an “O” in the routing table, or a “C”
• Why is there only just an”O” for this network and not the other
networks?
– Directly connected or via another area.

Your Turn -Discuss in groups (LSA 1s)
• Using the Multi-area OSPF Lab Handout: “Lab 1. – OSPF
Normal Areas” verify these results.
• Look at the link state database summary (show ip ospf
database) commands and the Router Links States (LSA1s) for
each router.
• Look at the routing tables (show ip route) and notice the routes
within that router’s area.
• Why do some routers have more than one set of Router Links
States?
• Where does “show ip ospf database” tell you the RouterID.
• Where does “show ip ospf database” tell you the Area.

LSA 2 - Network Link States
• LSA 2 – Network LSA
• Generated by the DR on every multi-access network
• Denoted by just an “O” in the routing table or “C” if the network is
directly connected.
• Flooded only within the originating area.
• LSA 2’s are in link state database for all routers within area, even those
routers on not on multi-access networks or DRs on other multi-access
networks in the same area.
• ABR may include a set of LSA 2s for each area it belongs to.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attached Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
LSA 2
LSA 2
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
No LSA 2’s for ABR-1
in Area 51, or for
Internal because no
other routers on multi-access
segment.
Lo - RouterID
192.168.2.1/32
DR
ABR-1 ABR-2
LSA 2
flooded LSA 2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
LSA 2
flooded
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
DR

ASBR#show ip ospf data
Net Link States (Area 0)
(LSA 2 - Generated by the DR)
Link ID ADV Router Age Seq# Checksum
172.16.1.2 192.168.2.1 201 0x8000000D 0xCFE8
• Link ID 172.16.1.2 = IP address of DR on MultiAccess Network
• ADV Router 192.168.2.1 = Router ID of DR
• Bottom line: Net Link States (LSA2’s) should display the RouterIDs of
the DRs on all multi-access networks in the area and their IP
addresses.
• Rick’s reminder: LSA 2 -> “Ethernet = Layer 2 or D R”
1 2

database) commands and the Net Links States (LSA2s) for
each router.
• Look at the routing tables (show ip route) and notice the multi-access
routes within that router’s area.
• Could a router have more than one entry in its listing of Net
Links States?
• Could an area with a broadcast segment, still have no LSA 2’s?

LSA 3 – Summary Net Link States
• LSA 3 – Summary LSA
• Originated by the ABR.
• Describes links between ABR and Internal Routers of the Local Area
• ABR will include a set of LSA 3’s for each area it belongs to.
• LSA 3s are flooded throughout the backbone (Area 0) and to other
ABRs.
• Routes learned via LSA type 3s are denoted by an “IA” (Inter-area) in
the routing table.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
LSA 1
LSA 1 LSA 1
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
LSA 1’s are sent as
LSA 3’s into other
areas by the ABRs.
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
LSA 3 172.16.1.0/24
LSA 3
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
LSA 3
LSA 3
LSA 3
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
areas by the ABRs.
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
LSA 1
LSA 3
172.16.51.0/24
.5
LSA 1
172.16.10.4/30
.6
LSA 3
LSA 1
LSA 3
Area 51 Internal
LSA 1
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
LSA 1’s are sent as LSA 3’s
into other areas by the
ABRs.
LSA 3
LSA 3
LSA 3
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
LSA 1
LSA 3
172.16.51.0/24
.5
LSA 1
172.16.10.4/30
.6
LSA 3
LSA 1
LSA 3
Area 51 Internal
LSA 1
.1
172.16.20.0/24
• Routers only see the topology of the area they belong to.
• When a link in one area changes, the adjacent routers originate in LSA 1’s and
Area 1
Lo - RouterID
192.168.4.1/32
Area 1
flood them within the area, causing intra-area (internal) routers to re-run the
SPF and recalculating the routing table.
• ABRs do not announce topological information between areas.
• ABRs only inject routing information into other areas, which is basically a
distance-vector technique.

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
LSA 1’s are sent as LSA
3’s into other areas by the
ABRs.
LSA 3
LSA 3
LSA 3
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
LSA 1
LSA 3
172.16.51.0/24
.5
LSA 1
172.16.10.4/30
.6
LSA 3
LSA 1
LSA 3
Area 51 Internal
LSA 1
.1
172.16.20.0/24
• ABRs calculate intra-area routes for directly attached areas and announce
them to all other areas as inter-area routes, using LSA Area 3’1
s.
• OSPF ABRs will only announce inter-area routes that were learned from the
Lo - RouterID
192.168.4.1/32
Area 1
backbone area, area 0.
• The backbone area serves as a repository for inter-area routes.
• This keeps OSPF safe from routing loops.

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
Don’t forget about the LSA
1’s from Area 0.
LSA 1
LSA 1 LSA 1
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
LSA 3
172.16.51.0/24
.5
172.16.10.4/30
.6
LSA 3
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

Area 0
Backbone Area
LSA 1’s
RTA RTB
LSA 3 LSA 3
Area 1 Area 51
RTC
Normal Areas
Not ABR
• ABRs calculate intra-area routes for directly attached areas and
announce them to all other areas as inter-area routes, using LSA 3’s.
• In normal operation, OSPF ABRs will only announce inter-area
routes that were learned from the backbone area, area 0.
• RTC does not forward LSA 3’s from Area 1 to Area 51, and does not
forward LSA 3’s from Area 51 to Area 1.
• The backbone area serves as a repository for inter-area routes.
• This keeps OSPF safe from routing loops.

Area 0
Backbone Area
LSA 3
RTA RTB
LSA 3
LSA 1’s
Area 1 Area 51
RTC
Normal Areas
Not ABR
• Example of an LSA 1 originated in Area 1, sent to Area 0 as an LSA 3,
and the sent to Area 51 as an LSA 3.
• RTC does not forward the LSA 3’s back into Area 1, or routing
loops may develop.
• Again, in normal operations, OSPF ABRs will only announce inter-area
routes that were learned from the backbone area, area 0.
• Note: RTC will create LSA 1’s and flood them within the appropriate
area.

Area 0
Backbone Area
LSA 3 LSA 3
RTA RTB
LSA 3 LSA 3
LSA 1’s LSA 1’s
Area 1 Area 51
RTC
Normal Areas
Not ABR
• RTC does not forward LSA 1’s from Area 1 as LSA 3’s into Area 51.
• RTC does not forward LSA 1’s from Area 51 as LSA 3’s into Area 1.
• Any LSA 3’s from RTC are not forwarded into Area 0 by RTA or RTB.
• OSPF specification states that ABRs are restricted to considering LSA
3’s only from the backbone area to avoid routing information loops.

Area 0
Normal Areas
Backbone Area
LSA 3
RTA RTB
Update is sent to Area 0 and Area
51 routers using a “distance
vector update technique.” SPF
not re-run, but routers update
routing table.
LSA 1’s
LSA 3
Area 1 Area 51
RTC
X
Area 1 routers re-run
SPF, creates new SPF
tree and updates
routing table.
Topology Change: Down Link
• When a router detects a topology change it immediately sends out LSA
1’s (Router LSAs) with the change.
• In the case of a down link, the age of the LSA is set to MaxAge (3,600
seconds) – Routers that receive LSAs with the age equal to MaxAge
remove this entry from their LSDB (Link State Data Base).
• Routers that receive the LSA 1’s, within the area of the change, re-run
their SPF algorithm, to build a new SPF tree and then make the
changes to their IP routing tables. (Continued next slide)

Area 0
Normal Areas
Backbone Area
LSA 3
RTA RTB
Update is sent to Area 0 and Area
51 routers using a “distance
vector update technique.” SPF
not re-run, but routers update
routing table.
LSA 1’s
LSA 3
Area 1 Area 51
RTC
X
Area 1 routers re-run
SPF, creates new SPF
tree and updates
routing table.
Topology Change: Down Link
• ABR RTA receives the LSA 1 and recalculate their SPF for that area,
Area 1.
• RTA floods the change as a LSA 3 within its other area, Area 0.
• RTB receives the LSA 3 and floods it within Area 51.
• Area 0 and Area 51 routers do not recalculate their SPFs, but inject the
change into their routing tables.
Note: LSA 3’s (and other Inter-Area routes) are viewed as “leaf nodes” in
the SPF tree.

Multi Area OSPF
Normal Areas
11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

ASBR
ASBR# show ip ospf database
Summary Net Link States (Area 0)
(Area 1 networks - Advertising Router ABR-2)
172.16.10.4 192.168.3.1 278 0x80000001 0xD126
172.16.20.0 192.168.3.1 278 0x80000001 0xA746
172.16.51.1 192.168.2.1 206 0x80000005 0xA832
• Link ID = IP network addresses of networks in other areas
• ADV Router = ABR Router ID sending the LSA-3
• Divided by ABR
• Bottom line: Should see networks in other areas and the ABR advertising that
route.
• Rick’s reminder: LSA 3 -> “networks sent by the A B R”
1 2 3

ASBR
ASBR# show ip route
O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:02:54, FastEthernet0/0
C 172.16.1.0/24 is directly connected, FastEthernet0/0
C 10.1.0.0 is directly connected, FastEthernet0/1
S 11.0.0.0/8 is directly connected, Null0
• Routes learned via LSA type 3s are denoted by an “IA” (Inter-Area
Routes) in the routing table.

