250601416-Advanced-Routing-Reference-Manual-Ver-0-9

Reference Manual ver. 1.0 (2012-14)
Created by Paul Nadstoga (pnadstoga@gmali.com)

Contents
EIGRP 1
OSPF 27
CONTROLLING ROUTING UPDATES 81
BGP 117
BRANCH OFFICE 150
IPv6 167
APPENDIXES 203

EIGRP
• EIGRP Basics
• EIGRP Packets
• EIGRP Stuck In Active
• EIGRP Timers
• EIGRP Metric
• EIGRP Tables
• EIGRP Over NBMA
• EIGRP Configurations
• EIGRP Verification and Tshooting

ADVANCED ROUTING ver. 1.0 CREATED BY PAWEL ‘PAUL’ N=DSTOGA (PNADSTOGA@GMAIL.COM) 2012-14 2
EIGRP BASICS
TYPE ALGORITHM INTERNAL AD EXTERNAL AD SUMMARY AD STANDARD PROTOCOLS TRANSPORT AUTHENTICATION MULTICAST IP TIMERS
Distance
Vector
DUAL 90 170 5 Cisco
 IP
 IPX
 AppleTalk
RTP:IP:88 MD5 224.0.0.10
 HELLO: 5 / 60
 HOLD: 15 / 180
The following conditions have to be met for two routers to form a neighbor relationship:
 Autonomous System values match
 source IP address of a received HELLO is in the same subnet as the primary IP address configured on the receiving interface (subnet mask does not need to be identical)
 K values match
 authentication key IDs + key strings match (if authentication is configured)

EIGRP PACKETS
PACKET OVERVIEW COMMENTS
 HELLO
 initially used to discover and verify neighbors
 later used to maintain the relationship (keep-alive mechanism)
 sent at interval specified by the HELLO timer
 multicasted on 224.0.0.10
 unreliable (delivery not acknowledged by the recipient)
 the default HELLO timer depends on the interface bandwidth
 neighbors learn each other’s timers through the HELLO
packets and use that information to forge a relationship
 more than one HELLO packets may be needed to convey all
routing information to a new neighbor
 UPDATE
 used to exchange routing information
 initially sent when forming a relationship and then only to affected routers
 unicasted to a specific router
 multicasted to a group of routers
 reliable (delivery acknowledged by the recipient)
Contains:
 prefix / prefix length
 metric components (bandwidth, delay, reliability, load)
 non-metric components (MTU, hop count)
 sent as multicast initially and when one ACK received from a
specific router the UPDATE is resent as an unicast
 also sent when a topology change is detected - in such case
the router sends a multicast UPDATE to all its neighbors
 UPDATE sent on an interface does not contain routes that
were learnt through the same interface because of the split
horizon rule
 QUERY
 sent when a specific information is required from one / all of its neighbors
 normally sent as multicast but can be retransmitted as unicast in certain cases
 if all outstanding QUERIES are not replied within the ACTIVE timer, the neighbor that failed to
reply is removed from the neighbor table
Also used when a router loses its successor and can’t find a feasible
successor for a route - in such case DUAL places the router in active
state and start sending multicasts in search for a successor.
 REPLY
 used to respond to a QUERY
Always sent as unicast to specifically inform the originator it does
not need to go into active state because it an alternative route is
available.
 ACK
 sent to acknowledge UPDATE, QUERY and REPLY
 unicast HELLO packets and contain a nonzero ack. number
 GOODBYE
 also known as graceful shutdown
 send to notify the neighbors when a router is shutting down the EIGRP process or removes a
network statement that included the neighbors in the EIGRP process (e.g. no network 10.0.0.0)
Sent as a HELLO packet with all K values set to 255.

EIGRP STUCK-IN-ACTIVE
 a situation that may take place when the successor is lost and a FS does not exist
 when the successor to a network is lost, QUERIES are sent to all the neighbors asking for an alternative route (note: the inactive link is not queried)
 if REPLIES are not received, the route is put into an ACTIVE state
 by default, the router will wait 180 sec. to receive replies to queries sent – any adjacency that hasn’t replied by then will be reset

EIGRP TIMERS
TIMER OVERVIEW COMMENTS
 HELLO
 specifies the time interval at which the HELLO packets are retransmitted
To adjust:
<Router(config-if)#ip hello-interval eigrp (AS) (5 | 60, 1-65535 sec.)>
To verify:
<Router#show ip eigrp interfaces detail>
Works independently in each direction -
neighbors don’t need to use the same
HELLO timer values
 HOLD
 specifies the time interval during which a router will consider a neighbor alive without receiving a HELLO from that neighbor
 by default equals to 3 x HELLO timer
To adjust:
<Router(config-if)#ip hold-timer eigrp (15 | 180, 1-65535 sec.)>
To verify:
<Router#show ip eigrp neighbors>
 changing the HELLO timer does not
automatically adjust the HOLD timer
 the HOLD timer is sent to the
neighbor in the HELLO packet i.e. a
router does not use locally
configured timer value be the value it
receives from the neighbor in the
HELLO packet
 the IOS does not prevent the user
from setting the HOLD timer to a
value lesser than HELLO!
 ACTIVE
 specifies the time interval the router waits after sending a QUERY before declaring the route stuck in active (SIA) and
resetting the neighbor relationship
To adjust:
<Router(config-router)#timers active-time (180, 1-65535 min.)>
<Router(config-router)#timers active-time disabled>
 increasing the timer might be useful
when troubleshooting EIGRP
 timers active-time disabled - disables
time limit for active states
DEFAULT TIMER VALUES
BANDWITDH EXAMPLE LINK DEFAULT HELLO TIMER DEFAULT HOLD TIMER ACTIVE
< 1.544 Mbps Multipoint Frame Relay 60 sec. 180 sec.
180 sec.
> 1.544 Mbps T1, Ethernet 5 sec. 15 sec.

