Exploring IP Routing and Ethernet Bridging.pdf

EXPLORING IP
ROUTING AND
ETHERNET BRIDGING
IPC_207
IP CONVERGENCE & IMS CURRICULUM
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Table of Contents
Chapter 1: Prologue 1
Chapter 2: Spanning Tree Protocol 7
Broadcast Storms and STP 9
Rapid Spanning Tree Protocol (RSTP) 14
Multiple Spanning Tree Protocol (MSTP) 19
Chapter 3: Routing Table 27
Routing Table Concepts 29
The Longest Match 36
Recursive Routing 40
Black Holes 42
Chapter 4: OSPF Key Concepts 47
Areas and Router Types 50
Link State Advertisements (LSA) 53
Link State Advertisements 57
Chapter 5: OSPF in Wireless Networks 67
Neighbor Discovery 70
Metrics 75
Areas 82
Chapter 6: BGPv4 Key Concepts 89
Autonomous Systems 92
BGP Messages 96
BGP Attributes and Routing Policy 100
Route Reflectors 111
Chapter 7: BGPv4 in Wireless Networks 117
Chapter 8: L3VPN in Wireless Networks 129
Interconnecting MTSOs 131
L3VPN Architecture 135
High-Level Operations 138
Chapter 9: L3VPN Routing 147
Provider-Customer Model 149
MPLS Router 151
VPN Routing and Forwarding 154
VPN Route Distribution using MP-iBGP 160
Route Distinguisher 162
VPNv4 and IPv4 Address Families 167
Route Target 169
IPC_207 Version 2.2c

L3VPN Routing 172
Acronyms 185
References 189

1 | Prologue
Chapter 1:
Prologue
Award Solutions Proprietary
1

1 | Prologue
Objectives
After completing this module, you will be able to:
• Explain why IP is the data transport of choice for
today’s carrier networks
• Explain the Customer-Provider Model and its use of
Layer 3 VPNs
• Describe the customers found in a carrier’s
implementation of the Customer-Provider Model
• List the routing protocols used in the typical
implementation of L3VPNs
2

The Customer-Provider model uses Layer 3 Virtual Private
Network (L3VPNs) to provide private network services to a
set of customers. The customer networks include a
Customer Edge (CE) router that connects to the service
provider’s Provider Edge (PE) router. The CE and PE
routers exchange routing information, using Border
Gateway Protocol (BGP). The PE routers use MPLS to
forward packets through the backbone and BGP to
distribute the customer routes to other VPN locations.
The L3VPN solution is widely deployed. It is easy for
customers to send data throughout their respective sites,
while at the same time being scalable and manageable
for the service provider. For these reasons, L3VPNs are
used in carrier networks to isolate traffic between
customers and services.
While the terms customer and provider evoke a
commercial relationship, this is not necessarily the case in
a carrier environment. The customer networks represent
subsystems within the larger network, interconnected
through a transport backbone.
1 | Prologue
Site A
Customer 1
CE
Customer 2
CE
Customer 3
CE
Site C
Site
B
Site
D
Customer-Provider Model
Customer 1
CE
Customer 2
CE
Customer 3
CE
Provider
PE PE
Customer 1
CE
Customer 2
CE
Customer 3
CE
PE
Customer 1
CE
Customer 2
CE
Customer 3
CE
PE
P
P
P
P
3

Wireless service providers adopt the L3VPN solution as a
means to manage their networks. The makeup of the
customer networks varies according to service provider
policies, but some generalities may be discussed. The
service provider may choose to implement different
wireless network technologies (e.g., UMTS, EVDO, LTE) as
customer networks. They may also choose to implement
different network services (e.g., voice, Internet access, IP
Multimedia Subsystem (IMS)) as customer networks. Or
they may also use them to segregate Operations,
Administration and Maintenance (OAM) connections or
inter-connectivity with Internet peering or roaming
partners.
The customer networks are present throughout the
service provider’s facilities, including local Mobile
Telephone Switching Offices (MTSO), regional Network or
Data Centers and national Network or Data Centers.
1 | Prologue
MTSO
MTSO
MTSO
MTSO
3G Voice
CE
3G Data
CE
4G Data
CE
Segregating Services
Core
PE
PE
PE
PE
P
P
P
P
IMS
CE
OAM
CE
Peering
CE
3G Voice
CE
3G Data
CE
4G Data
CE
IMS
CE
OAM
CE
Peering
CE
4

The customer networks run an interior gateway routing
protocol such as the Open Shortest Path First (OSPF). The
Core runs an interior gateway routing protocol as well to
distribute routes between the PE and P routers. The CE
and PE routers run eBGP to exchange customer routing
information. This routing information is subsequently
passed from one PE router to another PE using Multi-
Protocol BGP (MP-BGP) to distribute customer routes
throughout the network.
1 | Prologue
L3VPN Routing Protocols
5

1 | Prologue
Summary
• Wireless equipment at the cell site used T1s as transport before the
use of IP.
• IP interfaces in wireless networks carry user data, OA&M data and
signaling data.
• The Customer-Provider Model implements L3VPNs to segregate the
data traffic of individual customers.
• The customer networks found in a carrier’s network can include
networks belonging to individual customers, networks belonging to
technologies such as LTE and UMTS, and networks belonging to
particular traffic types such as OA&M.
• In a network that implements Layer 3 VPNs (L3VPNs), eBGP is used
between CEs and PEs to exchange customer routes, MP-BGP is used
between PEs to distribute the customer routes, and OSPF is used
between the PEs and P for internal core routing exchanges.
6

2 | Spanning Tree Protocol
Chapter 2:
Spanning Tree Protocol
7

Objectives
• Explain how the Spanning Tree Protocol protects
networks from broadcast storms
• Select the best Spanning Tree Protocol for a
given network
• List the differences between STP, RSTP, and
MSTP
• Describe the significance of the root switch to
the Spanning Tree Protocol
8

Broadcast Storms and
STP
9

Several version of spanning tree protocols have been
developed over the years. The most significant are:
Spanning Tree Protocol (STP)
The first Spanning Tree Protocol published as a standard
by the Institute of Electrical and Electronic Engineering
(IEEE) in 802.1d-1990. It is generally no longer used in
service provider environments as it takes too long to
discover and react to changes in the network topology. It
also predates the existence of virtual LANs (VLANs).
Rapid Spanning Tree Protocol (RSTP)
As its name implies, RSTP revamps the operation of the
spanning tree protocol to reduce the time it takes to
detect and react to changes in the topology. It does not,
however, incorporate support for VLANs. RSTP was first
published in 802.1w-2001, but was later incorporated
into 802.1d-2004.
Rapid Per-VLAN Spanning Tree Protocol Plus (Rapid-
PVST+)
Rapid-PVST+ represents the an evolution in a series of
spanning tree protocols developed by Cisco Systems, Inc.
It incorporates the performance improvements of RSTP
and includes support for VLANs. It is used in networks that
run only Cisco equipment, and is not widely supported by
other equipment vendors.
Multiple Spanning Tree Protocol (MSTP)
The latest version of the spanning tree protocol
standardized by the IEEE, originally published as 802.1s-
2002 and later incorporated into 802.1q-2005. MSTP
builds on the capabilities of RSTP and includes support for
multiple VLANs. One notable difference between MSTP
and Rapid-PVST+ regards the number of spanning tree
instances that run in the switch. Rapid-PVST+ requires
one spanning tree instance per VLAN, while MSTP allows
many VLANs to be mapped onto a single spanning tree.
MSTP therefore operates more efficiently when large
numbers of VLANs are configured. MSTP also supports
hierarchical network design with the concept of a region.
Topology changes affect the switches in the region, but
are invisible to switches outside of the region. This allows
for more efficient operation in large-scale networks.
Spanning Tree Protocols
•Legacy STP, now obsolete
•Recovery time < 1 minute
•Does not support VLANs
STP
•Recovery time < 1 second
•Does not support VLANs
RSTP
•Cisco proprietary
•One spanning tree per VLAN
Rapid-PVST+
•Latest open standard
•Multiple VLANs share one spanning tree
•Supports hierarchical network design
MSTP
10