Another example: non-area 0 router, Internal
Internal# show ip ospf database
LSA 3 - Generated by the ABR. Describes links between ABR and
Internal Routers of the Local Area
172.16.1.0 192.168.3.1 848 0x80000005 0xD339
172.16.51.1 192.168.3.1 843 0x80000001 0xB329
Internal# show ip route
O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:13:48, Serial0
O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:13:53, Serial0
O E2 11.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0
O E2 12.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0
O E2 13.0.0.0/8 [110/20] via 172.16.10.5, 00:14:42, Serial0

• Using the Multi-area OSPF Lab Handout: “Lab 1. –
OSPF Normal Areas” verify these results.
• Look at the link state database summary (show ip
ospf database) commands and the Summary Net
Links States (LSA3s) for each router.
• Look at the routing tables (show ip route) and notice
the Interarea (IA) routes.
• Why do some routers have more than one set of
Summary Net Links States?

LSA 4 – ASBR Summary Link States
• LSA 4 – ASBR Summary LSA
• Originated by the ABR.
• Flooded throughout the backbone area to the other
ABRs.
• Describes the reachability to the ASBRs
• Advertises an ASBR (Router ID) not a network
• Included in routing table as an “IA” route.
• Same format as a LSA 3 - Summary LSA, except LSA 4
ASBR Summary LSA the Network Mask field is always 0
Exceptions
• Not flooded to Stub and Totally Stubby networks.
• More on this later

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
Normal Areas
LSA 5’s flooded
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
LSA 4
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
LSA 4
LSA 4
Area 51 Internal
LSA 4
.1
172.16.20.0/24
• Flooded throughout the backbone area to the other ABRs.
Area 1
• How do the ABRs know about the ASBR? I am still researching this,
but I believe when routers receive an LSA 5 (AS External LSA) with
external route information, the routers denote the Router ID being the
ASBR.
Lo - RouterID
192.168.4.1/32
Area 1

ABR-2
ABR-2# show ip ospf database
Summary ASB Link States (Area 1)
LSA 4 - Reachability to ASBR.
Not flooded to Stub and Totally Stubby networks.
192.168.1.1 192.168.3.1 801 0x80000003 0x93CC
• Link ID 192.168.1.1 = Router ID of ASBR
• ADV Router 192.168.3.1 = Router ID ABR advertising route
• Bottom line: Routers in non-area 0, should see Router ID of ASBR
and its ABR to get there .
• Rick’s reminder: LSA 4 -> “Reachability to the A S B R”
1 2 3 4

Internal
LSA 4 - Reachability to ASBR.
Not flooded to Stub and Totally Stubby networks.
192.168.1.1 192.168.3.1 912 0x80000003 0x93CC
• Link ID 192.168.1.1 = Router ID of ASBR
• ADV Router 192.168.3.1 = Router ID ABR advertising route
• Note: No LSA 4s for Area 0 on Router B

database) commands and the Summary Net Links States
(LSA4s) for each router.
• Why do some routers have more than one set of Summary ASB
Links States and others may not (like RouterA and ASBR)?
• Which Area 0 routers have LSA 4’s in their LSDB?
• Why don’t some Area 0 routers have LSA 4’s in their LSDB?

LSA 5 - AS External Link States
• LSA 5 – AS External LSA
• Originated by the ASBR.
• Describes destination networks external to the Autonomous
System (This OSPF Routing Domain)
• Flooded throughout the OSPF AS except to stub and totally stubby
areas
• Denoted in routing table as E1 or E2 (default) route (soon)
• We will discuss default routes later.
• ASBR – Router which “redistributes” routes into the OSPF domain.
Exceptions
• Not flooded to Stub and Totally Stubby networks.
• More on this later

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| TOS | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Normal Areas
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
ASBR
router ospf 1
redistribute static
network 172.16.1.0 0.0.0.255 area 0
ip route 11.0.0.0 255.0.0.0 Null0
ip route 12.0.0.0 255.0.0.0 Null0
ip route 13.0.0.0 255.0.0.0 Null0

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Normal Areas
Lo - RouterID
192.168.1.1/32
LSA 5’s flooded
redistribute static
network 172.16.1.0 0.0.0.255 area 0
ip route 11.0.0.0 255.0.0.0 Null0
ip route 12.0.0.0 255.0.0.0 Null0
ip route 13.0.0.0 255.0.0.0 Null0
LSA 5
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
LSA 5
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
LSA 5
LSA 5
Area 51 Internal
LSA 5
.1
172.16.20.0/24
• “Redistribute” command creates an ASBR router.
Area 1
• Describes destination networks external to the OSPF Routing Domain
areas
Lo - RouterID
192.168.4.1/32
ASBR
router ospf 1

ABR-2
ABR-2# show ip ospf database
AS External Link States <- Note, NO Area!
LSA 5 - External Networks originated by the ASBR,
Flooded throughout A.S. except to Stub and Totally Stubby
Link ID ADV Router Age Seq# Checksum Tag
11.0.0.0 192.168.1.1 1191 0x80000001 0x3FEA 0
12.0.0.0 192.168.1.1 1191 0x80000001 0x32F6 0
13.0.0.0 192.168.1.1 1191 0x80000001 0x2503 0
• Link ID = External Networks
• ADV Router = Router ID of ASBR
• Note: For ABRs: There is only one set of “AS External Link States” in
database summary. In other words, an ABR router will only show one set of
“AS External Link States,” not one per area.
• Bottom line: All Routers should see External networks and the Router ID of
ASBR to get there .
• Rick’s reminder: LSA 5 -> O T H E R networks
1 2 3 4 5

ABR-2
ABR-2# show ip route
O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:11:44, FastEthernet0
O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:12:29, Serial0
• Designated by “E2”
• Notice that the cost is 20 for all three routes, we will see why later.
• It has to do with E2 routes and where the default cost is 20.
– Redistribute command (Route Optimization chapter): If a value is not
specified for the metric option, and no value is specified using the default-metric
command, the default metric value is 0, except for OSPF where
the default cost is 20.

Type-5 AS External Link States
LSA 5 - External Networks originated by the ASBR,
Flooded throughout A.S. except to Stub and Totally Stubby
11.0.0.0 192.168.1.1 1191 0x80000001 0x3FEA 0
12.0.0.0 192.168.1.1 1191 0x80000001 0x32F6 0
13.0.0.0 192.168.1.1 1191 0x80000001 0x2503 0
O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:13:48, Serial0
O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:13:53, Serial0
O E2 11.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0
O E2 12.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0
O E2 13.0.0.0/8 [110/20] via 172.16.10.5, 00:14:42, Serial0

E1 vs. E2 External Routes
• External routes fall under two categories:
– external type 1
– external type 2 (default)
• The difference between the two is in the way the cost (metric) of the
route is being calculated.
• The cost of a type 2 route is always the external cost, irrespective of
the interior cost to reach that route.
• A type 1 cost is the addition of the external cost and the internal cost
used to reach that route.
• A type 1 route is always preferred over a type 2 route for the same
destination.
• More later…

database) commands and the AS External Links States (LSA5s)
for each router.
• Also, look at the routing tables for each router.
• How many sets of LSA 5s does the ABRs have in their link state
summary database? Notice the ASBRs entries.

Stub Areas
Considerations for both Stub and Totally Stubby Areas
• An area could be qualified a stub when:
– There is a single exit point (a single ABR) from that area. More
than one ABR can be used, but be ready to “accept non-optimal
routing paths.”
– If routing to outside of the area does not have to take an optimal
path.
• The area is not needed as a transit area for virtual links (later).
• The ASBR is not within the stub area
• The area is not the backbone area (area 0)
• Stub areas will result in memory and processing savings depending
upon the size of the network.