EIGRP METRIC
FULL (ALL K VALUES USED)
𝟐𝟓𝟔 ∗ (𝑲𝟏 ∗ 𝐛𝐰 +
𝑲𝟐 ∗ 𝐛𝐰
𝟐𝟓𝟔 − 𝐥𝐨𝐚𝐝
+ 𝑲𝟑 ∗ 𝐝𝐞𝐥𝐚𝐲) ∗
𝑲𝟓
𝐫𝐞𝐥 + 𝑲𝟒
DEFAULT (ONLY K1 + K3 USED AND ARE EQUAL TO 1)
𝟐𝟓𝟔 ∗ ( 𝑲𝟏 ∗ 𝐛𝐰 + 𝑲𝟑 ∗ 𝐝𝐞𝐥𝐚𝐲)
bw = 107
/ minimum bandwidth in kbps (if the result is not a whole number the value is rounded down)
delay = sum of delays of outgoing interfaces in µsecs / 10
256 = multiplier used for compatibility with IGRP (EIGRP uses 32 bit metric while IGRP uses 24)

METRIC COMPONENTS
COMPONENT OVERVIEW COMMENTS
 BANDWIDTH
 the bandwidth of the interface
 static value
To modify:
<Router(config-if)#bandwidth (1-10000000 kbit.)>
Default values for:
 ethernet: 100000 Kbit/sec
 serial: 1544 Kbit/sec
 DELAY
 measure of time it takes for a packet to traverse a route
 static value
To modify:
<Router(config-if)#delay (1-16777215 usec.)>
Default value for:
 ethernet: 100 usec
 serial: 20000 usec
To view total delay for a route:
 show ip eigrp topology A.A.A.A/MM
 LOAD
 amount of traffic utilizing the link
 dynamic value (0-255)
 calculated on a 5 min. basis
 1/255 minimally loaded link
 255/255 fully saturated link
 RELIABILITY
 a measure of probability that the link will fail i.e. how often the link has experienced errors
 calculated on a 5 min. basis
 1/255 least reliable link
 255/255 fully reliable link
 MTU  not used anywhere in the metric calculation but sent for prefixes
 K VALUES
Defaults:
 K1=1, K2=0, K3=1, K4=0, K5=0
To modify:
<Router(config)# router eigrp (1-65535)>
<Router(config-router)#metric weights (tos 0-8) (k1 0-255) (k2 0-255) (k3 0-255) (k4 0-255) (k5 0-255)>
 identical K values are one of the conditions for two
routers to become an EIGRP neighbor
 TOS was never implemented so the value has to be
always set to 0
 TSHOOT
 show interface (interface)
 show ip protocols

EXAMPLE: DEFAULT METRIC CALCULATION
From R3 to 172.30.0.0 /24 through s1/1
 𝟐𝟓𝟔 ∗ ( 𝐊𝟏 ∗ 𝐛𝐰 + 𝐊𝟑 ∗ 𝐝𝐞𝐥𝐚𝐲)
 𝟐𝟓𝟔 ∗ (𝐊𝟏 ∗ (
𝟏𝟎,𝟎𝟎𝟎,𝟎𝟎𝟎
𝟏𝟓𝟒𝟒
) + 𝐊𝟑 ∗ (
𝟐𝟎,𝟎𝟎𝟎+𝟓𝟎𝟎𝟎
𝟏𝟎
)
 256 * (1 * (6476.6839 = 6476) + 1 * (2500)
 256 * (6476 + 2500)
 256 * 8976
 2297856
From R3 to 172.30.0.0 /24 through fa0/0
 𝟐𝟓𝟔 ∗ ( 𝐊𝟏 ∗ 𝐛𝐰 + 𝐊𝟑 ∗ 𝐝𝐞𝐥𝐚𝐲)
 𝟐𝟓𝟔 ∗ (𝐊𝟏 ∗ (
𝟏𝟎,𝟎𝟎𝟎,𝟎𝟎𝟎
𝟏𝟓𝟒𝟒
) + 𝐊𝟑 ∗ (
𝟏𝟎𝟎+𝟐𝟎,𝟎𝟎𝟎+𝟓𝟎𝟎𝟎
𝟏𝟎
)
 256 * (1 * (6476.6839 = 6476) + 1 * (2510)
 256 * (6476 + 2510)
 256 * 8986
 2300416
*Not a Feasible Successor since AD equals (needs to be less) than Feasible Distance of the
current Successor (via s1/1 - 172.1.34.1)

EIGRP TABLES
TABLE OVERVIEW COMMENTS
 NEIGHBOR TABLE  list of directly connected routers running EIGRP with which adjacencies are formed
To view the table content:
<R1#show ip eigrp neighbors>
 H (handle) - an IOS internally used number to track a
neighbor by recording the order in which the neighbours
were learnt
 Address - neighbor’s L3 address
 Interface - local interface on which the neighbor can be
reached
 Hold (hold time) - maximum time in seconds that the router
waits to hear from the neighbor before considering the link
down (any EIGRP packet received after the first HELLO from
that neighbor resets the timer)
 Uptime - time that has elapsed since the neighbor was added
to the table
 SRTT (smoothed round-trip time) - the average number of
milliseconds it takes for an EIGRP packet to be sent to this
neighbor and for the local router to receive an ACK for that
packet - this timer determines the RTO
 RTO (retransmission timeout) - the number of milliseconds
that the router waits for an ACK before retransmitting a
reliable packet from the retransmission quote to the
neighbor. If an UPDATE, QUERY or REPLY packet is sent, a
copy of packet is queued. If the RTO expires before an ACK is
received, another copy of the queued packet is sent
 Q Cnt (queue count) - number of packets waiting in the queue
to be sent out (if constantly higher than 0 a congestion
problem may exist)
 Seq Num - sequence number of the last UPDATE, QUERY or
REPLY packet that was received from the neighbor (used to
detect out-of-order packets)

 TOPOLOGY TABLE
 list of all routers learnt from each EIGRP neighbor  the table is updated when a directly connected router /
interface changes or a neighbor reports a route change
<Router#show ip eigrp topology (active | all-links | detail-links)>
 active - shows only active entries
 all-links - shows all links in topology table
 detail-links - more detailed version of the above
 P(Passive) - correct state for a stable network (network is
available and installation can occur in the routing table
 A(Active) - network is currently unavailable, and installation
cannot occur in the routing table (there are outstanding
queries for this network). A route will be put into Active state
when the current Successor is down and Feasible Successors
are not available
 U(Update) - network is being updated (placed in an UPDATE
packet); also applies if the router is waiting for an ACK for this
UPDATE
 Q(Query) - outstanding query packet for this network (also
applies if the router is waiting for an ACK for a QUERY)
 R(Reply status) - router is generating a REPLY for this
network or is waiting for an ACK for the REPLY
 S(Stuck-in-active status) - indicates EIGRP convergence
problem for the network with which it is associated
 successor - next-hop router with lowest cost and loop free
path (successors end up in the routing table)
 Feasible Successor - a backup router with loop-free path (to
become one a router has to meet the Feasible Condition)
 Feasible Condition - AD of Feasible Successor must be less
than the FD of the current Successor
 AD (Advertised Distance) - cost between the next-hop router
and the destination
 FD (Feasible Distance) - cost from a local router to the
destination