Broadcast storms may occur whenever a loop exists in the
Ethernet LAN topology. The figure illustrates a simple case
in which a loop exists between two Ethernet switches. The
sequence of events unfolds as follows:
1. The node on the left sends a broadcast frame. SW1
receives it on port gi0/1.
2. Whenever an Ethernet switch receives a broadcast
frame, it forwards the frame out all active ports,
except the one on which the frame was received. In
this case, SW1 forwards the frame out ports gi0/2
(shown in red) and gi0/3 (shown in green).
3. SW2 receives two broadcast frames. Ethernet does
not provide any means to determine that these are, in
fact, two copies of the same original frame, so SW2
must process each one independently. It receives one
frame (red) on port gi0/7 and forwards it out ports
gi0/6 and gi0/8. It receives the other frame (green)
on port gi0/8 and forwards it out gi0/6 and gi0/7.
The node on the right receives two copies of the
frame.
4. SW1 receives two broadcast frames. Ethernet
provides no means to determine that these are
copies of the frames that it recently forwarded on to
SW1, so it must process them again. It receives one
frame (green) on port gi0/2 and forwards it out ports
gi0/1 and gi0/3. It receives the other frame (red) on
port gi0/3 and forwards it out ports gi0/1 and gi0/2.
The node on the left receives two copies of the frame
that it originally sent.
The frames are now stuck in the loop. Ethernet has no
concept of a frame identifier, so the switches do not
recognize that they are processing the same frame again
and again. Ethernet also does not have a timeout
mechanism, so the switches continue processing the
frames in this manner forever. And each iteration through
the loop causes additional copies of the frame to be
made, so the storm consumes an increasing amount of
bandwidth, buffer space and processing time.
Broadcast Storms
Loops cause broadcast storms
bcast bcast
bcast bcast
bcast bcast
bcast bcast
bcast bcast
bcast bcast
gi0/2
gi0/1
gi0/3
gi0/7
gi0/6
gi0/8
bcast
bcast bcast
bcast bcast
bcast
bcast
bcast
bcast
bcast
bcast
bcast
bcast
SW1 SW2
11

Broadcast storms may be prevented (or managed) by
avoiding large, complex Ethernet networks in the first
place. Service providers may choose to strategically
deploy IP and MPLS to minimize the reach of the Ethernet
topology. Where Ethernet is used, designing loop-free
networks (i.e., tree topology) also eliminates the
conditions in which storms may occur. Most Ethernet
switches also implement some form of storm control
mechanisms wherein the broadcast traffic gets throttled.
Storm control features minimize the damage caused by
broadcast storms, but do not prevent them outright.
Finally, spanning tree protocols can also be used to
automatically patrol the network for loops. When a loop is
detected, the spanning tree protocol automatically
disables some links in order to break the loop.
Broadcast Storm Prevention
1. Prefer IP and MPLS over Ethernet
for large networks
2. Avoid loops in Ethernet topology
3. Storm Control
4. Spanning Tree Protocols
Backhaul
Router
CE
CE
CE
CE
PE
PE
12

When Spanning Tree Protocol is used, the switches in the
network exchange messages that enable them to detect
loops that would otherwise cause broadcast storms. They
then select one or more ports to be blocked in order to
simplify the logical topology of the network such that it
resembles a tree. The switches do not forward user traffic
through a blocked port. The net result is that the loop is
broken and the conditions for a broadcast storm are
eliminated.
Spanning Tree Protocol
Before After
13

Rapid Spanning Tree
Protocol (RSTP)
14

The Spanning Tree Protocol selects one switch to serve as
the root of the logical tree topology. The root switch
election process governs which switch gets selected as
the root and ensures that all switches make the same
selection. The procedure is relatively straightforward. Each
switch has a unique identifier. The switch with the lowest
identifier becomes the root. The identifier is a combination
of a priority value and the switch’s MAC address. By
default, all switches use the same priority value, so the
root switch election boils down to which switch has the
numerically lowest MAC address.
Choosing the root switch based solely on the MAC address
is fairly arbitrary, but the loop detection procedures work
regardless of which switch gets elected to be the root. The
administrator may influence the outcome, however, by
overriding the default priority value and setting it to
something lower than the default value. The other
switches then select it as the root switch.
The figure illustrates the process of overriding the default
priority value to select the root switch. If we let all of the
switches use the default priority value of 32768, then the
switch in the center would become the root switch, since it
has the numerically lowest address. By changing the
priority of the switch at the top of the figure, we ensure
that it wins the root switch election.
The Root Switch
Root Switch Election
• ID = Priority + Address
• By default, all switches use same priority value
• Override default priority value to specify root switch
32768
+ AABB:1122:0001
32768
+ AABB:1122:0003
32768
+ AABB:1122:0002
32768
+ AABB:1122:0005
32768
+ AABB:1122:0004
32768
+ AABB:1122:0006
8192 32768
+ AABB:1122:0007
15

The spanning tree algorithm assigns a cost to each switch
port based on the speed of the link. Faster links cost less.
The spanning tree algorithm also calculates the root path
cost for each switch port. This is defined as the sum of the
port costs between it and the root switch. When the
spanning tree detects a loop, it uses the root path cost to
determine which ports should carry traffic and which
should be blocked.
Shortest Path Selection
Shortest Path Selection
• Port Cost = assigned based on link speed (range: 1 to 200M)
• Root Path Cost = sum of port costs of shortest path to root switch
1
Cost: 2000 Cost: 2000
Cost: 2000
Cost: 2000
Root Path Cost: 6000
Root Path Cost:
6000
Root Path Cost:
4000
16

The spanning tree algorithm assigns each switch port a
specific role to play. It selects one port on each switch to
be the root port for that switch. This port provides the
most optimal path to the root switch (i.e., the lowest root-
path cost). The algorithm also selects one port on each
LAN segment to be the designated port for that segment.
Any remaining ports are designated alternate ports. Root
ports and designated ports forward user traffic. Alternate
ports discard user traffic.
The spanning tree algorithm only assigns the alternate
port role when a loop exists in the network’s topology. This
is the heart of the algorithm’s loop-detection function. The
loop is broken because the alternate port discards user
traffic. When a node in the LAN sends a broadcast frame,
it floods the network through root and designated ports,
but gets dropped by alternate ports.
Port Role Assignment
Port Roles
• Root Port = per switch, shortest path to root switch; forwards traffic
• Designated Port = per segment, shortest path to root; forwards traffic
• Alternate Port = alternate, but less optimal path to root switch; discards
traffic
1
Designated Port
Root Port
Alternate Port
32768
+ AABB:1122:0001
32768
+ AABB:1122:0003
32768
+ AABB:1122:0002
32768
+ AABB:1122:0005
32768
+ AABB:1122:0004
32768
+ AABB:1122:0006
8192
+ AABB:1122:0007
17

When a switch detects the loss of a Physical Layer signal,
the spanning tree algorithm re-assesses the port role
assignments in light of the new connectivity. If it
determines that a change is necessary, it sends a
proposal of the new role to its neighboring switch. The
neighboring switch re-assesses its own port roles based
on this new information and may agree or, if it believes it
has better insight into the topology, issue a counter-
proposal. Changing the role of one port on the switch may
cause a ripple effect throughout the network. The
proposal/agreement exchanges propagate in a link-by-link
fashion away from the failure toward the root switch until
it reaches a switch for whom the new proposal does not
require any changes.
In the figure shown, the link that connects the center
switch to the root switch fails. This had been the center
switch’s root port. The precise details of the operation go
beyond the scope of this discussion, but the loss of the
root port forces the center switch to change the remaining
port’s role from designated port to root port. The bottom-
most switch changes its root port to a designated port and
must therefore unblock one of its alternate ports. In this
example, the root path cost is the same for both alternate
ports, so the port priorities are considered. Similar to the
root switch priority, the administrator can influence the
selection of the root switch by modifying the default
priority value. If both ports have the same priority value,
then the numerical port identifier is used to break the tie.
The bottom-most switch sends a proposal to the
neighboring switch (on its left in the figure). The proposal
does not require it to change any of its port roles, so the
configuration changes stop there. The network has re-
converged on the new topology.
One task remains, however. Remember that each of the
switches in the network maintains a MAC table that
associated MAC addresses with port numbers. Some of
that information is now out of date. The root switch sends
a Topology Change Notification, which the other switches
use as a signal to flush stale data from their MAC tables.
Topology Changes
Topology Changes
• Nodes detect link failure through Physical Layer, or message timeout
• Proposal and agreement process updates port roles
• Topology Change Notification tells switches to flush MAC tables
1
Designated Port
Root Port
Alternate Port
Link Fails
DP becomes RP
RP becomes
DP
AP becomes RP
Proposal/
Agreement
Proposal/
Agreement
18