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
Stub Area

Stub Areas
Receives all routes from within A.S.:
• Within the local area - LSA 1s and LSA 2s (if appropriate)
• From other areas (Inter-Area) - LSA 3s
Does not receive routes from External A.S. (External Routes).
ABR:
• ABR blocks all LSA 4s and LSA 5s.
• ‘If LSA 5s are not known inside an area, LSA 4s are not necessary.’
• LSA 3s are propagated by the ABR.
Note: Default route is automatically injected into stub area by ABR
– External Routes: Once the ABR gets a packet headed to a default route, it
must have a default route, either static or propagated by the ASBR via
default information originate (coming!)
Configuration:
• All routers in the area must be configured as “stub”

Stub Areas
ABR-2
router ospf 1
network 172.16.1.0 0.0.0.255 area 0
network 172.16.10.4 0.0.0.3 area 1
area 1 stub << Command: area area stub
Internal
router ospf 1
network 172.16.0.0 0.0.255.255 area 1
area 1 stub << Command: area area stub
• All routers in the area must be configured as “stub”
including the ABR

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
LSA 4
ABR-1 ABR-2
LSA 5
LSA 5
172.16.1.0/24
Blocked X Area 0
172.16.0.0/16
.1
LSA 3
172.16.51.0/24
LSA .5
3
LSA 4
X Blocked
172.16.10.4/30
.6
Area 51 Internal
Default
route to
ABR
.1
injected Area 1
172.16.20.0/24
• LSA 3s (Inter-Area routes) are propagated by the ABR.
• ABR blocks all LSA 4s (reachabilitly to ASBR) and Area LSA 5s 1
(External routes)
• The ABR injects a default route into the stub area, pointing to the ABR. (This
Lo - RouterID
192.168.4.1/32
Stub Area
does not mean the ABR has a default route of its own.)
• Essentially, internal routers in a Stub Area only see Inter-Area OSPF routes
and the default route to the ABR – No External routes.

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
LSA 4
ABR-1 ABR-2
LSA 5
LSA 5
172.16.1.0/24
Blocked X Area 0
172.16.0.0/16
.1
LSA 3
172.16.51.0/24
LSA .5
3
LSA 4
X Blocked
172.16.10.4/30
.6
Area 51 Internal
Default
route to
ABR
.1
injected
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
Stub Area
• Changes in External routes no longer affect Stub Area routing tables.

Stub Areas
Internal
LSA 3 - Generated by the ABR.
Describes links between ABR and Internal Routers of the Local Area
Default Route – Advertised by ABR-1
0.0.0.0 192.168.3.1 243 0x80000001 0x8A46
Area 0 networks - Advertised by ABR-1
172.16.1.0 192.168.3.1 243 0x80000006 0xEF1E
Area 51 networks - Advertised by ABR-1
172.16.51.1 192.168.3.1 243 0x80000002 0xCF0E
• Notice that there are no LSA 4s or LSA 5s for stub area routers.
• Default Route injected by ABR (LSA 3)

Stub Areas
Internal
Gateway of last resort is 172.16.10.5 to network 0.0.0.0
O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:03:08, Serial0
O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:03:08, Serial0
O*IA 0.0.0.0/0 [110/782] via 172.16.10.5, 00:03:08, Serial0
NOTE on default route:
• ABR will advertise a default route with a cost of 1
• cost of 65 = 1 + 64 (serial link)
• Using bandwidth of 128K, not 64K: 782 = (100,000,000/128,000) + 1

Stub Areas
ABR-2
Gateway of last resort is not set
O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:01:59,FastEthernet0
O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:01:59, Serial0
• Notice, there is no automatic default route on ABR, as there are with the
internal stub routers.

Your Turn -Discuss in groups (Stub)
• Using the Multi-area OSPF Lab Handout: “Lab 2. – OSPF Stub
Area.
database) commands and the Summary Net Links States (LSA
3s).
Note: A Stub area may have more than one ABR, but because of
the default route, the internal routers will not be able to
determine which router is the optimal gateway outside the AS
and end up load balancing between the multiple ABRs.

Totally Stubby Areas
“Cisco proprietary”, however the RFC does make some provisions for this
as an optional feature..
Same considerations as with Stub areas:
• An area could be qualified a stub when there is a single exit point (a
single ABR) from that area or if routing to outside of the area does not
have to take an optimal path.
upon the size of the network. - This is even more true with Totally
Stubby areas

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
Totally Stubby Area

Receives routes from within A.S.:
• Only from within the local area - LSA 1s and LSA 2s (if appropriate)
• Does not receive routes from other areas (Inter-Area) - LSA 3s
Does not receive routes from External A.S. (External Routes)
ABR:
• ABR blocks all LSA 4s and LSA 5s.
• ABR blocks all LSA 3s, except propagating a default route.
• Default route is injected into totally stubby area by ABR.
Configuring:
• All routers must be configured as “stub”
• ABR must be configured as “stub no-summary”

ABR-2
router ospf 1
network 172.16.1.0 0.0.0.255 area 0
network 172.16.10.4 0.0.0.3 area 1
area 1 stub no-summary
^^ Command: area area stub no-summary
Internal
router ospf 1
network 172.16.0.0 0.0.255.255 area 1
area 1 stub
^^ Command: area area stub

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
LSA 4
ABR-1 ABR-2
LSA 5
LSA 5
172.16.1.0/24
Blocked X Area 0
172.16.0.0/16
.1
LSA 3
172.16.51.0/24
LSA .5
3
LSA 4
X Blocked
172.16.10.4/30
.6
X
Area 51 Internal
Default
route to
ABR
.1
injected Area 1
172.16.20.0/24
• LSA 3s (Inter-Area routes) are blocked by the ABR.
• ABR blocks all LSA 4s (reachability to ASBR) and LSA Area 5s 1
(External routes)
• The ABR injects a default route (LSA 3) into the stub area, pointing to the ABR.
Lo - RouterID
192.168.4.1/32
Totally Stubby Area
(This does not mean the ABR has a default route of its own.)
• Essentially, internal routers in a Totally Stubby Area only see the default route
to the ABR.

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
LSA 4
ABR-1 ABR-2
LSA 5
LSA 5
172.16.1.0/24
Blocked X Area 0
172.16.0.0/16
.1
LSA 3
172.16.51.0/24
LSA .5
3
LSA 4
X Blocked
172.16.10.4/30
.6
X
Area 51 Internal
Default
route to
ABR
.1
injected Area 1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
Totally Stubby Area
• Changes in any networks outside the Totally Stubby Area no longer
affects the Totally Stubby routing tables.

Internal
LSA 3 - Generated by the ABR.
Describes links between ABR and Internal Routers of the Local Area
Default Route – Advertised by ABR-2
0.0.0.0 192.168.3.1 205 0x80000003 0x8648
Default Route injected by ABR (LSA 3)
• Default route is injected into totally stubby area by ABR for all other networks
(inter-area and external routes)
• Does not receive routes from other areas (Inter-Area)
• Does not receive routes from External A.S. (External Routes)

Internal
Gateway of last resort is 172.16.10.5 to network 0.0.0.0
O*IA 0.0.0.0/0 [110/782] via 172.16.10.5, 00:03:09, Serial0
• Default route is injected into totally stubby area by ABR for all other networks
(inter-area and external routes)
• Does not receive routes from other areas (Inter-Area)
• Does not receive routes from External A.S. (External Routes)

ABR-2
O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:02:35,FastEthernet0
O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:02:35, Serial0
• ABR will forward Intra-Area routes (to other areas within AS)
• Notice, there is not an automatic default route in the ABR’s routing
table like there is with the internal Totally Stubby routers.

OSPF design considerations
OSPF Design Tips
• Different people have different approaches to designing OSPF
networks.
• The important thing to remember is that any protocol can fail
under pressure.
• “The idea is not to challenge the protocol but rather to work with
it in order to get the best behavior.” CCO

Number of Routers per Area
The maximum number of routers per area depends on several factors,
including the following:
· What kind of area do you have?
· What kind of CPU power do you have in that area?
· What kind of media?
· Will you be running OSPF in NBMA mode?
· Is your NBMA network meshed?
· Do you have a lot of external LSAs in the network?
· Are other areas well summarized?
For this reason, it's difficult to specify a maximum number of routers per
area.

Stub and Totally Stubby Areas:
• An area could be qualified a stub when there is a single exit point (a
single ABR) from that area or if routing to outside of the area does not
have to take an optimal path.
upon the size of the network. - This is even more true with Totally
Stubby areas
• Totally Stubby areas is a Cisco enhancement.