 ROUTING TABLE
 list of all best routes from EIGRP topology table and other routing processes
 the best route to a destination (successor) is chosen by comparing all FDs to that
destination and selecting the route with the lowest FD - which becomes the router’s
metric shown in the table
<Router#show ip route eigrp>
 [90/156160] - EIGRP’s Administrative Distance (believability)
 [90/156160] - the cost to reach the network (Feasible Distance)

EIGRP OVER NBMA
THINGS TO KEEP IN MIND:
 by default multicasts and broadcasts are denied on NBMA networks which requires special consideration for protocols such as EIGRP that rely on multicasts to establish and maintain
neighbor relationships
 in point-to-multipoint topologies, split horizon enabled on the hub may prevent updates from being propagated across all network
 pseudo broadcast must be enabled on the frame-relay interface OR EIGRP neighbors need to be statically configured if the pseudo broadcast cannot be used or is not supported
EXAMPLE:

CONFIGURATIONS COMMENTS
 CONFIGURE FR INTERFACES ---> R1(config)#interface s1/0
R1(config-if)#encapsulation frame-relay
R1(config-if)#ip address 172.16.124.1 255.255.255.0
R1(config-if)#no frame-relay inverse-arp
R1(config-if)#no arp frame-relay
R1(config-if)#bandwidth 128
R1(config-if)#ip bandwidth percent eigrp 1 40
R2(config)#interface s1/0
By default EIGRP uses 50% of the bandwidth specified with the
bandwidth command on a frame relay enabled interface.
 ip bandwidth-percent – defines how much percentage of the
interface bandwidth can be utilized the EIGPR
(*has to be configured on a per (sub)interface basis)
(** for multipoint interfaces the router further divides the
bandwidth according to the number of neighbours out that
interface)
 STATICALLY ADD FR MAPS ---> R1(config-if)#frame-relay map ip 172.16.124.2 102 broadcast
R1(config-if)#frame-relay map ip 172.16.124.3 103 broadcast
To confirm:
Router#show frame-relay map
 broadcast – (aka. pseudo broadcast) emulated broadcast -
acts as broadcast but the packets are sent as unicast
messages
 ENABLE EIGRP ---> R1(config)#router eigrp 1
R1(config-router)#no auto-summary
R1(config-router)#network 10.0.0.0

R2(config)#router eigrp 1
R4(config)#router eigrp 1
 SUMMARISE UPDATES ---> R1(config)#interface s1/0
R1(config)#ip summary-address eigrp 1 10.1.0.0 255.255.0.0
 DISABLE SPLIT-HORIZON ---> R1(config)#interface s1/0
R1(config-if)#no ip split-horizon eigrp 1
At this stage routes from R2 are not being propagated to R3 and
vice versa because split horizon will prevent R1 to advertise the
10.2.0.0/16 network via the same interface it was received on.
Disabling split horizon will generate on the local end:
*Oct 18 21:20:12.041: %DUAL-5-NBRCHANGE: EIGRP-IPv4 1: Neighbor
172.16.124.3 (Serial1/0) is resync: split horizon changed
 *ENABLE EIGRP NON-BROADCAST MODE ---> R1(config-router)#neighbor 172.16.124.2 s1/0
R1(config-router)#neighbor 172.16.124.3 s1/0
May be used as first solution or when the Frame Relay cloud does
not support pseudo broadcast. Changes the EIGRP packets
propagation mechanism from multicast to unicast.
(*the exit interface still has to be advertised with the network
command)
(** the mechanism change will only affect the interface via which
the routers communicated the EIGRP neighbor)
(*** both ends have to use the same mode)
Changing the mode will generate the following on the local end:
*Oct 18 21:39:23.961: %DUAL-5-NBRCHANGE: EIGRP-IPv4 1: Neighbor
172.16.124.2 (Serial1/0) is down: Static peer configured

EIGRP CONFIGURATIONS
ACTIVATION
STEP # COMMANDS COMMENTS
 START EIGRP PROCESS
<Router(config)#router eigrp (Autonomous System; 1-65535)> AS allows to start separate EIGRP processes on the same router
(the value has to be the same for all the routers within the same
processes).
 AUTOMATIC
SUMMARIZATION
<Router(config-router)#auto-summary>
<Router(config-router)#no auto-summary>
 auto-summary – when enabled, EIGRP automatically
summarize network updates to their classful boundaries
 HARDCODE ROUTER ID
<Router(config-router)#eigrp router-id (A.A.A.A)>
To verify:
<Router#show ip eigrp topology>
<Router#show ip protocols>
Mainly used in external routes as a loop prevention mechanism –
external routes are tagged with the RID and in case the advertising
router receives them back with its own RID they are dropped.
Unique for each AS.
1. use the configured value: eigrp router-id
2. use the highest IPv4 address on an UP|UP loopback
3. use the highest IPv4 address on an UP|UP non-loopback
 ADD NETWORKS
To add all networks:
<Router(config-router)#network 0.0.0.0 255.255.255.255>
To add individual networks:
<Router(config-router)#network (IP address)(wildcard)
OR
<Router(config-router)#network (IP address)(mask)
OR
<Router(config-router)#network (IP address)
To manually add a neighbor:
<Router(config-router)#neighbor A.A.A.A>
 auto-summary - enables automatic network summarization
on major network boundaries (enabled by default - it is
recommended to switch the command off and to do it before
adding networks - otherwise the EIGRP will have to re-
converge disturbing the network operation)
 network - specifies the range of addresses on which the
interfaces start sending / receiving HELLOs and advertise the
network the interface belongs to
 network (IP address)(mask) - the mask will be converted into
and displayed as wildcard in the running configuration
 network (IP address) - the IP address entered will be treated
as classful and the whole classful range will be included in the
EIGRP process

 PASSIVE INTERFACES
<Router(config-router)#passive-interface (default | (interface))>
To verify:
 passive-interface - no HELLOs are sent on the interface
(hence no relationship can be formed) but the network is still
advertised
 passive-interface default - sets all interfaces as passive
A passive interface is still part of the EIGRP process and the
network advertised but no HELLOs are sent to that interface.
 PROPAGATE DEFAULT
GATEWAY
<Router(config)#ip route 0.0.0.0 0.0.0.0 (IP address | exit interface)>
<Router(config-router)#network 0.0.0.0>
 network 0.0.0.0 - can also be used to include any static route
in the updates
 ip default-network - sets and redistributes given network as
default (has to be classful and has to be reachable by the
router)