Multiple Spanning Tree
Protocol (MSTP)
19

The figure illustrates a hypothetical Ethernet backhaul
network design. In this design, each cell site is assigned a
unique VLAN ID. The cell site links enter the Mobile
Telephone Switching Office (MTSO) and connect to a set
of access switches. Each access switch carries traffic for
ten cell sites. Two access switches, designated SWA and
SWB are shown in the diagram. The others are omitted
due to lack of space. The access switches connect to a
pair of aggregation switches, SWC and SWD, which in turn
connect to a Multi-Layer Switch (MLS) that acts as
backhaul router. The spanning tree protocol manages the
redundant links between the access and aggregation
switches.
Multiple VLAN Sample Topology
MLS
SWC
SWA
SWB
SWD
Aggregation switches
Access switches
Spanning Tree manages
redundant links
20

Let’s assume that the MLS is the root switch in our sample
topology. When the spanning tree algorithm runs, each
access switch selects a root port and an alternate port.
The root path costs are the same for both ports (assuming
of the same links are the same speed), so the selection of
the root and alternate ports is fairly arbitrary. In the figure
shown, the top-most port on SWA remains active, while the
bottom-most port gets blocked. SWB does just the
opposite of this.
Spanning Tree Results
MLS
SWC
SWA
SWB
SWD
Designated Port
Root Port
Alternate Port
21

When redundant links are deployed in the network,
service providers often prefer a load-balancing solution to
a strict active/standby scheme. When multiple VLANs are
used, this can be accomplished by directing some VLAN
traffic (for example, the even-numbered VLANs) across
one of the redundant links and other VLAN traffic (for
example, the odd-numbered VLANs) across the other
redundant link.
Referring back to our sample topology, let us focus on the
links that connect access switch SWA to the aggregation
switches SWC and SWD. SWA carries traffic for 10 VLANs
(one per cell site). If RSTP is used to manage these links,
then load balancing is not possible. When RSTP computes
the spanning tree, the traffic for all 10 VLANs is carried
over the active link. The blocked link remains idle. This is
because RSTP does not support the notion of computing
multiple spanning trees.
Alternatively, one could use PVSTP+ instead. As its name
implies, Rapid PVSTP+ computes one spanning tree
solution for each VLAN configured on the switch. There
would therefore be 10 spanning trees in our example. By
overriding the default port priorities, we could arrange for
half of the spanning trees to select the top-most port to be
the root port and the bottom-most port to be the alternate
port. We could configure the other half of the spanning
trees to do just the opposite. The net effect is that the
traffic would be evenly divided (assuming the cell sites
were equally loaded) across the two links. If either of the
links failed, then the spanning trees would react by
unblocking the alternate port and all traffic would be
directed over the remaining link.
Load Balancing with VLANs
• MSTP
– Efficient load balancing
– Two spanning trees (one blocks port 1, the other blocks port 2)
– Half of the VLANs get mapped to one spanning tree; half get mapped to the other
• PVSTP+
– Less efficient load balancing ; one spanning tree per VLAN
– Half of the VLANs block port 1; the other half block port 2
• RSTP
– Single spanning tree, active/idle configuration, no load balancing
MLS
MSTP = Two Spanning Trees
SWC
SWA
SWD
MLS
SWC
SWA
SWD
22

While Rapid PVSTP+ achieves the goal of balancing the
traffic load, it does so at a certain cost. SWA needs to
maintain 10 separate spanning tree instances. Each
spanning tree instance introduces some processing,
memory and messaging overhead. Note, however, that in
our sample topology, there are really only two possible
spanning trees. The fact that Rapid PVSTP+ requires one
spanning tree per-VLAN means that it creates five
separate copies of each of the two possible spanning
trees. This is where MSTP offers some advantages over
RSTP and Rapid PVSTP+.
MSTP allows us to define spanning tree instances and
then associate those instances with a set of VLANs. In our
case, we would create the two spanning tree instances,
overriding the port priorities so that they choose different
root ports, and then map five VLANs onto each of the two
instances.
Load Balancing with VLANs (Continued)
• MSTP
– Efficient load balancing
– Two spanning trees (one blocks port 1, the other blocks port 2)
– Half of the VLANs get mapped to one spanning tree; half get mapped to the other
• Rapid PVSTP+
– Less efficient load balancing ; one spanning tree per VLAN
– Half of the VLANs block port 1; the other half block port 2
• RSTP
– Single spanning tree, active/idle configuration, no load balancing
MLS
MSTP = Two Spanning Trees
SWC
SWA
SWD
MLS
SWC
SWA
SWD
23

MSTP supports a form of hierarchical network design by
introducing the concept of a region. A region is a collection
of switches that share a common view of their physical
and logical topology. Collectively, the switches in the
region appear as a single, virtual switch to the switches
outside of the region. When a topology change occurs
within the region, the switches inside the region re-
converge on the new topology. The switches outside the
region remain unaware of, and unaffected by, the topology
change.
MSTP regions allow the Ethernet LAN to grow in scale and
complexity without adversely affecting the performance of
the error detection and recovery procedures.
MSTP Regions
24

Summary
• Spanning Tree Protocol (STP) prevents loops
from triggering broadcast storms in Layer 2
networks.
• Rapid Spanning Tree Protocol (RSTP)
significantly reduces the time to detect and
recover from a failure.
• Multiple Spanning Tree Protocol (MSTP) adds
support for VLANs.
• MSTP provides an efficient means to load
balance VLAN traffic across multiple links.
25

The figure illustrates a hypothetical Ethernet backhaul
network. Based on the priority and addresses shown,
which switch is selected as root switch by the Spanning
Tree Protocol? What could you do to ensure that the MLS
becomes the root switch?
Review Exercise
MLS
SWC
SWD
SWA
32768 + e05f:b903:5280
32768 + e05f:b903:535a
32768 + e05f:b903:4653
SWB
32768 + e05f:b903:1121
32768 + e05f:b903:9863
32768 + e05f:b903:af3c
32768 + e05f:b903:753a
32768 + e05f:b903:5280
32768 + e05f:b9c3:a2d0
32768 + e05f:b93f:d9ea
32768 + e05f:b9c3:7719
26

3 | Routing Table
Chapter 3:
Routing Table
27

3 | Routing Table
Objectives
• Read and understand IP routing tables
• Understand the factors that go into the route
selection algorithm
• Identify the preferred route from a set of
alternates
• Employ recursive routing techniques
• Explain how black holes impact routing and why
a packet might be deliberately discarded
28

3 | Routing Table
Routing Table
Concepts
29

The routing table is essentially a collection of network
addresses that the router knows how to reach. For a given
inbound packet, the routing table indicates to which node
it should forward the packet (the next hop) and/or the
outbound interface it should use to get there. The routing
table entries may contain additional information used to
prioritize routes, indicate their origin and how long they’ve
been around.
Routing table entries come from one of three possible
sources. The most straightforward source of routes are
the router’s own network interfaces. If we configure an
interface with the address 192.168.1.1/24, then it stands
to reason that the router knows how to reach the nodes
on the 192.168.1.0/24 subnet. Similarly, so-called static
routes can be created by explicitly configuring the routes
on the router. Finally, the router can use a dynamic routing
protocol such as OSPF and BGPv4 to exchange routing
information with other routers.
It’s worth noting that the routing table is only one
repository of routing information. Individual routing
protocols maintain databases that contain routing
information they’ve learned from neighboring routers. And
while it’s convenient to talk about the router searching the
routing table when it receives a packet, modern routers
copy the contents of the routing table into a more
optimized data structure, typically called the Forwarding
Information Base (FIB).
3 | Routing Table
Routing Table Concepts
Routing Table
Destination Source Cost Next Hop
0.0.0.0/0 Static 1 192.168.1.1
192.168.1.0/24 Connected 2 Gi 0/0
10.1.1.0/22 OSPF 10 10.5.3.8
172.16.0.0/16 BGP 20 192.168.7.30
Forwarding
Information
Base
Sources
Static
Connected
Dynamic
30

This sample routing table is used to discuss the concepts
and structure that make up the routing table.
3 | Routing Table
Routing Table Example
R1# show ip route
B 216.238.54.0/24 [20/0] via 12.123.1.236, 4w4d
O 216.221.5.0/24 [110/1] via 12.123.1.236, 7w0d
136.141.0.0/24 is subnetted, 2 subnets
S 136.141.6.0 [1/0] via 12.122.125.4, 7w0d
R 136.141.2.0 [120/0] via 12.122.125.4, 7w0d
B 170.162.32.0 [20/0] via 12.123.1.236, 7w0d
O 170.160.32.0 [110/1] via 12.122.125.4, 7w0d
O 170.160.64.0 [110/1] via 12.123.1.236, 3d15h, fa0/1/1
187.179.0.0/16 is variably subnetted, 3 subnets, 2 masks
B 187.179.0.0/19 [20/0] via 12.123.1.236, 13:51:40, gi0/1
B 187.179.32.0/19 [20/0] via 12.123.1.236, 4w3d
S 187.179.64.0/18 [1/0] via 12.123.1.236, 4w3d
S 34.254.244.0/22 [1/0] via 12.122.125.4, 7w0d
S 34.254.112.0/21 [1/0] via 12.122.125.4, 7w0d
S 34.253.0.0/19 [1/0] via 12.123.1.236, 7w0d
31