Quick Review
• Areas
• LSAs
• Stub Area
• Totally Stubby Area

LSA-1 - Router LSA

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
LSA 1 .1
LSA 1 LSA 1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
within Area 0
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
within other areas
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
LSA 1
.1
172.16.51.0/24
.5
LSA 1
172.16.10.4/30
.6
LSA 1
Area 51 Internal
LSA 1
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
LSA 1’s are flooded out
other interfaces within
the same area.
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
LSA 1
Originated
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
LSA 1
flooded

LSA-2 - Network LSA

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
LSA 2
LSA 2
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
No LSA 2’s for ABR-1
in Area 51, or for
Internal because no
other routers on multi-access
segment.
Lo - RouterID
192.168.2.1/32
DR
ABR-1 ABR-2
LSA 2
flooded LSA 2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
LSA 2
flooded
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
DR

LSA-3 - Summary LSA

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
LSA 1
LSA 1 LSA 1
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
areas by the ABRs.
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
LSA 3 172.16.1.0/24
LSA 3
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
LSA 3
LSA 3
LSA 3
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
areas by the ABRs.
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
LSA 1
LSA 3
172.16.51.0/24
.5
LSA 1
172.16.10.4/30
.6
LSA 3
LSA 1
LSA 3
Area 51 Internal
LSA 1
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Multi Area OSPF
Normal Areas
Lo - RouterID
192.168.1.1/32
Don’t forget about the LSA
1’s from Area 0.
LSA 1
LSA 1 LSA 1
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
LSA 3
172.16.51.0/24
.5
172.16.10.4/30
.6
LSA 3
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

LSA-4 – ASBR Summary LSA

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
Normal Areas
LSA 5’s flooded
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
LSA 4
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
LSA 4
LSA 4
Area 51 Internal
LSA 4
.1
172.16.20.0/24
• Flooded throughout the backbone area to the other ABRs.
Area 1
Lo - RouterID
192.168.4.1/32
Area 1

LSA-5 - External LSA

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Normal Areas
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
Area 51 Internal
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
ASBR
router ospf 1
redistribute static
network 172.16.1.0 0.0.0.255 area 0
ip route 11.0.0.0 255.0.0.0 Null0
ip route 12.0.0.0 255.0.0.0 Null0
ip route 13.0.0.0 255.0.0.0 Null0

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Normal Areas
Lo - RouterID
192.168.1.1/32
LSA 5’s flooded
redistribute static
network 172.16.1.0 0.0.0.255 area 0
ip route 11.0.0.0 255.0.0.0 Null0
ip route 12.0.0.0 255.0.0.0 Null0
ip route 13.0.0.0 255.0.0.0 Null0
LSA 5
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
ABR-1 ABR-2
172.16.1.0/24
Area 0
172.16.0.0/16
LSA 5
.1
172.16.51.0/24
.5
172.16.10.4/30
.6
LSA 5
LSA 5
Area 51 Internal
LSA 5
.1
172.16.20.0/24
• “Redistribute” command creates an ASBR router.
Area 1
• Describes destination networks external to the OSPF Routing Domain
areas
Lo - RouterID
192.168.4.1/32
ASBR
router ospf 1

Stub Area

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
LSA 4
ABR-1 ABR-2
LSA 5
LSA 5
172.16.1.0/24
Blocked X Area 0
172.16.0.0/16
.1
LSA 3
172.16.51.0/24
LSA .5
3
LSA 4
X Blocked
172.16.10.4/30
.6
Area 51 Internal
Default
route to
ABR
.1
injected
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
Stub Area

Totally Stubby Area

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
LSA 4
ABR-1 ABR-2
LSA 5
LSA 5
172.16.1.0/24
Blocked X Area 0
172.16.0.0/16
.1
LSA 3
172.16.51.0/24
LSA .5
3
LSA 4
X Blocked
172.16.10.4/30
.6
X
Area 51 Internal
Default
route to
ABR
.1
injected Area 1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
Totally Stubby Area

Next
• E1 and E2 routes
• Default Routes
• Route Summarization
• NSSA (Not So Stubby Areas)
• Multiple ABR Scenario
• Multiple ASBR Scenario
• Virtual Links
• Load Balancing
• show commands

NSSA (Not So Stubby Area)
N S S A
A r e a 2
B a c k b o n e A r e a
A r e a 0
R I P
R T G
A S B R
R T B R T A
A B R ( P o s s i b l e
A S B R )
R T D
R T C
R T E
R T F
R T H

• Relatively new, standards based OSPF enhancement, RFC 1587.
• NSSA allows an area to remain a stub area, but carry external routing
information (Type 7 LSAs) from its stubby end back towards the OSPF
backbone.
• ASBR in NSSA injects external routing information into the backbone
and the NSSA area, but rejects external routing information coming
from the ABR.
• The ABR does not inject a default route into the NSSA.
– This is true for a NSSA Stub, but a default route is injected for a
NSSA Totally Stubby area.
• Note: RFC 1587, “A default route must not be injected into the NSSA
as a summary (type-3) LSA as in the stub area case.”
• What???
• Following scenario is only example of how NSSA works. For the
purposes of learning about NSSAs, don’t get hung up on the why’s and
what if’s.

N S S A
A r e a 2
A r e a 0
Default route via RTG
R I P
R T G
A S B R
R T B R T A
A S B R )
R T D
R T C
R T E
R T F
R T H
NSSA Stub Area
• Area 2 would like to be a stub network.
• RTH only supports RIP, so RTG will run RIP and redistribute those routes in
OSPF.
• Unfortunately, this makes the area 2 router, RTG, an ASBR and therefore
area 2 can no longer be a stub area.
• RTH does not need to learn routes from OSPF, a default route to RTG is all it
needs.
• But all OSPF routers must know about the networks attached to the RIP
router, RTH, to route packets to them.

N S S A
A r e a 2
A r e a 0
R I P
R T G
A S B R
LSA 5
R T B R T A
A S B R )
LSA 7
R T D
R T C
R T E
LSA 7
R T F
R T H
LSA 7
LSA 7
LSA 7
LSA 7
LSA 7s
Blocked
NSSA Stub Area (cont.)
• NSSA allow external routes to be advertised into the OSPF AS while retaining the
characteristics of a stub area to the rest of the OSPF AS.
• ASBR RTG will originate Type-7 LSAs to advertise the external destinations.
• These LSA 7s are flooded through the NSSA but are blocked by the NSSA ABR.
• The NSSA ABR translates LSA 7s into LSA 5s and flood other areas.

N S S A
A r e a 2
A r e a 0
R I P
R T G
A S B R
LSA 5
R T B R T A
A S B R )
LSA 7
R T D
R T C
R T E
LSA 7
R T F
R T H
LSA 7
LSA 7
LSA 7
LSA 7
LSA 7s
Blocked
Type 7 LSA NSSA External Link Entry
• Originated by an ASBR connected to an NSSA.
• Type 7 messages can be flooded throughout NSSAs and translated
into LSA Type 5 messages by ABRs.
• Routes learned via Type-7 LSAs are denoted by either a default
“N1” or an “N2” in the routing table. (Relative to E1 and E2).

N S S A
A r e a 2
NSSA Generic
R I P
R T G
A S B R
LSA 7
R T D
R T C
R T E
LSA 7
R T F
R T H
LSA 7
LSA 7
Configuring NSSA Stub Area
Configured for all routers in Area 2:
router ospf 1
LSA 5
R T B R T A
LSA 7
LSA 7
network 172.16.2.0 0.0.0.255 area 2
area 2 nssa
A r e a 0
A S B R )
LSA 7s
Blocked

NSSA Stub and NSSA Totally Stubby
• There are two flavors in NSSA:
– Stub
– Totally Stubby
• Area 2 routers may or may not receive Inter-area routes from RTA, depending
upon NSSA configuration
• NSSA areas have take on the same characteristics as stub and totally stubby
areas, along with the characteristics of NSSA areas.

NSSA –Stub
NSSA stub areas:
• NSSAs that block type 4 and 5, but allow type 3.
• To make a stub area into an NSSA, use the following command under
the OSPF configuration.
• This command must be configured on all routers in area 2.
router ospf 1
area 2 nssa

N S S A
A r e a 2
A r e a 0
NSSA Stub Areas
R I P
R T G
A S B R
LSA 4s & LSA 5s X
X RTH
LSA 5
LSA 3s
0.0.0.0/0
routes:
E1/E2
R T B R T A
A S B R )
RTH routes:N1/N2
LSA 7
R T D
R T C
R T E
LSA 7
R T F
R T H
LSA 7
LSA 7
LSA 7
LSA 7
LSA 7s
Blocked
NSSA Stub Area Routing Tables:
• RTG: Area 2 routes, Area 0 routes (IA), RTH RIP routes
– No 0.0.0.0/0 (IA) route from RTB (ABR), despite documentation
• Area 2 Internal Routers: Area 2 routes, RTH routes (N1/N2), Area 0 routes (IA)
– No 0.0.0.0/0 (IA) route from RTB (ABR), despite documentation
• RTB: Area 2 routes, Area 0 routes, RTH routes (N1/N2), External routes if redistributed from
RTA ASBR (E1/E2)
• RTA: Area 0 routes, Area 2 routes, RTH routes (E1/E2), External routes if redistributed from
RTA (E1/E2)
• Note: Area 2 routers may or may not receive E1/E2 routes from RTA, depending upon NSSA
configuration (next).

NSSA Stub Areas
R I P
R T G
A S B R
R T H
Area 2 routers:
router ospf 1
N S S A
A r e a 2
RTH routes:N1/N2
LSA 7
R T D
R T C
R T E
LSA 7
R T F
LSA 7
LSA 7
LSA 3s
LSA 4s & LSA 5s X
X RTH
0.0.0.0/0
LSA 7
LSA 7
network 172.16.2.0 0.0.0.255 area 2
area 2 nssa
A r e a 0
LSA 5
routes:
E1/E2
R T B R T A
A S B R )
LSA 7s
Blocked

NSSA – Totally Stubby
NSSA Totally Stubby Area
• NSSA totally stub areas: Allow only summary default routes and filters everything
else.
• To configure an NSSA totally stub area, use the following command under the
OSPF configuration on the NSSA ABR:
router ospf 1
area 2 nssa no-summary
• Configure this command on NSSA ABRs only.
• All other routers in area 2 (internal area 2 routers):
router ospf 1
area 2 nssa
• After defining the NSSA totally stub area, area 2 has the following characteristics (in
addition to the above NSSA characteristics):
· No type 3 or 4 summary LSAs are allowed in area 2. This means no
inter-area routes are allowed in area 2.
· A default route is injected into the NSSA totally stub area as a type 3
summary LSA by the ABR.