TUNING
FEATURE COMMANDS COMMENTS
 ADJUST AD
Globally:
<Router(config-router)#distance eigrp (internal (1-255)) (external (1-255))>
Per routes:
<Router(config-router)#distance (AD 1-255) (source A.A.A.A W.W.W.W) (*1-99 | 1300-
1999 | ACL name)>
 distance (AD 0-255) - returns Incomplete command
 distance (AD 0-255) 0.0.0.0 255.255.255.255 - assigns AD = 0
 ADJUST K VALUES
<Router(config-router)#metric weights (tos; 0-8) (k1; 0-255) (k2; 0-255) (k3; 0-255) (k4;
0-255) (k5 0-255)>
 TIMERS
o HELLO
<Router(config-if)#ip hello-interval eigrp (AS) (1-65535 sec)>
To verify:
o HOLD
<Router(config-if)#ip hold-timer eigrp (1-65535 sec)>
To verify:
<Router#show ip eigrp neighbors>
o ACTIVE
<Router(config-router)#timers active-time (1-65535 min)>
<Router(config-router)#timers active-time disabled>
 BANDWIDTH LIMIT
<Router(config-if)#ip bandwidth-percent eigrp (AS) (1-999999)>  ip bandwidth-percent - maximum bandwidth % EIGRP may
use on the interface
 SPLIT-HORIZON
<Router(config-if)#ip split-horizon eigrp>
To verify:
<Router#show ip eigrp interface detail>

 LOAD BALANCING
Equal load balancing:
<Router(config-router)#maximum-paths (1-32)>
Unequal load balancing:
<Router(config-router)#variance (1-128)>
To verify:
Load balancing is the ability to forward traffic over all its network
ports that are the same metric from the destination address.
When a packet is process-switched, equal load balancing occurs on
a per-packet basis. When packets are packet-switched, load
balancing occurs on a per-destination basis.
 maximum-paths - installs routes with a metric equal to the
minimum metric in the routing table (the default is 4; set to
1 to disable load balancing)
 variance - a multiplier that is applied to a successor’s metric -
any path with a metric that fits within the range can be
unequal balanced over (default is 1 meaning only equal load
balancing is enabled)
The command affects which routes end up in the routing
table but does not affect the routes’ roles i.e. successor,
feasible successor etc.
 STUB ROUTING
<Router(config-router)#eigrp stub (connected) (static) (summary)>
<Router(config-router)#eigrp stub (receive-only)>
To verify local settings:
<Router#show ip protocols
To verify neighbor’s settings:
<Router#show ip eigrp neighbors detail>
 eigrp stub - configures the spoke router as a stub and
prevents the hub router from sending queries under any
condition (the stub sends a notification packet to all its
neighbours to report its status as a stub and all queries
issued by stubs are answered by the hubs on their behalf)
 receive-only - prevents the stub from sending any type of
routes
 connected - permits stub to send connected routes (if the
network command does not include a given network it needs
to be redistributed using the redistribute connected
command)
 static - permits stub to send static routes (static routes must
be redistributed!)
 summary - permits stub to send summary routes
The stub router still receives all routing updates from its
neighbours. Only the outgoing packets undergo control.

 MANUAL
SUMMARIZATION
<Router(config-if)#ip summary-address eigrp (AS) A.A.A.A M.M.M.M>  the AS specifies that summarization will only be sent out to
neighbors in within that AS
 while summarizing it has to be remembered that routes will
always prefer more specific routes
 the summary route will use a metric equal to the lowest
metric of a subordinate route
 advertising a summary will take down and bring up all
neighbor relationships established via that interface
 summarization should be avoided if the priority is for the
routes to always take the shortest paths
The following will be generated on the local end:
*Oct 18 21:03:05.482: %DUAL-5-NBRCHANGE: EIGRP-IPv4 1:
Neighbor 172.16.124.1 (Serial1/0) is resync: summary configured
The following will be generated on the far end:
*Oct 18 21:03:15.810: %DUAL-5-NBRCHANGE: EIGRP-IPv4 1:
Neighbor 172.16.124.2 (Serial1/0) is resync: peer graceful-restart

AUTHENTICATION
 DEFINE KEYS
<Router(config)#key chain (key chain name)>
<Router(config-keychain)#key (key id; 0-2147483647)>
<Router(config-keychain-key)#key string (authentication string)>
*<Router(config-keychaing-key)#accept-lifetime (hh:mm:ss) (1-31) (month) (year) infinite |
duration (1-2147483646) | (hh:mm:ss) (1-31) (month) (year)
*<Router(config-keychaing-key)#send-lifetime (hh:mm:ss) (1-31) (month) (year) infinite |
duration (1-2147483646) | (hh:mm:ss) (1-31) (month) (year)
To verify:
<Router#show key chain>
<Router#debug eigrp packet>
 key chain - enters the sub-configuration mode for key chain
 key - enters the sub-configuration mode for a given key
 key string - defines the password for a given key (by default
the key is stored in plain form in the running configuration -
unless the service password-encryption is enabled)
NOTE: the key numbers and key strings have to match on
both ends - otherwise authentication will fail!
 accept-lifetime - specifies the period of time during which the
key is accepted; the key can be accepted always (infinite),
within a given time limit in sec. (duration) or during given
period (default start time and the earliest acceptable date is 1
Jan 1993)
 send-lifetime - same as above but with regards to the time a
key can be used for encryption
 ip authentication mode - enables md5 authentication for
EIGRP packets sent on that interface (entering this command
only on one side will tear the relationship down -
authentication mode changed)
 ip authentication key-chain - specifies the keys to be used for
EIGRP packets encryption (entering this command only on
one side will NOT tear the relationship down)
The authentication feature does encrypts packets but rather
prevents the router from accepting non-authenticated EIGRP
packets and therefore from forming relationship with non-
authenticated routers.
Sending EIGRP messages: use the lowest key number among all
currently valid keys.
Receiving EIGRP messages: check the MD5 digest using ALL
currently valid keys.
 ACTIVATE
AUTHENTICATION
<Router(config)#interface (interface)>
<Router(config-if)#ip authentication mode eigrp (AS) md5>
<Router(config-if)#ip authentication key-chain eigrp (AS) (key chain name)>
To verify:
<Router#debug eigrp packet>