The routing table is organized into two tiers.
Level 1 routes serve as routes only as shown in the first
line of the example above or they may serve as indexes to
the Level 2 routes listed below them as seen in the third
line of our example. When a router searches for a match
to a particular destination address, it first searches the
Level 1 routes. If it finds a match, then it searches the
Level 2 routes for a more specific match.
A Parent Route is a Level 1 route that has Level 2 routes
beneath it. A Child Route is a Level 2 route. The router
automatically adds Parent Routes to the routing table
when Child Routes are created.
The basis of this two-tier hierarchy is classful routing which
has been replaced by Classless Inter-Domain Routing
(CIDR). This hierarchy is useful for quickly searching the
routing table and does not mean that the network
employs classful addressing or that the router uses
classful routing.
3 | Routing Table
Parent and Child Routes
R1# show ip route
B 216.238.54.0/24 [20/0] via 12.123.1.236, 4w4d
O 216.221.5.0/24 [110/1] via 12.123.1.236, 7w0d
S 136.141.6.0 [1/0] via 12.122.125.4, 7w0d
R 136.141.2.0 [120/0] via 12.122.125.4, 7w0d
B 170.162.32.0 [20/0] via 12.123.1.236, 7w0d
O 170.160.32.0 [110/1] via 12.122.125.4, 7w0d
O 170.160.64.0 [110/1] via 12.123.1.236, 3d15h, fa0/1/1
B 187.179.0.0/19 [20/0] via 12.123.1.236, 13:51:40, gi0/1
B 187.179.32.0/19 [20/0] via 12.123.1.236, 4w3d
S 187.179.64.0/18 [1/0] via 12.123.1.236, 4w3d
S 34.254.244.0/22 [1/0] via 12.122.125.4, 7w0d
S 34.254.112.0/21 [1/0] via 12.122.125.4, 7w0d
S 34.253.0.0/19 [1/0] via 12.123.1.236, 7w0d
Level 1 Route
Level 1/Parent Route
Level 2/Child Route
32

The prefix length sets the range of addresses that a route
applies to. For example, if a network is listed as
10.10.10.0/24, the route applies to destination IP
addresses in the range from 10.10.10.0 to
10.10.10.255. If the network is listed as 10.10.10.0/28,
the route applies to IP addresses in the range of
10.10.10.0 – 10.10.10. 15. All routes in a routing table
must have a prefix length in order to determine the range
of addresses that the route applies to. The range for each
route is not necessarily unique. The ranges for
10.10.10.0/28 is wholly contained in the range for
10.10.10.0/24. If you examine the sample routing table,
some routes do not have route prefixes, for example,
136.141.6.0 [1/0] via 12.122.125.4, 7w0d and
136.141.2.0 [120/0] via 12.122.125.4, 7w0d.
<COLUMN BREAK HERE>
The presence or absence of the prefix length in the
example table is determined by the rules below.
Level 1 with no Children
Level 1 route has its own prefix length.
Level 1 with Children
• Case 1: All Children have same prefix length.
― Level 1 route lists the prefix length, Child Routes
do not list prefix lengths.
• Case 2: Children have different prefixes.
― Prefix lengths are listed for each Child and the
Level 1 route uses the classful prefix for the
network.
3 | Routing Table
Prefix Length
R1# show ip route
B 216.238.54.0/24 [20/0] via 12.123.1.236, 4w4d
O 216.221.5.0/24 [110/1] via 12.123.1.236, 7w0d
S 136.141.6.0 [1/0] via 12.122.125.4, 7w0d
R 136.141.2.0 [120/0] via 12.122.125.4, 7w0d
B 170.162.32.0 [20/0] via 12.123.1.236, 7w0d
O 170.160.32.0 [110/1] via 12.122.125.4, 7w0d
O 170.160.64.0 [110/1] via 12.123.1.236, 3d15h, fa0/1/1
B 187.179.0.0/19 [20/0] via 12.123.1.236, 13:51:40, gi0/1
B 187.179.32.0/19 [20/0] via 12.123.1.236, 4w3d
S 187.179.64.0/18 [1/0] via 12.123.1.236, 4w3d
S 34.254.244.0/22 [1/0] via 12.122.125.4, 7w0d
S 34.254.112.0/21 [1/0] via 12.122.125.4, 7w0d
S 34.253.0.0/19 [1/0] via 12.123.1.236, 7w0d
Prefix in Level 1 Route
Prefix in Level 2 Route
33

Route Code
The route code indicates the source of the routing
information. The meaning of the codes is given at the top
of the routing table when working with the device. In our
example here, B=BGP, O=OSPF, S=Static, and R=RIP.
Connected routes (not shown here) are listed with a C and
Local routes (AKA Host routes) are listed with an L. Host
routes represent the router’s own IP addresses and carry
a 32-bit prefix length.
Administrative Distance
The administrative distance is a precedence value
assigned to the route, based on the source of the routing
information. Some sources of routing information are
considered more trustworthy than others, and have a
lower administrative distance. If the same route is learned
through multiple sources, the one with the lowest
administrative distance gets installed into the routing
table.
Metric
Each route is assigned a metric to show the relative cost
of using the particular route. Metrics act as tie breakers
when the router must select between routes for the same
destination network, which were learned from the same
dynamic routing protocol. Most routing protocols allow
Equal Cost Multi-Path (ECMP) and allow multiple routes to
the same destination network having the same metric to
be installed in the routing table. Traffic matching these
routes is load-balanced across the ECMP routes. ECMP
adds overhead to the forwarding function of a router as
the router must track which data stream is using which of
the routes. Administrators can set the maximum number
of ECMP routes that are allowed per destination network.
The metric may also be manipulated by an administrator
to prefer one ECMP route over another.
Next-Hop Address
The next-hop address identifies the IP address of the
device to which the router sends packets that match the
route. The next-hop address is typically the IP address of
the router that provided the route.
3 | Routing Table
What’s in a Route?
R1# show ip route
B 216.238.54.0/24 [20/0] via 12.123.1.236, 4w4d
O 216.221.5.0/24 [110/1] via 12.123.1.236, 7w0d
S 136.141.6.0 [1/0] via 12.122.125.4, 7w0d
R 136.141.2.0 [120/0] via 12.122.125.4, 7w0d
B 170.162.32.0 [20/0] via 12.123.1.236, 7w0d
O 170.160.32.0 [110/1] via 12.122.125.4, 7w0d
O 170.160.64.0 [110/1] via 12.123.1.236, 3d15h, fa0/1/1
B 187.179.0.0/19 [20/0] via 12.123.1.236, 13:51:40, gi0/1
B 187.179.32.0/19 [20/0] via 12.123.1.236, 4w3d
S 187.179.64.0/18 [1/0] via 12.123.1.236, 4w3d
S 34.254.244.0/22 [1/0] via 12.122.125.4, 7w0d
S 34.254.112.0/21 [1/0] via 12.122.125.4, 7w0d
S 34.253.0.0/19 [1/0] via 12.123.1.236, 7w0d
Route Code
Admin.Distance/Metric
Next-Hop Address
34

In general, when a router knows more than one route to a
particular destination, it prefers the one with the lowest
metric. This can be a problem when the routes were
learned from different sources because the metrics that
one routing protocol uses may not be consistent with the
metrics used by other routing protocols. For example, BGP
uses the number of Autonomous Systems (ASs) in its Path
Vector as a metric, while OSPF metrics are based on
bandwidth. Relying solely on route metrics, the router
would not be able to select the best possible routes.
The router uses the Administrative Distance to select the
best possible route from among a set of alternate routes
that were learned from different routing sources. The
administrative distance is determined by the source of
routing information. Routing information sources with
lower administrative distance values are preferred over
those with higher administrative distance values.
Unlike the route metric, the administrative distance is not
set on a per-route basis. All routes from a particular
routing information source share the same administrative
distance value. For example, all routes learned by OSPF
have 110 as the administrative distance value. The
administrative value is a configuration parameter set
within the router and is not exchanged by the routing
protocols (also unlike the route metrics).
Routing protocols with lower administrative distances are
deemed to be more reliable, accurate or trustworthy than
those with higher administrative distances. The table
shown here lists the default administrative values used by
Cisco routers. The defaults can be overridden, however, if
one wishes to change the relative rankings.
Finally, it’s worth noting that the administrative distance
determines which routes actually get installed in the
routing table. Typically multiple routes to the same
destination network (as specified by network address and
prefix length) from different routing protocols are not
found in routing tables. Whichever one has the lower
administrative distance causes the others to be bumped
from the routing table.
Source: “What is Administrative Distance?”, Document ID:
15986, Cisco.
3 | Routing Table
Administrative Distance
Source Administrative Distance
Connected 0
Static 1
EIGRP summary route 5
eBGP 20
Internal EIGRP 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EGP 140
ODR 160
External EIGRP 170
iBGP 200
<Unknown> 255
35