NSSA Totally Stubby Areas
N S S A
A r e a 2
A r e a 0
R I P
R T G
A S B R
LSA 4s & LSA 5s X
LSA 5
LSA 3s X
0.0.0.0/0
RTH
routes:
E1/E2
R T B R T A
A S B R )
RTH routes: N1/N2
LSA 7
R T D
R T C
R T E
LSA 7
R T F
R T H
LSA 7
LSA 7
LSA 7
LSA 7
LSA 7s
Blocked
RTB (ABR):
router ospf 1
network 172.16.1.0 0.0.0.255 area 0
network 172.16.2.0 0.0.0.255 area 2 ...
area 2 nssa no-summary
Area 2 routers:
router ospf 1
network 172.16.2.0 0.0.0.255 area 2
area 2 nssa

NSSA Totally Stubby Areas
N S S A
A r e a 2
A r e a 0
R I P
R T G
A S B R
LSA 4s & LSA 5s X
LSA 5
LSA 3s X
0.0.0.0/0
RTH
routes:
E1/E2
R T B R T A
A S B R )
RTH routes: N1/N2
LSA 7
R T D
R T C
R T E
LSA 7
R T F
R T H
LSA 7
LSA 7
LSA 7
LSA 7
LSA 7s
Blocked
NSSA Totally Stubby Area Routing Tables:
• RTG: Area 2 routes, RTH RIP routes, 0.0.0.0/0 (IA) route from RTB (ABR)
– Totally Stubby: No Area 0 routes or external routes from RTA
• Area 2 Internal Routers: Area 2 routes, RTH routes (N1/N2), 0.0.0.0/0 (IA) route from RTB
(ABR)
– Totally Stubby: No Area 0 routes or external routes from RTA
• RTB: Area 2 routes, Area 0 routes, RTH routes (N1/N2), External routes from RTA ASBR
(E1/E2) if redistributed by ASBR
• RTA: Area 0 routes, Area 2 routes, RTH routes (E1/E2), External routes (E1/E2)

For More on NSSA
For more examples
• See NSSA document on my web site for more info.
A r e a 2
N S S A A r e a 0
2 0 0 . 2 0 0 . 2 0 0 . 0 / 2 4
1 0 . 0 . 0 . 0 / 8
R T D R T C R T B R T A
R T E
R I P
D e f a u l t R o u t e
1 7 2 . 1 6 . 3 . 0 / 2 4 1 7 2 . 1 6 . 2 . 0 / 2 4 1 7 2 . 1 6 . 1 . 0 / 2 4
2 2 2 . 2 2 2 . 2 2 2 . 0 / 2 4

Virtual Links

Virtual Links
• All areas in an OSPF autonomous system must be physically
connected to the backbone area (area 0).
• In some cases where this is not possible, you can use a virtual link to
connect to the backbone through a non-backbone area.
• As mentioned above, you can also use virtual links to connect two
parts of a partitioned backbone through a non-backbone area.
• The area through which you configure the virtual link, known as a
transit area, must have full routing information.
• Must be configured between two ABRs.
• The transit area cannot be a stub area.

Virtual Links
• A virtual link has the following two requirements:
– It must be established between two routers that share a common
area and are both ABRs.
– One of these two routers must be connected to the backbone.
• Doyle, “should be used only as a temporary fix to an unavoidable
topology problem.”

Virtual Links
The command to configure a virtual link is as follows:
area <area-id> virtual-link <remote-router-id>
RTA(config)#router ospf 1
RTA(config-router)#network 192.168.0.0 0.0.0.255 area 51
RTA(config-router)#network 192.168.1.0 0.0.0.255 area 3
RTA(config-router)#area 3 virtual-link 10.0.0.1
...
RTB(config)#router ospf 1
RTB(config-router)#network 192.168.1.0 0.0.0.255 area 3
RTB(config-router)#network 192.168.2.0 0.0.0.255 area 0
RTB(config-router)#area 3 virtual-link 10.0.0.2

Virtual Links
• OSPF allows for linking discontinuous parts of the backbone using a
virtual link.
• In some cases, different area 0s need to be linked together. This can
occur if, for example, a company is trying to merge two separate
OSPF networks into one network with a common area 0.
• In other instances, virtual-links are added for redundancy in case
some router failure causes the backbone to be split into two. (CCO)
• Whatever the reason may be, a virtual link can be configured
between separate ABRs that touch area 0 from each side and having
a common area.

Virtual Links – Another Example

Route Summarization
Inter-Area Route Summarization - Area Range
• By default ABRs do not summarize routes between areas.
• Route summarization is the consolidation of advertised addresses.
• This feature causes a single summary route to be advertised to other areas by
an ABR.
• In OSPF, an ABR will advertise networks in one area into another area.
• If the network numbers in an area are assigned in a way such that they are
contiguous, you can configure the ABR to advertise a summary route that
covers all the individual networks within the area that fall into the specified
range.
On the ABR (Summarizes routes before injecting them into different area)
Router(config-router)# area area-id range network-address
subnet-mask
• area-id - Identifier of the area about which routes are
to be summarized. (From area)

Route Summarization
• RTB is summarizing the range of subnets from 128.213.64.0 to 128.213.95.0
into one range: 128.213.64.0 255.255.224.0.
• This is achieved by masking the first three left most bits of 64 using a mask of
255.255.224.0.
128.213.64.0/24 - 010 00000
128.213.95.0/24 – 010 11111
-----------------------------------------
128.213.64.0/19 - 01000000

Route Summarization
• In the same way, RTC is generating the summary address 128.213.96.0
255.255.224.0 into the backbone.
• Note that this summarization was successful because we have two distinct
ranges of subnets, 64-95 and 96-127.
128.213.96.0/24 - 011 00000
128.213.127.0/24 – 011 11111
-----------------------------------------
128.213.96.0/19 - 01100000

Route Summarization
128.213.64.0/24 - 010 00000
128.213.95.0/24 – 010 11111
-----------------------------------------
128.213.64.0/19 - 01000000
RTB
router ospf 100
area 1 range 128.213.64.0 255.255.224.0

Route Summarization
128.213.96.0/24 - 011 00000
128.213.127.0/24 – 011 11111
-----------------------------------------
128.213.96.0/19 - 01100000
RTC
router ospf 100
area 2 range 128.213.96.0 255.255.224.0

Route Summarization
External Route Summarization - summary-address
• When redistributing routes from other protocols into OSPF (later), each route is
advertised individually in an external link state advertisement (LSA).
• However, you can configure the Cisco IOS software to advertise a single route
for all the redistributed routes that are covered by a specified network address
and mask.
• Doing so helps decrease the size of the OSPF link state database.
On the ASBR only (Summarizes external routes before injecting them into the
OSPF domain.)
Router(config-router)# summary-address network-address
subnet-mask

Route Summarization
RTA
router ospf 100
summary-address 128.213.64.0 255.255.224.0
redistribute bgp 50 metric 1000 subnets (later)
RTD
router ospf 100
summary-address 128.213.96.0 255.255.224.0
redistribute bgp 20 metric 1000 subnets (later)

Injecting Default Routes into OSPF
• By default, 0.0.0.0/0 route is not propagated from the ASBR to other
routers.
• An autonomous system boundary router (ASBR) can be forced to
generate a default route into the OSPF domain.
• As discussed earlier, a router becomes an ASBR whenever routes are
redistributed into an OSPF domain.
• However, an ASBR does not, by default, generate a default route into
the OSPF routing domain.

The way that OSPF generates default routes (0.0.0.0) varies depending
on the type of area the default route is being injected into.
Stub and Totally Stubby Areas
• For stub and totally stubby areas, the area border router (ABR) to the
stub area generates a summary link-state advertisement (LSA) with the
link-state ID 0.0.0.0.
• This is true even if the ABR doesn't have a default route.
• In this scenario, you don't need to use the default-information
originate command.