REDISTRIBUTION
To set a default-metric (NOTE: this command does not affect the metric of directly connected networks)
<Router(config-router)#default-metric (bandwidth kb; 1-4294967295) (delay 10-microsec; 0-255) (reliability; 0-255) (load; 0-255) (MTU; 1-65535)>
ROUTING PROTOCOLS
PULL ROUTES FROM: COMMANDS COMMENTS
 RIP
<Router(config)#router eigrp (AS)>
<Router(config-router)#redistribute rip (*metric (bandwidth) (delay) (reliability) ( load) (MTU) (*route-map
(route map name))>
 default-metric - overridden by the
redistribute metric command
 metric - redistribute router with the specified
metric (by default it is set to infinite
(unreachable) for all redistributed protocols
except for EIGRP with different AS - in such
case the it takes the metric from the source
of the routing information)
 match internal - redistribute the OSPF
internal routes
 match external - redistribute OSPF external
Type 1/2 routes
 match nssa-external - redistribute OSPF NSSA
external routes
 route-map - applies a route map to
redistributed routes
EIGRP was designed to automatically redistribute
IGRP route from the same AS.
Good practice to make redistributed routes appear
as links e.g. 100Mb:
#default-metric 100000 10 255 1 1500
 OSPF
<Router(config-router)#redistribute ospf (process ID 1-65535) (*match (external (1-2)) (*internal) (*nssa-
external) (*metric (bandwidth) (delay) (reliability) ( load) (MTU) (*route-map (route map name))>
Example:
 BGP
<Router(config-router)#redistribute bgp (AS #) (*metric (bandwidth) (delay) (reliability) (load) (MTU) (*route-
map (route map name))>

DIRECTLY CONNECTED NETWORKS
COMMANDS COMMENTS
<Router(config-router)#redistribute connected (*metric (bandwidth) (delay) (reliability) (load) (MTU) (*route-map (route map name))>
STATIC ROUTES
COMMANDS COMMENTS
<Router(config-router)#redistribute static (*metric (bandwidth) (delay) (reliability) (load) (MTU) (*route-map (route map name))>

EIGRP VERIFICATION AND TSHOOTING
 show ip eigrp neighbors
 show ip eigrp topology (all-links)
 show ip eigrp interface
 show ip eigrp interface detail
 show ip eigrp traffic
 show ip route eigrp
 debug ip eigrp neighbors
 debug ip eigrp packet
 clear ip eigrp neighbors
COMMAND VERIFIES / DISPLAYS EXAMPLE
 show ip eigrp neighbors
 EIGRP neighbors for a given process
 neighbors’ IP addresses
 the local interface the neighbors are reachable through
 HOLD timers
 how long the adjacency have been active
 show ip eigrp neighbors detail
 detailed information about neighbors

 show ip eigrp topology
 EIGRP router-id
 successors, feasible distances, feasible successors, advertised distances
 networks’ states
 show ip eigrp interfaces
 interfaces participating in a given EIGRP process
 number of peers on a given interfaces
 does not display information about passive-interfaces
 show ip eigrp interface detail
 detailed information about interfaces enabled for EIGRP
 does not include passive interfaces

 show ip eigrp traffic
Displays EIGRP traffic statistics
 show ip route eigrp
Displays routing table’s entry learnt via EIGRP

Displays IP routing protocol process parameters and statistics
 debug ip eigrp neighbors Displays events associated with EIGRP neighbors
 debug ip eigrp packet Displays events associated with EIGRP packets
 clear ip eigrp neighbors Purges EIGRP neighbor table

OSPF
• OSPF Basics
• OSPF Routers
• OSPF Packets
• OSPF Tables
• OSPF Metric
• OSPF Areas
• OSPF Virtual Links
• OSPF Timers
• OSPF Routers ID
• OSPF Link ID
• OSPF DR / BDR
• OSPF Adjacencies States
• OSPF Networks
• OSPF Over NMBA
• OSPF Configurations
• OSPF Verification and Tshooting

ADVANCED ROUTING ver. 1.0 CREATED BY PAWEL ‘PAUL’ NADSTOGA (PNADSTOGA@GMAIL.COM) 2012-14 28
OSPF BASICS
TYPE ALGORITHM AD STANDARD PROTOCOLS TRANSPORT AUTHENTICATION DROHTERS DR/BDR TIMERS
Link State Dijkstra 110
 RFC 2328
 RFC 2740
IP IP:89
 plain text
 MD5
224.0.0.5 224.0.0.6
 10/40
 30/120
The following conditions have to be met for two routers to form a neighbor relationship:
 Area ID match on both ends
 stub flag match (on/off)
 route-IDs are unique
 primary IP addresses of the routers must be on the same subnet
 hello timer match on both ends
 hold timer match on both ends
 authentication modes and passwords match (if authentication is configured)

OSPF ROUTERS
To view a router type:
ROUTER OVERVIEW COMMENTS
 INTERNAL  routers that have all their interfaces in the same area and have identical LSDBs
 BACKBONE
 routers that sit on the perimeter of the backbone area and have at least one interface connected to
Area 0
 maintain OSPF routing information using the same algorithms and rules as the internal routers
 ABR
 Area Border Router
 routers that have interfaces attached to multiple areas
 maintain separate LSDBs for each area they are connected to
 serve as exit points for the area (routing information destined to another area can get there only via
the ABR of the local area)
 to identify itself as an ABR, the router sends Type 1 LSA
with a border bit (b bit) set
 ABR containing a NSSA area will also become an ASBR
 CISCO recommends no more than 2 areas per ABR (in
addition to Area 0)
 ASBR
 Autonomous System Border Router
 routers that have at least one interface attached to an external internetwork (another AS) e.g. a
non-OSPF network
 capable of importing non-OSPF network information to the OSPF network and vice-versa (route
redistribution)
 to identify itself as an ASBR, the router sends Type 1
LSA with an external bit (e bit) set
 any form of redistribution enabled on a router will mark
it as an ASBR (it doesn’t even have to be working i.e.
redistributing RIP when it’s not activated)

OSPF PACKETS
 all OSPF packet types are encapsulated directly into an IP payload
 a protocol ID of 89 defines all OSPF packets
PACKET OVERVIEW COMMENTS
 HELLO
 sent to discover neighbors and form adjacencies with them Sent to:
 DROTHER - 224.0.0.5
 DR/BDR - 224.0.0.6
 DBD
 Database Description
 contains LSA headers only and describes the content of the entire link-state database
 each DBD has a sequence number which can be incremented only by the master (which in turn is
explicitly acknowledged by the slave)
Exchanged during EXTSTART + EXCHANGE adjacency
establishment phases.
 LSR
 Link-State Request
 requests specific link-state records from a router
 LSU
 Link-State Update
 sends specifically requested link-state records
 all LSUs are acknowledged
 LSAck
 Link-State Acknowledgement
 send to acknowledge the receipt of the other packets
 LSA
 Link-State Advertisement
 11 types
 all have 20-byte headers
 the LSA includes a link ID field that identifies (by network number and mask) the object that this link
connects to
 sequence number
 each router link is defined as an LSA type
Each LSA has their own age timer and waits 30 min before
requiring an update.
Sequence numbers – if the seq. in the update is:
 same as local – ignore the update
 higher than local – accept and propagate
 lower than local – ignore the update, send back local