3 | Routing Table
The Longest Match
36

A packet has arrived with destination address
66.134.50.11. The router has the following three routes
installed in its routing table.
• 66.0.0.0/8 next-hop 10.10.10.1 gi0/1
• 66.134.0.0/18 next-hop 12.13.14.5 gi0/22
• 66.134.48.0/20 next-hop 62.34.24.33 gi0/3
The prefix length indicates the range of addresses
covered by the route. Here is the range for each of the
routes.
• 66.0.0.0/8 has range 66.0.0.0 – 66.255.255.255
• 66.134.0.0/18 has range 66.134.0.0 –
66.134.63.255
• 66.134.48.0/20 has range 66.134.48.0 –
66.134.63.255
Our destination address, 66.134.50.11 falls into each of
these three ranges. When a router discovers more than
one possible route for the packet it bases the forwarding
decision on the Longest-Match Rule.
3 | Routing Table
Finding the Right Route
Route
Prefix
Length
Range of Addresses
Is
66.134.50.11
in this range?
66.0.0.0 /8 66.0.0.0 – 66.255.255.255
66.134.0.0 /18 66.134.0.0 – 66.134.63.255
66.134.48.0 /20 66.134.48.0 – 66.134.63.255
Source IP: 12.12.134.19
Destination IP: 66.134.50.11
37

The Longest-Match Rule says that when multiple routes
contain the destination address in their ranges, the one
with the longest prefix length is the route to use.
Examining our prefix lengths we have /8, /18/ and /20.
The /20 is the longest prefix length and the route
66.134.48.0/20 is the route used for forwarding the
packet.
A route is said to match the packet’s destination address
if the packet’s destination address has the same network
prefix as the route. In other words, the route matches if
the first n bits of the packet’s destination address match
the first n bits of the route entry, where n is the route’s
prefix length. The bits in the destination address must
match exactly the bits in the network prefix of the route.
3 | Routing Table
Longest-Match Rule
Destination IP
66.134.50.11
01000010 10000110 00110010 00001011
S 66.0.0.0/8 01000010 00000000 00000000 00000000
Destination matches 8 bits
B 66.134.0.0/18 01000010 10000110 00000000 00000000
O 66.134.48.0/20 01000010 10000110 00110000 00000000
Longest-Match Rule: the route matches if the first n bits of the
packet’s destination address match the first n bits of the route
entry, where n is the route’s prefix length.
38

The Longest-Match requires that the destination’s prefix
match the network prefix exactly. Enter the route number
in the space provided for each of the destination IP
addresses.
What happens if there is no matching route for the
destination IP address?
3 | Routing Table
Longest-Match Exercise
1 66.0.0.0/8 0100 0010 0000 0000 0000 0000 0000 0000
2 66.134.0.0/18 0100 0010 1000 0110 0000 0000 0000 0000
3 66.134.48.0/20 0100 0010 1000 0110 0011 0000 0000 0000
Which Route (above) do these Destination IPs use?
66.128.12.31 0100 0010 1000 0000 0000 1100 0001 1111
66.134.64.222 0100 0010 1000 0110 0100 0000 1101 1110
66.134.46.76 0100 0010 1000 0110 0010 1110 0100 1100
39

3 | Routing Table
Recursive Routing
40

The router needs two pieces of information to forward a
packet to the next device in the path toward its
destination: the next-hop address and the exit interface. If,
after searching the routing table, the preferred route
identifies only the next-hop address, but not the exit
interface, the router searches the routing table a second
time to locate a suitable exit interface.
In the example given above, the router receives a packet
with a destination address of 10.1.0.5. Its first pass
through the routing table selects the 10.1.0.4/30 route as
the preferred route. This entry includes the next-hop
address, 172.16.4.2, but not the exit interface. The router
searches the routing table a second time for a route to
172.16.4.2. It finds the 172.16.4.0/30 route and
determines that fa0/1/2 is the exit interface.
This recursive search could have been avoided if the
10.1.0.4/30 route had included both a next-hop address
and an exit interface. This is useful to remember,
especially when configuring static routes. Specifying both
the next-hop address and the exit interface reduces the
amount of processing required to forward the packets.
3 | Routing Table
Recursive Routing
P1#show ip route
10.0.0.0/30 is subnetted, 2 subnets, 2 masks
10.1.0.4 [110/2] via 172.16.4.2, 00:02:56
172.16.4.0 is directly connected, fa0/1/2
P1#show ip route
10.0.0.0/30 is subnetted, 2 subnets, 2 masks
10.1.0.4 [110/2] via 172.16.4.2, 00:02:56, fa0/1/2
172.16.4.0 is directly connected, fa0/1/2
fa 0/1/2
Dst: 10.1.0.5
172.16.4.2 10.1.0.5
1
2
1
Next Hop
address
Exit
Interface
Next Hop
address
Exit
Interface
41

3 | Routing Table
Black Holes
42

The term black hole refers to the case when a router
receives a packet, but cannot forward the packet and the
packet is dropped. This action results in a routing black
hole, analogous to the theoretical astronomical
phenomenon of a region in space with a gravitational pull
such that nothing, not even light, can escape.
There are several reasons why black holes – in the IP
networking sense – might occur.
• Routing Errors – A packet arrives, the router finds no
route and discards the packet. If the router is allowed
to send ICMP messages, it notifies the sender using
an Internet Control Message Protocol (ICMP)
“Destination Network Unreachable” message.
• MTU Problems – If the Maximum Transmission Unit
(MTU) of the outbound link is too small to
accommodate the packet, and if the packet’s “Don’t
Fragment” bit is set, the router drops the packet and
send an ICMP “Fragmentation Required” message
indicating what the MTU must be to send the packet
through the interface.
• Intentional Discard – In some circumstances, the
routing table includes a null route (also known as
black hole route). Any incoming packets that that
match that route are discarded. This is an appropriate
way to deal with malicious traffic, for example. An
Intrusion Prevention System (IPS) may dynamically
create a null route when it detects suspect activity.
ICMP messages are usually suppressed in this case,
so as to conceal the countermeasures from the
attacker.
3 | Routing Table
Black Holes
• Network forwards
packets to a router
• Router drops the
packets
• Possible reasons:
– Routing Error
– MTU Error
– Intentional Discard
• “Black Hole” Route:
R(config)#ip route 10.1.1.1 255.255.255.0 null0
43

3 | Routing Table
Summary
• There are three sources of routes: directly connected
interfaces, static routes, and dynamic routing protocols.
• Routing table organization is as follows: Level 1 and Level 2
routes; Parent and Child Routes.
• Administrative distance, the longest-match rule and route
metrics are used to select from among multiple alternate
routes.
• Routing table entries have a next-hop address, metric and
(optionally) exit interface.
• Recursive routing may be used when the route table entry
does not specify an exit interface.
44

3 | Routing Table
Review Questions
Use the table on the following page to answer the
questions below.
1. How many Level 1 routes are in this table?
2. How many routes were learned through dynamic
routing protocols?
3. The router receives a packet with the destination
address 172.16.0.1. Which interface does the
router send the packet out?
4. The router receives a packet with destination
address 20.15.0.4 Which interface does the
router send the packet out?
45

3 | Routing Table
Review
P1#show ip route
Codes: C - connected, S - static, R - RIP, B – BGP, O - OSPF
C 140.30.19.0 is directly connected, gi0/1
B 20.0.0.0/8 [20/0] via 140.30.19.110, 00:04:17
C 66.134.48.0 is directly connected, gi0/0
C 172.16.60.0 is directly connected, fa0/1/6
O 172.16.0.0 [110/2] via 172.16.4.2, 00:02:56, fa0/1/0
O 10.1.4.1/32 [110/2] via 172.16.4.2, 00:02:56, fa0/1/0
O 10.1.0.4/30 [110/2] via 172.16.4.2, 00:02:56, fa0/1/0
46

4 | OSPF Key Concepts
Chapter 4:
OSPF Key Concepts
47

Objectives
After completing the module, you will be able to:
• Describe how OSPF relates to Autonomous
Systems and other routing protocols.
• Describe the design of scalable networks using
OSPF areas
• Describe the role of Area Border Routers (ABRs)
• List the types of Link-State Advertisements that
OSPF uses to communicate routing information
48