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Stub Area
Lo - RouterID
192.168.2.1/32
LSA 4
ABR-1 ABR-2
LSA 5
LSA 5
172.16.1.0/24
Blocked X Area 0
172.16.0.0/16
.1
LSA 3
172.16.51.0/24
LSA .5
3
LSA 4
X Blocked
172.16.10.4/30
.6
Area 51 Internal
Default
route to
ABR
.1
injected Area 1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
Stub Area
All routers in the area must be configured as “stub” including the ABR:
router ospf 1
area 1 stub

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
ASBR
.1
Lo - RouterID
192.168.1.1/32
Totally Stubby Area
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
Lo - RouterID
192.168.2.1/32
LSA 4
ABR-1 ABR-2
LSA 5
LSA 5
172.16.1.0/24
Blocked X Area 0
172.16.0.0/16
.1
LSA 3
172.16.51.0/24
LSA .5
3
LSA 4
X Blocked
172.16.10.4/30
.6
X
Area 51 Internal
Default
route to
ABR
.1
injected Area 1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32
Totally Stubby Area
All routers in the area must be configured as “stub” except the ABR “stub no summary”:
ABR: router ospf 1 Other: router ospf 1
area 1 stub no-summary area 1 stub

Normal Areas
• By default, in normal areas routers don't generate default routes.
• To have an OSPF router generate a default route, use the default-information
originate command.
• This generates an external type-2 link with link-state ID 0.0.0.0 and
network mask 0.0.0.0.
• This command should only be used on the ASBR.
– Some documentation states this command works only on an
ASBR while other documentation states this command turns a
router into an ASBR.

To have OSPF generate a default route use the following:
router ospf 10
default-information originate [always] [metric metric-value]
[metric-type type-value] [route-map map-name]

There are two ways to generate a default.
1) default-information originate
• If the ASBR already has the default route (ip route 0.0.0.0 0.0.0.0),
you can advertise 0.0.0.0 into the area.
2) default-information originate always
• If the ASBR doesn't have the route (ip route 0.0.0.0 0.0.0.0), you can
add the keyword always to the default-information originate
command, and then advertise 0.0.0.0.
• You should be careful when using the always keyword. If your router
advertises a default (0.0.0.0) inside the domain and does not have a
default itself or a path to reach the destinations, routing will be
broken.

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
.1
Lo - RouterID
192.168.1.1/32
0.0.0.0/0
0.0.0.0/0 0.0.0.0/0
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
0.0.0.0/0
172.16.1.0/24
Area 0
172.16.0.0/16
Lo - RouterID
192.168.2.1/32
.1
0.0.0.0/0
172.16.51.0/24
.5
172.16.10.4/30
.6
0.0.0.0/0 0.0.0.0/0
Area 51 Internal
ASBR
router ospf 1
ASBR
ABR-1 ABR-2
redistribute static
network 172.16.1.0 0.0.0.255 area 0
default-information originate
ip route 0.0.0.0 0.0.0.0 10.0.0.2
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

11.0.0.0/8
12.0.0.0/8
13.0.0.0/8
10.1.0.0/24
.1
No 0.0.0.0/0 route, but
propagated anyway or
“always”
Lo - RouterID
192.168.1.1/32
0.0.0.0/0 0.0.0.0/0
.2 .3
Lo - RouterID
192.168.3.1/32 Pri 200 Pri 100
0.0.0.0/0
172.16.1.0/24
Area 0
172.16.0.0/16
Lo - RouterID
192.168.2.1/32
.1
0.0.0.0/0
172.16.51.0/24
.5
172.16.10.4/30
.6
0.0.0.0/0 0.0.0.0/0
Area 51 Internal
ASBR
router ospf 1
ASBR
ABR-1 ABR-2
redistribute static
network 172.16.1.0 0.0.0.255 area 0
default-information originate always
ip route 0.0.0.0 0.0.0.0 10.0.0.2
.1
172.16.20.0/24
Area 1
Lo - RouterID
192.168.4.1/32

Redistributing External Routes
E1 vs. E2 External Routes
• External routes fall under two categories, external type 1 and
external type 2.
• The difference between the two is in the way the cost (metric) of the
route is being calculated.
• A type 1 (E1) cost is the addition of the external cost and the internal
cost used to reach that route.
• The cost of a type 2 (E2) route is always the external cost,
irrespective of the interior cost to reach that route.
• Type 2 (E2) is the default!

router ospf 1
redistribute routing-protocol metric-type [1|2]
• metric-type 1 - A type 1 cost is the addition of the external cost and
the internal cost used to reach that route.
redistribute rip [metric value] metric-type 1
• metric-type 2 - The cost of a type 2 route is always the external
cost, irrespective of the interior cost to reach that route.
• We will look at this command, along with internal/external costs, later
in the chapter discussion route redistribution.

metric-type 2 - The cost of a type 2 route is always the external cost,
irrespective of the interior cost to reach that route.
More later, but here is a taste of the metric value option …
• If a value is not specified for the metric value option, and no value is
specified using the default-metric command, the default metric value
is 0, except for OSPF where the default cost is 20.
• 0 is only understood by IS-IS and not by RIP, IGRP and EIGRP.
• RIP, IGRP and EIGRP must have the appropriate metrics assigned to
any redistributed routes, or redistribution will not work.
• Use a value consistent with the destination protocol.

L o o p 1 6 2 . 1 0 . 5 . 1 / 1 6
R I P
574 510
. 1 . 4
R o u t e r E
A S - R e m o t e
192.10.10.0
L o o p 1 . 1 0 . 2 0 2 . 2 0 6 / 2 4
. 2
10.0.0.0/8
R I P
. 1
metric-type 1
510
A S B R
. 3
L o o p 1 . 5 . 2 0 2 . 2 0 6 / 2 4
510
RIP routes redistributed with a
metric (cost) of 500 plus the
outgoing cost of the interface
and a metric-type 1
. 1 . 2
R o u t e r A
S w i t c h
2 0 6 . 2 0 2 . 0 . 0 / 2 4
O S P F
A r e a 0
510
574
L o o p 1 . 0 . 2 0 2 . 2 0 6 / 2 4 R o u t e r B
. 1
L o o p 2 . 0 . 2 0 2 . 2 0 6 / 2 4
2 0 6 . 2 0 2 . 1 . 0 / 2 4
574 . 2
584
R o u t e r C
. 1
L o o p 1 . 2 . 2 0 2 . 2 0 6 / 2 4
S w i t c h
O S P F
A r e a 1
2 0 6 . 2 0 2 . 2 . 0 / 2 4
. 2
R o u t e r D
584
L o o p 2 . 2 . 2 0 2 . 2 0 6
/24
O S P F
A r e a 5 1
. 2
. 1
574
R o u t e r F
L o o p 2 . 1 0 . 2 0 2 . 2 0 6 / 2 4
1 9 2 . 1 0 . 5 . 0 / 2 4
ASBR
router ospf 1
redistribute rip metric 500 metric-type 1
network 206.202.0.0 0.0.0.255 area 0

L o o p 1 6 2 . 1 0 . 5 . 1 / 1 6
R I P
500
. 1 . 4
R o u t e r E
A S - R e m o t e
192.10.10.0
L o o p 1 . 1 0 . 2 0 2 . 2 0 6 / 2 4
. 2
10.0.0.0/8
R I P
. 1
metric-type 2
RIP routes redistributed with a
metric (cost) of 500 and a
metric-type 2 (default)
500
A S B R
. 3
L o o p 1 . 5 . 2 0 2 . 2 0 6 / 2 4
500
. 1 . 2
R o u t e r A
S w i t c h
2 0 6 . 2 0 2 . 0 . 0 / 2 4
O S P F
A r e a 0
L o o p 1 . 0 . 2 0 2 . 2 0 6 / 2 4 R o u t e r B
. 1
L o o p 2 . 0 . 2 0 2 . 2 0 6 / 2 4
2 0 6 . 2 0 2 . 1 . 0 / 2 4
. 2
500 500
R o u t e r C
. 1
500
L o o p 1 . 2 . 2 0 2 . 2 0 6 / 2 4
S w i t c h
O S P F
A r e a 1
2 0 6 . 2 0 2 . 2 . 0 / 2 4
. 2
R o u t e r D
L o o p 2 . 2 . 2 0 2 . 2 0 6
/24
O S P F
A r e a 5 1
. 2
. 1
500
R o u t e r F
L o o p 2 . 1 0 . 2 0 2 . 2 0 6 / 2 4
1 9 2 . 1 0 . 5 . 0 / 2 4
ASBR
router ospf 1
redistribute rip metric 500 metric-type 2
network 206.202.0.0 0.0.0.255 area 0

So when should you redistribute a Type-1 (E1) External route?
• If there is more than one ABR for the area and the area is not a stub or
totally stubby area.
– In this case one of the ABRs may provide a shorter path
for certain non-area 0 internal routers, than other ABRs.
– E1 routes will include all internal costs from the internal
router to the ABR and to the ASBR, allowing each router
to choose which ABR provides the shorter path.
• Multiple ASBRs redistributing the same networks.
– In this case the routers’ cost to each ASBR can be used
to choose the shortest path to the destination.