LSAs
 TYPE 1: Router LSAs
 advertised by every router in the area
 flooded within its area only (does not cross ABR)
 includes list of directly attached links
 contains (O) intra-area routes
 each link is identified by IP prefix assigned
to link and link type
 TYPE 2: Network LSAs
 advertised by the DR
 generated for every transit broadcast and NBMA network within the area (intra-area)
 flooded to all routers within the transit network area (does not cross ABR)
 lists each of the attached routers that make up the transit network (including the DR itself +
subnet mask used on the link)
 contains (O) intra-area routes
 the link-state ID for a network LSA is the IP
address of the advertising DR interface
 TYPE 3: Summary LSA
 advertised by the ABR
 used to flood network information outside the originating area (inter-area)
 describes network number and subnet mask of the link
 flooded throughout a single area only but are regenerated by ABRs to flood into other areas
 contains (IA) intra-area routes
 it is advised to perform manual
summarization at the ABR (by default Type
3 LSA is advertised into the backbone area
for every subnet defined in the originating
area)
 TYPE 4: Summary LSA
 advertised by the ABR (but only when ASBR exist within an area)
 used to advertise an ASBR to all other routers in the AS (router ID and route to it)
 flooded throughout a single area only but are regenerated by ABRs to flood into other areas
 TYPE 5: External LSA
 advertised by the originating ASBR
 used to advertise networks from outside the OSPF AS
 flooded to the entire AS
 advertising router ID (ABSR) remains unchanged throughout the AS
 contains (E1/E2) external routes
 Type 4 LSA is needed to find the ASBR
 TYPE 6: Group Summary  NOT SUPPORTED BY CISCO ROUTERS
 TYPE 7: NSSA External Link LSA
 originated by the ASBR within NSSAs
 flooded only within the NSSA in which they originated
 contains (E1/E2) external routes
 converted into Type 5 LSA by the ABR when
leaving the area
 TYPE 9, 10, 11: Opaque  DESIGNED FOR FUTURE USE

OSPF TABLES
TABLE OVERVIEW COMMENTS
 NEIGHBOUR TABLE
 also known as adjacency database
 list of directly connected routers running OSPF with which adjacencies are
formed
<R1#show ip ospf neighbors ((type | number) (neighbor-id) detail))>
 type - interface type (FastEthernet, Serial etc.)
 number - interface number
 neighbor-id - neighbor’s router ID
 detail - displays all neighbors given in detail
 Neighbor ID - neighbor’s router ID
 Pri - priority of the neighbor’s interface on which adjacency is
formed
 State - adjacency state
 Dead Time - if the router doesn’t receive a HELLO packet from the
neighbor before the timer expires, the adjacency is considered dead
 Address - IP address of the neighbor’s interface on which adjacency
is formed
 Interface - local interface on which adjacency is formed

 TOPOLOGY TABLE
 typically referred to as LSDB (Link State Database)
 contains all routers and their attached links in the area or network
 all routers within an area have an identical LSDB
<R1#show ip ospf database>
 Link ID - name given to the entity on the link’s far end (see page 46)
 ADV Router - advertising router ID
 Age - the time that has passed since the last link update
 Seq# - link-state sequence number (detects old/duplicate LSAs)
 Checksum - fletcher checksum of the complete contents of the LSA
 Link count - number of interfaces detected for router

 ROUTING TABLE
 also known as forwarding database
 contains list of best paths to destinations
<Router#show ip route ospf>
 [110/65] - OSPF’s Administrative Distance (believability)
 O - OSPF intra-area route (from within the area)
 IA - OSPF inter-area route (from outside the area but from local AS)
 N1 - OSPF NSSA external type 1 route
 N2 - OSPF NSSA external type 2 route
 E1 - OSPF external type 1 route (from outside of local AS)
 E2 - OSPF external type 2 route (from outside of local AS)
For the same prefix/prefix length, OSPF always prefers routes in the
following order:
 O
 IA
 E1
 E2

OSPF METRIC
𝒓𝒆𝒇𝒆𝒓𝒆𝒏𝒄𝒆 𝒃𝒂𝒏𝒅𝒘𝒊𝒅𝒕𝒉
𝒍𝒊𝒏𝒌 𝒔𝒑𝒆𝒆𝒅 (𝒃𝒑𝒔)
 reference bandwidth (default) = 100 Mbps
 COST
 OSPF term for metric
 route’s metric is the sum of all costs along the path
 the lower the metric the more preferred the route is
To hardcode cost on an interface:
<Router(config-if)#ip ospf cost (1-65535)>
 ip ospf cost - the command hardcodes the cost and overrides
the value that normally would be calculated using the
formula
The COST is advertised in the LSAa that are advertised within an
OSPF area. When the COST is calculated to a destination then it’s
based on the exit interface of each router in the path to the
destination. Not consistent values along the path can lead to
asymmetric routing and the path one way may not be the same as
the return path.
 REFERENCE BANDWIDTH
 defaults to 100Mbps
To modify:
<Router(config-router)#auto-cost reference (bandwidth in Mbps)>
To verify:
<Router#show ip ospf interface (interface)>
 100Mbps = 100,000Kbps = 100,000,000bps
Cisco recommends keeping the value constant throughout the
entire OSPF AS to avoid sub-optimal routing decisions.
Interface Type Bandwidth COST
Loopback 8,000,000,000 1
Serial 56,000 1785
T1 1,544,000 64
Ethernet 10,000,000 10
Fast Ethernet 100,000,000 1
Gigabit Ethernet 1,000,000,000 1

OSPF AREAS
 an area is a logical collection of OSPF networks, routers, and links that share area ID
 a router within a given area maintains a topological database only for the area to which it belongs
 an router does not have detailed information about network topology beyond of the area it belongs to
 OSPF uses 2-layer hierarchy: transit and regular (the underlying physical connectivity must map to the two-layer area structure with all non-backbone areas directly attaching to Area 0)
 the purpose of dividing networks into sub-domains is to restrict the propagation of routes and reduce the amount of resources required by each router to maintain its link database
 recommended maximum number of routers in an OSPF area: 50
AREA OVERVIEW COMMENTS
 BACKBONE (AREA 0)
 a standard area that has been designated to as the central point to which all areas connect
 all traffic moving from one area to another area must traverse the backbone
 all characteristics of the STANDARD area apply also to AREA 0
 STANDARD
 contains LSA Types: 1/2, 3 , 4, 5
 contains route types: O, IA, E1/2
 STUBBY
 contains LSA Types: 1/2, 3
 contains route types: O, IA
 E1/2 external routes are not allowed
 a default route (Type 3 LSA) is injected by the ABR (0.0.0.0/0 via ABR)
To create:
<Router(config-router)#area (area ID) stub>
 for an area to become STUBBY, all routers belonging
to it must be configured to operate as such
 area cannot be converted to STUBBY if it contains a
virtual link
STUB routers and non-STUB routers will not form
adjacencies!