The Open Shortest Path First (OSPF) protocol is a widely
used interior gateway protocol. It is a mature protocol,
defined by the Internet Engineering Task Force (IETF)
standard specification, Request for Comment (RFC) 2328.
OSPF runs within a single Autonomous System (AS). OSPF
routers collect information about the AS topology, known
generically as Link State Advertisements (LSAs). The
routers store this information in a Link State Database
(LSDB) and use it to calculate least-cost routes to each of
the destinations in the AS. These routes loaded into
routing tables.
OSPF quickly detects changes to the AS topology (such as
interface failures), updates the LSDB, recalculates the
cost of the routes and updates the routing tables.
OSPF Areas allows OSPF to perform more efficiently over
large networks. Without OSPF areas, OSPF would not scale
well as the size and complexity of the Autonomous System
grows. The Link State Database (LSDB) would expand,
increasing the amount of memory that OSPF needs to
function. The number of Link State Advertisements (LSA)
sent would increase, worsening the messaging overhead
added by the protocol. The Shortest Path First algorithm
would run more frequently and need to consider greater
numbers of alternate paths, increasing the amount of
processing that OSPF performs.
OSPF areas provide scalability by dividing the autonomous
system into logical subsystems. Each router learns the
detailed topology of its own area, but not that of other
areas. Instead, the Area Border Router (ABR) originates a
Summary-LSA into the area on behalf of all the routers
outside of the area.
All routers in a single area have identical LSDBs. ABRs
have a LSDB for each area that they participate in.
Open Shortest Path First (OSPF)
• Interior Gateway Protocol
• Link State Protocol
• Bandwidth → Link cost
• IETF Standard (RFC 2328)
• Uses Areas to subdivide the
autonomous system
OSPF Area 1
OSPF Area 0
OSPF Area 2
AS 100
BGP
AS 300
BGP
49

Areas and Router
Types
50

OSPF defines a special area called the backbone. The
backbone is always be area 0 (alternately written as
0.0.0.0). All other areas must connect to the backbone.
This acts as a transit network for inter-area traffic. The
non-backbone areas send routing information to the
backbone, which in turn distributes the information to the
other non-backbone areas.
If an area does not contain a physical interface to the
backbone network, a logical backbone connection may be
formed by configuring what’s called a virtual link.
The Backbone Area: Area 0
• Backbone Area = Area 0
• All areas must connect to backbone area
• Distribute inter-area routes
Area 1
Area 0
Area 2
51

OSPF defines four types of routers: Internal Routers,
Backbone Routers, Area Border Routers, and Autonomous
System Boundary Routers. Depending on their function, a
given router may be of one or more of these types.
The simplest type of router is an internal router. If all of a
router’s interfaces are part of the same area, then that
router is said to be an internal router.
A router with at least one interface that is part of the
backbone area is a backbone router. If all of the router’s
interface are part of the backbone area, then that router
could be considered both a backbone and internal router.
The Area Border Router (ABR) contains interfaces on at
least two different areas. The ABR originates Summary-
LSAs into the backbone containing a summary of the non-
zero area’s intra-area routes. It originates Summary-LSAs
into the non-zero area containing a summary of the AS’s
inter-area routes.
The Autonomous System Boundary Router (ASBR)
exchanges routing information with other Autonomous
Systems (ASs). This may be through a different routing
protocol, such as the Border Gateway Protocol (BGP) or
through static configuration. They originate External-AS
Link State Advertisements (LSAs) to communicate these
external routes throughout the AS.
OSPF Router Types
AS 300
Area Border Router
(ABR)
Internal Router
ASBR, ABR and
Backbone Router
Backbone Router
AS 400
AS 100
Backbone Router
Internal and
Autonomous
System Boundary
Router (ASBR)
ASBR and
Backbone Router
Area 1 Area 2
Area 0
AS 200
52

Link State
Advertisements (LSA)
53

The router maintains a Link State Database (LSDB) for
each area to which it belongs. The contents of the LSDB
are flooded to all of the routers in the area. Each of the
routers in the area eventually build an exact replica of the
area’s LSDB. The LSDB is a representation of the topology
of the area. It identifies the routers and networks in the
area, the links that interconnect them and the cost of
those links.
The contents of the LSDB are Link State Advertisements
(LSA). The LSDB holds four types of LSAs:
1. Router-LSAs. Each router originates a Router-LSA that
describes the state of the its interfaces.
2. Networks-LSAs. The network’s designated router
originates a Network-LSA that contains a list of
routers connected to that network.
3. Summary-LSAs. The area border router originates a
Summary-LSA that contains routes to destinations
outside the area, but inside the autonomous system.
4. External-LSAs. The autonomous system boundary
router originates an External-LSA that contains routes
to destinations outside the autonomous system.
Using the information in the chart, draw the network
topology for the local area that contains 11 routers and
three networks. The numbers indicate the metrics on the
given links between the connected nodes or networks. For
example, the 0 in the row and column that link Network 3
with Router 7 indicate that Router 7 connects to Network
3 and from Network 3 to Router 7 the cost is 0. However,
look at the link from Router 7 to Network 3 and the cost is
2. There is no cost associated with coming out of a
network. Costs are incurred when leaving routers.
Link State Database
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 N1 N2 N3
R1 3 0 0
R2 5 5 5 0
R3 5
R4 5
R5 5
R6 3 0 0
R7 4 4 4 0
R8 4
R9 4
R10 4
R11 3 3
N1 3 3
N2 1 1
N3 2 2
Using the information in
this LSDB, draw the
topology of the local
network.
54

Draw the network based on the LSDB on the previous
page.
Draw the Network
55

This network is based on the LSDB given in the exercise. It
is in an OSPF network. Your drawing on the previous page
should look similar.
The Network
N3 2
2 4
4
4
1
R2
R4
R3
R1
R5
R7 R9
R8
R6
R10
N2
N1 3
3
5
5
5
1
R11
3 3
56

Link State
Advertisements
57

All OSPF routing information is exchanged in the form of a
Link State Advertisements (LSA). An LSA is a piece of
routing information that may describe a router, a physical
interface or a range of addresses, along with an
associated cost. The LSA specifies its originating router, its
age in seconds (which allows old routing information to
expire) and a sequence number (used to detect duplicate
LSAs).
There are 11 types of LSAs. The type of LSA depends on
the type of router creating the LSA.
Link State Advertisement
Link State
Advertisement
(LSA)
Router-LSA
(Type 1)
Network-LSA
(Type 2)
Summary-LSA
(Type 3, 4)
External-LSA
(Type 5, 7)
Opaque-LSA
(Type 9,10,11)
58

Each router in the Autonomous System originates a
Router-LSA. The Router-LSA indicates whether the router
is an ASBR or ABR. It includes a list of the router’s links,
the cost of each link, as well as other information such as
the link’s address. The Router-LSA gets distributed to
each of the routers in the area, but the ABR does not
forward the Router-LSA across area boundaries.
Type 1 Router-LSA
Area 1
Router-LSA = Type 1
Link State ID: 192.168.50.1
No of Links=2
Link 1 Cost , Type and other Details
Link 2 Cost , Type and other Details
Neighbor List
Generated by each router in the AS Area-scope
Link State ID is set to Router ID
59

A Network-LSA is originated for each broadcast or Non-
Broadcast Multiple Access (NBMA) network in the
autonomous system (but not for point-to-point links). It
includes a list of routers attached to the network and the
subnet address and mask assigned to the network. When
there are multiple OSPF router connected to the network,
only one of them, the Designated Router, sends the
Network-LSA. The Network-LSA gets distributed to each of
the routers in the area, but the Area Border Router (ABR)
does not forward the Network-LSA across area
boundaries.
Type 2 Network-LSA
Area 1
Network-LSA = Type 2
Subnet Mask = 255.255.255.0
RID of Attached Router-R1
Generated by Designated Router Area-scope
Link State ID is set to DR Interface Address
R1 (DR)
R3 R4
Point-to-Point Link
Network-LSA = Type 2
Subnet Mask = 255.255.255.0
R2
10.2.1.100/24
NW: 10.2.1.0/24
60