Know your outputs
• show ip route
• show ip ospf
• show ip ospf neighbor
• show ip ospf border-router
• show ip database
• show ip interface

show ip route
Internal#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
<text omitted>
O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:13:48, Serial0
O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:13:53, Serial0
O E2 11.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0
O E2 12.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0
O E2 13.0.0.0/8 [110/20] via 172.16.10.5, 00:14:42, Serial0
Internal#
• LSA 1 and LSA 2: Denoted by “O” or “C”
• LSA 3: Denoted by “IA”
• LSA 5: Denoted by “E1” or “E2” (default)

show ip ospf
ABR-2#show ip ospf
Routing Process "ospf 1" with ID 192.168.3.1
Supports only single TOS(TOS0) routes
It is an area border router
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 3. Checksum Sum 0x97E3
Number of DCbitless external LSA 0
Number of DoNotAge external LSA 0
Number of areas in this router is 2. 2 normal 0 stub 0 nssa
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 8 times
<text omitted>
Area 1
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 5 times
<text omitted>

show ip ospf neighbor
ASBR#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.3.1 100 FULL/BDR 00:00:37 172.16.1.3 FastEthernet0/0
192.168.2.1 200 FULL/DR 00:00:33 172.16.1.2 FastEthernet0/0
• Displays a list of neighbors and their link state status

show ip ospf border-router
• To display the internal OSPF routing table entries to an Area Border Router (ABR) and
Autonomous System Boundary Router (ASBR), use the show ip ospf border-routers
privileged EXEC command.
• LSA 4’s (routes to ASBRs) are not installed in the main IP routing table but in the special
internal OSPF routing table.
This command will displays any ABRs in the area or any ASBRs in the OSPF routing domain.
• Destination - Router ID of the destination.
• Next Hop - Next hop toward the destination.
• Cost - Cost of using this route.
• Type - The router type of the destination; it is either an ABR or ASBR or both.
• Rte Type - The type of this route; it is either an intra-area or interarea route.
• Area - The area ID of the area from which this route is learned.
• SPF No - The internal number of the shortest path first (SPF) calculation that installs this route.
ABR-1#show ip ospf border
OSPF Process 1 internal Routing Table
Codes: i - Intra-area route, I - Inter-area route
i 192.168.1.1 [1] via 172.16.1.1, FastEthernet0/0, ASBR, Area 0, SPF 38
i 192.168.3.1 [1] via 172.16.1.3, FastEthernet0/0, ABR, Area 0, SPF 38
ABR-1#

show ip ospf database
Internal#show ip ospf data
Router Link States (Area 1)
192.168.3.1 192.168.3.1 898 0x80000003 0xCE56 2
192.168.4.1 192.168.4.1 937 0x80000003 0xFD44 3
172.16.1.0 192.168.3.1 848 0x80000005 0xD339
172.16.51.1 192.168.3.1 843 0x80000001 0xB329
192.168.1.1 192.168.3.1 912 0x80000003 0x93CC
Type-5 AS External Link States
11.0.0.0 192.168.1.1 1302 0x80000001 0x3FEA 0
12.0.0.0 192.168.1.1 1303 0x80000001 0x32F6 0
13.0.0.0 192.168.1.1 1303 0x80000001 0x2503 0

Router Link States (LSA 1)
• Router Link States (LSA1’s) should display all the RouterIDs of routers in that area, including its own.
• Link State ID is always the same as the Advertising Router.
• ADV Router is the Router ID of the router that created this LSA 1.
Net Link States (LSA 2)
• Net Link States (LSA2’s) should display the RouterIDs of the DRs on all multi-access networks in the area
and their IP addresses.
• Link ID is the IP address of DR on MultiAccess Network.
• ADV Router is the Router ID of the DR.
Summary Link States (LSA 3)
• Should see networks in other areas and the ABR advertising that route.
• Link ID is the IP network addresses of networks in other areas.
• ADV Router is the ABR Router ID sending the LSA-3.
Summary ASB Link States (LSA 4)
• Routers in non-area 0, should see Router ID of ASBR and its ABR to get there.
• Link ID is the Router ID of ASBR
• ADV Router is the Router ID of the ABR advertising route
Type-5 AS External Link States (LSA 5)
• All Routers should see External networks and the Router ID of ASBR to get there
• Link ID is the External Network
• ADV Router is the Router ID of ASBR advertising the LSA 5.

show ip ospf interface
SanJose3#show ip ospf interface fa 0
FastEthernet0 is up, line protocol is up
Internet Address 192.168.1.3/24, Area 0
Process ID 1, Router ID 192.168.31.33, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 192.168.31.33, Interface address 192.168.1.3
Backup Designated router (ID) 192.168.31.22, Interface address 192.168.1.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:08
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 2
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 2, Adjacent neighbor count is 2
Adjacent with neighbor 192.168.31.11
Adjacent with neighbor 192.168.31.22 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
SanJose3#
Do you know these?

OSPF Extra’s, FAQs, and FYIs
• The following sections contain information to help you understand
OSPF.
• This information is not necessarily on the CCNP Advanced Routing
Exam.

Extra: OSPF over ISDN
• OSPF Hello traffic can keep an ISDN link up indefinitely.
• By entering the command “ip ospf demand-circuit” on one side of a BRI,
adjacencies will be formed and:
– Ongoing OSPF Hellos will be suppressed
– The DNA (Do-Not-Age) bit is set in the LSA so that this entry is not aged in
the router’s LSDB.
• LSA is not flooded when reaching LSRefresh
• LSA is not flooded if there is a new version but the contents are the
same
show ip ospf interface bri 0
• “Run as demand circuit”
• “(Hello Suppressed)”
show ip ospf neighbor
• Dead Time: “-”

Extra: OSPF over ISDN
Router1
interface BRI1/1
ip address 192.158.254.13 {/30}
ip ospf demand-circuit
router ospf 20
network 192.158.254.0 0.0.0.255 area 0
Router2
interface BRI1/0
ip address 192.158.254.14 {/30}
router ospf 20
network 192.158.254.0 0.0.0.255 area 0
Note: You need to configure the demand circuit at one end of the link only.
However, if you configure this command on both ends it does not cause any
harm.
Suggestion: To reduce the affect of link flaps on the demand circuit, configure
the area that contains the demand circuit as totally stub.
• In this case configure Area 1 to be a totally stubby area.
• Summarizing routes on Router 1 can also help if the flapping link is within the
summarized range.

Extra: OSPF and Load Balancing
• OSPF only supports equal-cost load balancing.
• By default, four equally good routes to the same destination are
kept in the routing table for load balancing.
• This can be increased up to six with the maximum-paths
command.
• The bandwidth and/or ip ospf cost (or in the case of serial
links [1.544 Mbps] the lack of) commands can be used to make
unequal-cost links look like equal-cost links to OSPF for load
balancing.
– This should be done with caution, as it may burden slower
links and/or not make efficient use of faster links.

Extra: OSPF and DNS Lookups
• Loopback interfaces simplify the management and
troubleshooting of OSPF routing domains by providing
predictable Router Ids.
• This can be taken one step further by recording the Router Ids
in a Domain Name Service (DNS) database.
• The router can then be configured to consult the server
address-to-name mappings, or Reverse DNS lookups, and then
display the routers by name instead of by Router ID.

Extra: OSPF and DNS Lookups
ASBR#show ip ospf data
Router Link States (Area 0)
172.16.10.5 ABR-1 412 0x8000000F 0x6F9C 1
192.168.1.1 ABR-2 201 0x80000012 0x8D3D 1
192.168.2.1 ABR-2 205 0x80000016 0x7E46 1
192.168.3.1 ABR-2 205 0x80000005 0x9C36 1
ASBR was configured to perform DNS lookups as follows:
ip name-server 172.16.1.100
ip ospf name-lookup
• The first command specifies the DNS server.
• The second command enables the OSPF process to perform DNS lookups.
• This can also be used for identifying router interfaces such as ABR-1 and
ABR-2

Extra: IOS 12.01(T) – router-id
router-id
• To use a fixed router ID, use the router-id router configuration
command.
• To force OSPF to use the previous OSPF router ID behavior, use the
no form of this command.
• Takes precedence over Loopback address
router ospf 1
router-id ip-address

OSPF and Redistribution (later)
• “Before Cisco IOS Software Release 12.1.3, when redistributing connected
routes into OSPF, connected networks included in the network statements
under router OSPF advertised in Type-1, Type-2, or Type-3 link-state
advertisements (LSAs) were also announced in Type-5 LSAs.”
• In other words, if you are using the redistributed connected command, any
connected networks included using the OSPF network command, were not
only advertised as normal using LSA Type 1, 2, or 3, but also as an external
LSA Type-5.
• “Memory is required to store those Type-5 LSAs. The storage also requires a
CPU to process the LSAs during full or partial Shortest Path First (SPF) runs
and to flood them when some instability occurs.”
• “In Cisco IOS Software Release 12.1(3) and later, the Type-5 LSAs are no
longer created for connected networks included in the network statements
under router OSPF.”
Redistributing Connected Networks into OSPF
• http://www.cisco.com/warp/public/104/redist-conn.html

Question from another instructor (later)…
• I was teaching redistribution of RIP into OSPF and couldn't explain the Baypoint output on p. 132 of
the lab manual. Since both 172.16.8.0 and 172.16.7.0 are being redistributed into OSPF with a
default metric of 10, why does the .7.0 network have a metric of 20?
• The reason for the difference is that when using the redistribute connected command, those
connected networks are not affected by the default-metric command. So once the ospf, 'redistribute
connected subnets' command is entered on SJ1, those directly connected networks, being
redistributed into rip, are no longer affected by the default-metric value.
For more info see:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fiprrp_r/ind_r/1rfindp2.htm
"You cannot use the 'default-metric' command to affect the metric used to advertise 'connected'
routes."
Usually, it is not advisable to redistribute connected networks into area 0, but thats another story.
• In other words, if you are using the redistributed connected command, any connected networks
included using the OSPF network command, were not only advertised as normal using LSA Type 1,
2, or 3, but also as an external LSA Type-5.
We saw this in our multi-area OSPF labs when some students were using 12.0.5 and others were
using 12.2. With 12.0.5 an internal OSPF route, LSA Type-1, was also being advertised as a LSA
Type-5, when they used the "redistribute connected" command.