 TOTALLY STUBBY
 contains LSA Type: 1/2 and 3 (LSA Type 3 is only used to advertise 0.0.0.0/0)
 contains route types: O
 E1/2 external routes are not allowed
 a default route (via Type 3 LSA) is injected by the ABR (0.0.0.0/0 via ABR)
 because LSA Type 4 and 5 are not permitted, STUBBY and TOTALLY STUBBY areas cannot
contain ASBR
 only the ABR configuration needs to be modified to transform STUBBY to TOTALLY STUBBY area
To create (on ABR only):
<Router(config-router)#area (area ID) stub no-summary>
 STUBBY and TOTALLY STUBBY areas can be used to
reduce the resource utilization of routers in portion of
the network not requiring full routing knowledge
 area cannot be converted to TOTALLY STUBBY if it
contains a virtual link
 NOT SO STUBBY
 contains LSA Types: 1/2, 3, 7
 contains route types: O, IA, N1/2
 implements STUBBY or TOTALLY STUBBY functionality yet contains an ASBR
 allows LSA Type 7 (originated by ASBR) to advertise N1/2 external routes
 the ABR converts it into LSA Type 5 before flooding them to the rest of OSPF domain (if there
are multiple ABRs in an NSSA, the ABR with the highest router ID performs the translation)
 LSA Type 3 will pass into and out of the area
 ABR will not inject a default route into an NSSA unless explicitly configured to do so
To create NSSA (allows N1/2 external routes + allows IA inter-area routes):
<Router(config-router)#area (area ID) nssa)>
To create NSSA with stub functionality (allows N1/2 external routes + allows IA inter-area routes +
injects default route (Type 7 LSA with 0.0.0.0/0 via ABR):
<Router(config-router)#area (area ID) nssa default-information originate>
To create NSSA with totally stub functionality (allows N1/2 external routes + injects default route
(Type 3 LSA with 0.0.0.0/0 via ABR):
<Router(config-router)#area (area ID) nssa no-summary>
 *default-information originate - ensures that ABR
injects a default route into a STUBBY NSSA (by default
it doesn’t but does in TOTALLY STUBBY NSSA areas)
 area cannot be converted to a NSSA if it contains a
virtual link
 while all routers in the NSSA have to be configured as
such, additional functions (default-information, no-
summary) need to be only configured on the ABR

EXAMPLES: OSPF AREAS
 AREA 0 is the BACKBONE area
 AREA 23 is a STANDARD area
 R1 is a BACKBONE router
 R2 is an ABR
 R3 is an ASBR
<R1(config)#router ospf 1>
<R1(config-router)#network 10.1.1.1 0.0.0.0 area 0>
<R2(config-router)#network 10.1.12.2 0.0.0.0 area 0
<R3(config-router)#redistribute connected subnets>
(O) INTRA-AREA ROUTES (IA) INTER-AREA ROUTES (E1/2) EXTERNAL ROUTES (N1/2) NSSA EXTERNAL ROUTES DEFAULT ROUTE Type LSAs ACCEPTED
R1    n/a n/a 1/2, 3, 4, 5
R2: AREA 0    n/a n/a 1/2, 3, 4, 5
R2: AREA 23    n/a n/a 1/2, 3, 4, 5
R3    n/a n/a 1/2, 3, 4, 5

SCENARIO 1: STUBBY AREA
 AREA 23 is a STUBBY area
 E1/E2 external routes (Type 5 LSA) are not allowed
 ABR (R2) injects default route: Type 3 LSA with 0.0.0.0/0 via ABR into AREA 23
 (note: 172.20.200.1/24 redistributed by R3 is no longer advertised)
<R1(config-router)#router ospf 1>
<R2(config-router)#area 23 stub>
<R3(config-router)#area 23 stub>
R1    n/a n/a 1/2, 3, 4, 5
R2: AREA 0    n/a n/a 1/2, 3, 4, 5
R2: AREA 23   X n/a n/a 1/2, 3
R3   X n/a 0.0.0.0/0 via ABR 1/2, 3

SCENARIO 2: TOTALLY STUBBY AREA
 AREA 23 is a TOTALLY STUBBY area
 E1/E2 routes (Type 5 LSA) are not accepted from ASBR (R3)
 IA routes (Type 3 LSA) are not advertised by ABR (R2) into AREA 23
 ABR injects default route: Type 3 LSA with 0.0.0.0/0 via ABR into AREA 23
R1(config-router)#router ospf 1>
<R2(config-router)#area 23 stub no-summary>
R1    n/a n/a 1/2, 3, 4, 5
R2: AREA 0    n/a n/a 1/2, 3, 4, 5
R2: AREA 23  X X n/a n/a 1/2, *3 (only for default)
R3  X X n/a 0.0.0.0/0 via ABR 1/2, *3 (only for default)

SCENARIO 3: NOT SO STUBBY AREA
 AREA 23 is a NOT SO STUBBY area
 N1/N2 routes (Type 7 LSA) are accepted from ABSR (R3)
 IA routes (Type 3 LSA) are advertised by ABR (R2) to AREA 23
 the ABR coverts Type 7 LSA into Type 5 LSA before flooding them to the rest of the OSPF
domain (if there are multiple ABRs in an NSSA, the ABR with highest router-id performs the
translation)
<R2(config-router)#no area 23 stub no-summary>
<R2(config-router)#area 23 nssa>
<R3(config-router)#no area 23 stub>
<R3(config-router)#area 23 nssa>
R1    X n/a 1/2, 3, 4, 5
R2: AREA 0    X n/a 1/2, 3, 4, 5
R2: AREA 23   X  n/a 1/2, 3, 7
R3   X  n/a 1/2, 3, 7

SCENARIO 4: NOT SO STUBBY AREA (STUB FUNCTIONALITY)
 AREA 23 is a NOT SO STUBBY area with STUB functionality
 all characteristics of a NSSA plus:
o ABR (R2) injects default route: Type 7 LSA with 0.0.0.0/0 via ABR (R2)
<R2(config-router)#area 23 nssa default-information originate>
R1    X n/a 1/2, 3, 4, 5
R2: AREA 0    X n/a 1/2, 3, 4, 5
R2: AREA 23   X  n/a 1/2, 3, 7
R3   X  0.0.0.0/0 via ABR 1/2, 3, 7