The Area Border Router (ABR) does not forward Router-
LSAs or Network-LSAs from one area to the next. Instead,
the ABR originates a Summary-LSA that, as its name
implies, summarizes the routing information for that area.
The ABR may originate separate Summary-LSAs for each
of the networks to be advertised, or it may condense
multiple networks into a single Summary-LSA by
advertising a single network address with a shorter
network prefix. When a router receives a Summary-LSA, it
computes a new cost to the target by adding in the cost of
the path between that router and the ABR. When an ABR
receives a Summary-LSA sent from some other ABR, it
does not forward the Summary-LSA across area
boundaries.
In the example shown, the ABR receives four Network-
LSAs from within Area 1. The ABR has been configured to
condense these into a single Summary-LSA that it
originates into Area 0. It sets the cost of the summary
route to be the largest cost of its individual component.
Type 3 Summary-LSA
Area 0
Summary-LSA = Type 3
Subnet Mask = 255.255.252.0
Metric = 10
Generated by ABR Area-scope
Link State ID is set to Advertised Network Address
Cost = Advertised Cost + Cost to ABR
Area 1
10.1.1.0/24 Cost: 3
10.1.2.0/24 Cost: 5
10.1.3.0/24 Cost: 10
10.1.4.0/24 Cost 8
Network-LSA
Subnet Mask = 255.255.255.0
ABR
61

The Area Border Router (ABR) generates an Autonomous
System Boundary Router (ASBR) Summary-LSA for each
ASBR in its area. This includes the cost of the path from
the ABR to the ASBR. The router may choose to use this
information to calculate the total cost to reach a
destination outside of the Autonomous System. When an
ABR receives an ASBR Summary-LSA from some other
ABR, it does not forward it across area boundaries.
Type 4 ASBR Summary-LSA
Area 1
ASBR Summary-LSA = Type 4
Subnet Mask = 0.0.0.0
Metric = 22
Generated by ABR Area-scope
Link State ID is set to ASBR Router ID
For External Type 1 routes, add cost to ASBR
For External Type 2 routes, store as-is in Routing Table
Area 0
R7
(RID 192.168.50.7)
AS 100
ASBR
62

When the Autonomous System Boundary Router (ASBR)
wants to import a route from other routing processes to an
external destination into the Autonomous System (AS), it
originates an AS-External-LSA. This includes the network
address and mask of the external destination and the next
hop address of the external link. The AS-External-LSA gets
flooded throughout the AS.
The cost of the external route may be one of two types.
Type 1 costs use the same units as other OSPF LSAs. The
Shortest Path First algorithm may make a meaningful
comparison between the Type 1 cost of the AS-External-
LSA and the costs learned through other types of LSAs.
Type 2 costs are expressed using some other type of units
that are not compatible with the metrics used by the AS.
These other units are assumed to be significantly larger
than those used by the AS, such that the Shortest Path
First algorithm always selects a path with a cost expressed
in Type 1 units over a path expressed in Type 2 units.
Type 5 AS-External-LSA
AS External-LSA = Type 5
Link State ID: 10.0.0.0/16
Metric = 20
Metric = 20
Area 0
Metric = 20
Link State ID is set to Advertised Network Address
ASBR generates 1 AS External-LSA per route imported for the whole AS
Distributed transparently without any modification within AS
Includes the Next Hop IP address of the external link in the LSA
Area 1
AS 100
eBGP
10.0.0.0/16
10.1.0.0/16
10.2.0.0/15
ASBR
Metric = 20
Metric = 20
Metric = 20
63

The Opaque-LSA type was introduced as a means to
extend OSPF to carry application-specific data. It provides
the means to carry a generic chunk of data, the exact
form and function of which is known only to the
application that defines it. For example, the Traffic
Engineering application defines a type of Opaque-LSA. In
this application, the LSA carries bandwidth reservation
information associated with the link.
Three types of Opaque-LSAs have been defined. Link-local
opaque-LSAs (type 9) is sent to routers attached to the
specified network and not forwarded beyond that point.
Area-Local opaque-LSAs (Type 10) are forwarded to all
routers in the area, but not forwarded across area
boundaries. AS-local opaque-LSAs (Type 11) are
forwarded to all routers in the Autonomous System.
Types 9 – 11 Opaque-LSA
AS 300
AS 400
AS 100
Area 1
Area 2
Area 0
AS 200
N1
Opaque-LSA
Network-scope
Opaque-LSA
Area-scope
Opaque-LSA
AS-scope
64

Summary
• OSPF is used to exchange routing information
between routers within an Autonomous System.
• OSPF uses Link-State Advertisements to
communicate information about the topology of the
network.
• Routers store Link-State Advertisements in the
Link-State Database.
• Each router in an area maintains its own copy of
the Link-State Database.
• OSPF routers learn details about the topology of
their area, but not that of other areas.
65

The figure illustrates a sample OSPF network. It highlights
four of the routers (R4, R8, R12 and R13). What type of
routers are these? (Hint: a single router may be of more
than one type.)
Your choices are: internal router, backbone router, Area
Border Router (ABR), Autonomous System Boundary
Router (ASBR).
Orlando St. Petersburg
Tampa
Review Exercise
R1
R4
R6
R5
R3
R7
N1 N2 R9 R11
R10
R8
R12
R2
R14
R16
R15
R13
R17
N3 R18
Area 1 Area 2
Area 3
BGP
R8:
R13:
BGP
R19
R12:
R4:
66

5 | OSPF in Wireless Networks
Chapter 5:
OSPF in Wireless
Networks
67

Objectives
After completing the module, you will be able to:
• Describe the operations of an OSPF router
• Troubleshoot OSPF adjacency issues
• Analyze the contents of the link-state database
• Calculate OSPF link metrics to influence routing
decisions
• Use OSPF areas to build scalable networks
68

An OSPF-capable router spends its life performing the
following functions.
• OSPF routers automatically discover the existence of
neighboring routers.
• Having discovered a new neighbor, the two routers
synchronize their Link State Databases (LSDBs).
• Once the topology of the network is known (through
the LSDB), the router runs a Shortest Path First (SPF)
algorithm to calculate the optimal path to reach each
of the known destinations in the network.
• The “best” path information is merged with other
routing information (e.g., static routes and other
dynamic routing protocols) to create routing table.
• When changes occur in the network topology (e.g.,
links are enabled or shutdown), the router updates its
neighbors by sending a Link State Advertisement
(LSA). Of course, the router may also re-run the SPF
calculations to determine the impact of the topology
change and update the routing table accordingly.
• OSPF routers keep tabs on their neighbors. If the
router suddenly stops receiving keep-alive messages,
it assumes that the neighbor has gone off-line,
updates the LSDB accordingly, re-runs the SPF
algorithm and adjusts the routing table as necessary.
Life of an OSPF Router
Neighbor
Discovery
Synchronize
LSDB
Compute SPF
Populate
Routing Table
Send LSA
Keep-Alive
69

Neighbor Discovery
70

Whenever possible, OSPF relies on the automatic
discovery of neighboring routers, rather than having them
administratively configured. An OSPF router periodically
sends a “Hello” message to the multi-cast address
224.0.0.5. This is a well-known address to which all OSPF
routers are supposed to be listening.
The router does not necessarily exchange routing
information with each one of its neighbors. It only does so
with so-called adjacent neighbors. The Hello message
includes several configuration data: the router identifier,
area identifier, hello interval, dead interval and subnet
prefix. The two neighboring routers must agree on each of
these parameters if they are to form an adjacency and
exchange routing information. The Hello message also
includes the list of neighboring routers that are already
known to this router. This is used to confirm that the
neighbor heard your Hello message (i.e., to confirm that
two-way communication exists).
Neighbor Discovery
• Hello message
– Send periodically
– Multicast: 224.0.0.5
• Includes
– Router ID
– Area ID
– Hello Interval
– Dead Interval
– Subnet Prefix
– Neighbor List
71

The Hello protocol is used to discover new OSPF
neighbors, verify that two-way communication is in place,
communicate information about the router’s configuration
and confirm that the neighbor remains active over time.
The router sends a Hello message every HelloInterval
seconds (10 seconds is typical). The router sends the
Hello message to the well-known multicast address
224.0.0.5. The router includes the Area ID, the sending
router’s own Router ID and a list of Router IDs from whom
the sending router has received a Hello message. The
message also contains authentication credentials, the
network mask of the interface, the value of the
HelloInterval and RouterDeadInterval timers, the Router ID
of the Designated Router and a priority value used in the
Designated Router election procedure.
In the example shown, Router R1 has been sending Hello
messages every HelloInterval seconds. Router R2 comes
online starts running OSPF. It starts sending its own Hello
messages. Router R1 hears the Hello message that
Router R2 has sent. It adds R2 to its list of neighbors and
includes R2’s Router ID when it sends its next Hello
message. Likewise, Router R2 hears the Hello message
that Router R1 has sent. It notes that its own Router Id
was included in the message. Router R2 now knows that
two-way communication is possible with this neighbor. R2
adds R1 to its list of the neighbors and includes R1’s
Router Id in the next Hello message that it sends. Router
R1 notes that its Router ID has been included in Router
R2’s hello message. Router R1 now knows that two-way
communication is possible with R2.
The two routers analyze the fields in the Hello messages.
If their respective area ID, network mask, HelloInterval,
and RouterDeadInterval fields match, they may attempt to
form an adjacency with each other and begin to
synchronize their Link State Databases.
Hello Protocol
R1 R2
Hello
Area ID, Router ID, Neighbor List
Hello
Area ID, Router ID, Neighbor List (R2)
Hello
R2 powers on and
sends first Hello
R1 hears Hello
and adds R2 to
neighbor list
Hello
Area ID, Router ID, Neighbor List (R1)
R2 hears Hello
and adds R1 to
neighbor list
Hello
72