OSPF FAQs and FYIs
Q: Why are loopbacks advertised as /32 host routes in OSPF?
A: Loopbacks are considered host routes in OSPF, and they're
advertised as /32. For more information, see section 9.1 of RFC
2328. In Cisco IOS ® version 11.3T and 12.0, if the ip ospf network
point-to-point command is configured under loopbacks, then
OSPF advertises the loopback subnet as the actual subnet
configured on loopbacks.
http://www.cisco.com/warp/public/104/9.html
Q: Can a virtual link cross more than one area.
A: No.

OSPF FAQs and FYIs
Q: What happens within OSPF if there is more than one route to a
destination? What is the preference of OSPF in choosing a best
route?
A: Here is the OSPF preference rules:
• Intra-area routes area always most preferred.
• Inter-area routes are preferred over AS or NSSA external routes.
• AS-external routes and NSSA-external routes are of equal preference.
Within these routes, preferences are as follows:
– External Type-1 routes are always preferred
• If equal, route-metric (cost) is the tie-breaker
– External Type-2 routes
• If equal, route metric and distance to the originating router are
used as tie-breakers.
– If still a tie (Type-1 or Type-2), AS-external (LSA 5) routes are
preferred over NSSA external (LSA 7) routes.
• If these rules do not solve the tie, routes are installed as parallel routes.

OSPF FAQs and FYIs
OSPF Packet Pacing
• Introduced in Cisco IOS 11.3
• Helps avoid packet drops at the receiving side, caused by uncontrolled
bursts of link-state updates.
• The receiving router may not be able to queue and process all of the
packets so some packets are dropped.
• To make matters worse, when the sending router does not receive
LSAcks for all of the LSAs sent, so retransmits along with other LSAs
needed to be sent.
• Currently Cisco IOS Packet Pacing, every 33 milliseconds (non-configurable)
the router builds a link-state update and sends it to its
neighbors.
• The next group of LSAs is transmitted after another 33 milliseconds.
• This speeds up convergence and decreases the length of the transition
period.

OSPF FAQs and FYIs
OSPF Group Pacing
• Introduced in Cisco IOS 11.3
• Every LSA is aged whiled stored in the LSDB.
• ALL LSAs are aged independently of one another.
• When an LSA reaches LSRefreshTime (30 minutes) the router that
originated the it floods the LSA.
• When an LSA reaches MaxAge (60 minutes) the router floods the LSA,
even if it did not originate the LSA.
• If a router has a lot of LSAs, maintaining a separate timer can be
expensive.
• With Cisco OSPF Group Pacing, LSAs are collected into groups by
their ages, with ages within 4 minutes by default (can be configured).
• The router maintains timers for LSA groups instead of individual LSAs.
• This is used for all LSA operations including LSA aging and LSA
refreshing.

OSPF FAQs and FYIs (Know this!)
Cisco SPF Scheduling (Review)
• SPF algorithm is CPU intensive and takes some time depending upon
the size of the area (coming next week), the number of routers, the size
of the link state database.
• A flapping link can cause an OSPF router to keep on recomputing a
new routing table, and never converge.
• To minimize this problem:
– SPF calculations are delayed by 5 seconds after receiving an
LSU (Link State Update)
– Delay between consecutive SPF calculations is 10 seconds
• You can configure the delay time between when OSPF receives a
topology change and when it starts a shortest path first (SPF)
calculation (spf-delay).
• You can also configure the hold time between two consecutive SPF
calculations (spf-holdtime).
Router(config-router)#timers spf spf-delay spf-holdtime

OSPF Design Issues – Some extra info!
• Number of Routers per Area
• Number of Neighbors
• Number of Areas per ABR
• Full Mesh vs. Partial Mesh
• Memory Issues

OSPF Design Issues
• The following information is taken from Cisco CCO.
• http://www.cisco.com/warp/public/104/3.html
• The OSPF RFC (1583) did not specify any guidelines for the number of routers
in an area or number the of neighbors per segment or what is the best way to
architect a network.
• Different people have different approaches to designing OSPF networks.
• The important thing to remember is that any protocol can fail under pressure.
• The idea is not to challenge the protocol but rather to work with it in order to get
the best behavior.
• The following are a list of things to consider.
– Number of Routers per Area
– Number of Neighbors
– Number of Areas per ABR
– Full Mesh vs. Partial Mesh
– Memory Issues

OSPF Design Issues
Number of Routers per Area
The maximum number of routers per area depends on several factors,
including the following:
• What kind of area do you have?
• What kind of CPU power do you have in that area?
• What kind of media?
• Will you be running OSPF in NBMA mode?
• Is your NBMA network meshed?
• Do you have a lot of external LSAs in the network?
• Are other areas well summarized?
• For this reason, it's difficult to specify a maximum number of routers
per area.

OSPF Design Issues
Number of Neighbors
• The number of routers connected to the same LAN is also important.
• Each LAN has a DR and BDR that build adjacencies with all other
routers.
• The fewer neighbors that exist on the LAN, the smaller the number of
adjacencies a DR or BDR have to build.
• That depends on how much power your router has. You could always
change the OSPF priority to select your DR.
• Also if possible, try to avoid having the same router be the DR on
more than one segment.
• If DR selection is based on the highest RID, then one router could
accidentally become a DR over all segments it is connected to.
• This router would be doing extra effort while other routers are idle.

OSPF Design Issues
Number of Areas per ABR
• ABRs will keep a copy of the database for all areas they service.
• If a router is connected to five areas for example, it will have to keep a list of
five different databases.
• The number of areas per ABR is a number that is dependent on many
factors, including type of area (normal, stub, NSSA), ABR CPU power,
number of routes per area, and number of external routes per area.
• For this reason, a specific number of areas per ABR cannot be
recommended.
• Of course, it's better not to overload an ABR when you can always spread
the areas over other routers.
• The following diagram shows the difference between one ABR holding five
different databases (including area 0) and two ABRs holding three databases
each.
• Again, these are just guidelines, the more areas you configure per ABR the
lower performance you get. In some cases, the lower performance can be
tolerated.

OSPF Design Issues
Full Mesh vs. Partial Mesh
• Non Broadcast Multi-Access (NBMA) clouds such as Frame
Relay or X.25, are always a challenge.
• The combination of low bandwidth and too many link-states is a
recipe for problems.
• A partial mesh topology has proven to behave much better than
a full mesh.
• A carefully laid out point-to-point or point-to-multipoint network
works much better than multipoint networks that have to deal
with DR issues.

OSPF Design Issues
Memory Issues
• It is not easy to figure out the memory needed for a particular OSPF configuration. Memory
issues usually come up when too many external routes are injected in the OSPF domain. A
backbone area with 40 routers and a default route to the outside world would have less
memory issues compared with a backbone area with 4 routers and 33,000 external routes
injected into OSPF.
• Memory could also be conserved by using a good OSPF design. Summarization at the area
border routers and use of stub areas could further minimize the number of routes
exchanged.
• The total memory used by OSPF is the sum of the memory used in the routing table (show
ip route summary) and the memory used in the link-state database. The following
numbers are a rule of thumb estimate. Each entry in the routing table will consume between
approximately 200 and 280 bytes plus 44 bytes per extra path. Each LSA will consume a
100 byte overhead plus the size of the actual link state advertisement, possibly another 60
to 100 bytes (for router links, this depends on the number of interfaces on the router). This
should be added to memory used by other processes and by the IOS itself. If you really
want to know the exact number, you can do a show memory with and without OSPF being
turned on. The difference in the processor memory used would be the answer (keep a
backup copy of the configs).
• Normally, a routing table with less than 500K bytes could be accommodated with 2 to 4 MB
RAM; Large networks with greater than 500K may need 8 to 16 MB, or 32 to 64 MB if full
routes are injected from the Internet.

Whew!

OSPF- Multi area

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