Having established an adjacency with each other, the two
neighboring routers synchronize their LSDBs. They first
send a series of Database Description messages. These
messages summarize the contents of the LSDB and
include enough information for the neighbor to determine
whether it already knows the Link State Advertisements
(LSAs) contained within. If the router learns about a new
LSA that is not in its LSDB, or sees that a more recent
copy of an LSA is available, it sends a Link State Request
message, indicating that it wants to know the details
about the LSAs. The first router then sends the requested
LSAs in a Link State Update message.
The figure illustrates the process by which router R1
synchronizes its database with router R2. Router R2 also
synchronizes its database with router R1, but this is not
shown in the illustration.
Database Synchronization
Link State Request
Please send me these LSAs…
R1 R2
LSDB LSDB
Database Description
These are the LSAs in my LSDB
Link State Update
Here are the LSAs you asked for…
73

Consider the effects of the neighbor discovery procedure
on a multiple-access network with six routers. Each router
would discover five neighbors and form five adjacencies.
Fifteen adjacencies would be formed overall. The
Designated Router (DR) concept reduces the number of
adjacencies required in this circumstance, and by
extension reduces the size of the Link State Database
(LSDB) and the number of OSPF messages sent over the
network.
The Hello protocol includes a mechanism by which the
routers autonomously determine which router functions as
the DR. The administrator can influence this decision by
assigning certain priorities to each of the routers. Once a
DR has been elected for the network, it generally does not
change unless the DR goes out of service. To facilitate a
fast recovery from that circumstance a Backup
Designated Router (BDR) is elected at the same time as
the DR. If the DR should go out of service, the BDR takes
over as DR and a new BDR is elected.
Instead of forming adjacency with every neighbor on the
network, a router instead forms an adjacency with only the
DR and BDR. In the example configuration shown, the
total number of adjacencies that form is reduced from 15
to 9.
Designated Router (DR)
R1 R2 R3
R4 R5 R6
R1 (DR) R2 (BDR)
R3
R4
R5 R6
Multi-Access
Network with
6 OSPF
Routers
With DR and BDR Election
Adjacencies = 2N-3
Adjacencies = (2*6)-3=9
R1 R2
R3
R4
R5 R6
Designated Router is
Elected to Reduce
Adjacencies
Backup Designated Router
is also elected for fast
takeover from DR’s Failure
No DR and BDR Election
Adjacencies = N*(N-1)/2
Adjacencies = 6*(6-1)/2 = 15
74

Metrics
75

OSPF automatically assigns a cost to each of the links in
the network based on the bandwidth of the link. It
calculates this cost by selecting a fixed value known as
the reference bandwidth, and then dividing the reference
bandwidth by the bandwidth of the link. The minimum cost
that can be assigned to a link is 1. It is important that
each of the routers in the network use the same reference
bandwidth in order to accurately compare routes.
OSPF Metric
Interface
Type
Speed
(Mbps)
Cost
ref = 100 Mbps ref = 1 Gbps ref = 10 Gbps
10GE 10000 1 1 1
OC-192c 9953.28 1 1 1
OC-48c 2488.32 1 1 4
GigE 1000 1 1 10
OC-12c 622.08 1 1 16
OC-3c 155.52 1 6 64
Fast Ethernet 100 1 10 100
Ethernet 10 10 100 1000
Reference Bandwidth
Link Speed
OSPF Cost =
76

Exercise
Calculate the cost for the T3 and T1 links.
Interface Type
Speed
(Mbps)
Cost
ref = 100 Mbps ref = 1 Gbps ref = 10 Gbps
10GE 10000 1 1 1
OC-192c 9953.28 1 1 1
OC-48c 2488.32 1 1 4
GigE 1000 1 1 10
OC-12c 622.08 1 1 16
OC-3c 155.52 1 6 64
Fast Ethernet 100 1 10 100
Ethernet 10 10 100 1000
T3 44.736
T1 1.544
77

Each router in the network uses the information in its link
state database to execute the Shortest Path First (SPF)
algorithm and calculate the most optimal route to each
destination in the network. The SPF algorithm uses the
costs that were assigned to each of the links. The cost of
the route is the sum of the link costs that make up the
route.
The figure illustrates four switching offices: Orlando, St.
Petersburg, Tampa and Boca. Each switching office has
an edge router (R3, R8, R13 and R18) that connects to
the backbone for the region (R1 and R2). Additionally,
each edge router connects directly to two neighboring
offices (for example, R3 in Orlando connects to R8 in St.
Petersburg and R13 in Tampa).
Shortest Path First
Orlando
1
R4
R6
R5
R3
R7
N1 3
3
5
5
5
1
St. Petersburg
N2 2
2 4
4
4
R9 R11
R10
R8
R12
2
Tampa
1
R14
R16
R15
R13
R17
N3 3
3
5
5
5
Boca
2
2 4
4
4
N4 R19 R21
R20
R18
R22
1 1
1 1
2
R1
R2
78

The Shortest Path First algorithm produces a tree of
shortest paths to each router and network in the area,
with the router itself as the root of the tree. The router
only needs to know the next hop for each of the
destinations to forward packets. These next hops are
loaded into the routing table.
The figure illustrates the results of the Shortest Path First
algorithm when run on router R3 (we’ve also abbreviated
the diagram by only showing the paths to each office’s
edge router). The table illustrates the set of next hops that
are loaded into the routing table.
Shortest Path First
R3
1
R18
R13
R8
R2
R1
1
1
2
1
1
2
1
1
R3
1
R18
R13
R8
R2
R1
1
1
1
1
Before After
Destination Next Hop
R8 R8
R1 R1
R13 R13
R2 R13
R18 R13
79

It is possible that the Shortest Path First algorithm
identifies multiple paths to a destination with the same
cost value. When that happens, multiple paths may be
loaded into the routing table (assuming they have
different next hops) and traffic is distributed equally
across each of them.
We can illustrate this by changing the cost of the R1-R2
and R8-R18 links. If these two links each have a cost of
one, then router R3 calculates multiple equal-cost paths
to routers R2 and R18. To send packets to router R2, R3
could forward packets to router R1 or R13. Each path has
a cost of two. To send packets to R18, R3 could forward
packets to R8 or R13. Each path has a cost of two.
Equal-Cost Multipath
After
R3
1
R18
R13
R8
R2
R1
1
1
1
1
1
1
Destination Next Hop
R8 R8
R1 R1
R13 R13
R2 R13
R2 R1
R18 R13
R18 R8
80

The figure illustrates four switching offices: Orlando, St.
Petersburg, Tampa and Boca. Each switching office has
an edge router (R3, R8, R13 and R18) that connects to
the backbone for the region (R1 and R2). Additionally,
each edge router connects directly to two neighboring
offices (for example, R3 in Orlando connects to R8 in St.
Petersburg and R13 in Tampa).
Assign a cost to each of the links indicated such that the
following two conditions are met:
1. Each edge router (R3, R8, R13 and R18) processes
only traffic that originates or terminates inside its
respective office. They do not process so-called
transit traffic that both originates and terminates
outside their respective offices.
2. Each edge router shall direct traffic destined for an
adjacent office over the direct link to that office.
When properly configured, traffic that runs from Orlando
to St. Petersburg goes over the R3-R8 link. Traffic that
runs from Orlando to Tampa traverses the R3-R13 link.
Traffic that runs from Orlando to Boca passes through the
backbone, R3-R1-R2-R18.
Orlando St. Petersburg
Tampa Boca
SPF Exercise
R1
R4
R6
R5
R3
R7
N1 3
3
5
5
5
N2 2
2 4
4
4
R9 R11
R10
R8
R12
R2
R14
R16
R15
R13
R17
N3 3
3
5
5
5
2
2 4
4
4
N4 R19 R21
R20
R18
R22
81

Areas
82

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