SCENARIO 5: NOT SO STUBBY AREA (TOTALLY STUB FUNCTIONALITY)
 AREA 23 is a NOT SO STUBBY area with TOTALLY STUB functionality
 all characteristics of a NSSA plus:
o IA routes (Type 3 LSA) are not propagated by ABR (R2) into AREA 23
o ABR (R2) injects default route: Type 3 LSA with 0.0.0.0/0 via ABR
<R2(config-router)#area 23 nssa default-information originate no-summary>
R1    X n/a 1/2, 3, 4, 5
R2: AREA 0    X n/a 1/2, 3, 4, 5
R2: AREA 23   X  n/a 1/2,*3 (only for default),7
R3  X X  0.0.0.0/0 via ABR 1/2,*3 (only for default),7

OSPF VIRTUAL LINKS
 used when an area cannot be directly connected to the backbone
 act as a tunnel formed to join two areas across an intermediate area
 both end routers must share a common area
 at least one end must reside in Area 0
 HELLOs are sent every 10 sec. by default
 LSAs learnt through a virtual link have the DoNotAge (DNA) option set so that they do not age out (required to avoid excessive flooding over the virtual link)
 cannot traverse stub areas

VIRTUAL LINKS CONFIGURATION
COMMANDS COMMENTS
<Router(config-router)#area (transit area ID) virtual-link (router ID of the far end router) (*hello-interval (sec.)) (*dead-interval (sec.))>
To verify:
<Router#show ip ospf virtual-links>
 both ends of a virtual links need to be
configured
 hello-interval - specifies the time between
the HELLO packets that are sent on the
interface
 dead-interval - specifies the time that must
pass without HELLO packets being seen
before the neighbor declares the router
down
EXAMPLE:

OSPF TIMERS
TIMER OVERVIEW COMMENTS
 HELLO
 specifies the time interval at which the HELLO packets are retransmitted
To adjust:
<Router(config-if)#ip ospf hello-interval (1-65535 sec)>
To verify:
Matching timer value is a condition of forming an
adjacency.
 DEAD
 specifies the time interval during which a router will consider a neighbour alive without receiving a
HELLO from that neighbour
 by default equals to 4 x HELLO timer
To adjust:
<Router(config-if)#ip ospf dead-interval (1-65535 sec)>
<Router(config-if)#ip ospf dead-interval minimal hello-multiplier (3-20)>
To verify:
Matching timer value is a condition of forming an
adjacency
 ip ospf dead-interval minimal hello-multiplier -
sets the dead interval to 1 sec. with HELLOs sent at
the rate of multiplier per second

OSPF ROUTER ID
 router’s name in the OSPF process
 duplicate router-id’s will prevent two routers from becoming neighbors!
 determined in the following order:
1. ID hardcoded using the <(config-router)#router-id A.A.A.A> command
2. highest IP of an UP|UP local loopback interface
3. highest IP of an UP|UP physical local physical interface (doesn’t have to be OSPF enabled)
 if the router-id cannot be determined (no IP addresses assigned to interfaces) the OSPF process will not start (router-id = 0.0.0.0) and the following error will be generated:
 the ID doesn’t change unless:
o the router is rebooted
o the OSPF process is cleared e.g. with #clear ip ospf process
 flood war - an error message generated when a router in a different area has the same router ID as the one the message is displayed on and is advertising a network that the local router
isn’t advertising
OSPF LINK ID
 Link ID is a name given to the entity that is on the other end of the link
LINK TYPE DESCRIPTION LINK ID
1 Point-to-point Neighbor Router ID
2 Link to transit network Interface address of the DR
3 Link to stub area IP network number
4 Virtual link Neighbor Router ID
To view Link ID:
 show ip ospf database

OSPF DR / BDR
 on a Multipoint Broadcast networks routers form adjacencies with DR (Designated Routers) and BDR (Backup Designated Router)
 a router that is neither DR nor BDR is called DROTHER
 DROTHERs only form FULL adjacencies with DR and BDR
 DROTHERS form 2-WAY adjacencies with themselves
 adjacencies have synchronized LSDBs
 BDR does not perform any DR functions when the DR is operating
 BDR receives all information, but it is the DR that performs LSA forwarding and LSDB synchronization
 a router can have interface belonging to different networks behaving as both DR and BDR
 DROTHERS listen on 224.0.0.5
 DR & BDR listen on 224.0.0.6
 the DR/BDR improve network functionality by reducing routing update traffic
DR / BDR ELECTION PROCESS COMMENTS
 routers view the OSPF priority value of the other routers during HELLO exchange
 the router with the highest priority becomes the DR
 the router with the second highest priority becomes the BDR
 router ID acts as a tie breaker
 the only time DR/BDR change is when one of them is out of service
 adding routers with higher priority than current BD/BDR does not preempt current selection
 BDR uses the wait timer to determine whether the DR is out of service (if the DR is not confirmed to be forwarding LSAs
before the timer expires it is consider down)
 should the DR fail the BDR becomes the new DR and new BDR is elected
To modify interface priority:
<Router(config-if)#ip ospf priority (0-255)>
To view interface priority:
<Router#show ip ospf neighbor>
<Router#show ip ospf neighbor detail>
To view current DB/BDB:
<Router#show ip ospf neighbor> <-- displays states per neighbor
<Router#show ip ospf neighbor detail> <-- displays states for the whole segment
 priority range: 0 - 255 (if 0 the router never becomes
the DR/BDR)
 default priority = 1
Sample output of show ip ospf neighbor detail:
To influence the DB/BDB election:
 boot up DR first followed by DBD and DROTHERS
 shut and un-shut interface in the above order
 use clear ip ospf process command

OSPF ADJACENCIES STATES
 when an adjacencies are formed the routers go through several state changes before they become fully adjacent
STATE OVERVIEW COMMENTS
 DOWN
HELLO packets have been sent but none have been received Events that can cause this state:
 starting an OSPF process on a router
 RouterDeadInterval he expiration
 KillNbr
 InactivityTimer
 LLDown
 ATTEMPT
The router sends unicast HELLO packets every poll interval to the neighbor from which HELLO packets have
not been received within the DEAD interval
This state is only valid for manually configured neighbors
in an NBMA environment.
 INIT
 the router has received HELLO packet from its neighbor, but the receiving router’s ID was not included in
the incoming HELLO packet
 one-way HELLO
When a router receives a HELLO from a neighbor, it
should be able to find own router-id in the content
which acknowledges that the packet came as a reply to
locally generated HELLO.
 2-WAY
 a bi-directional communication has been established between two routers (each router has seen the
other router’s HELLO packet)
 at this stage it is decided whether two routers should become neighbors (based on whether the required
conditions have been met)
 on broadcast and non-broadcast multi-access networks DROTHERS form only 2-WAY relationship with
each other and FULL relationship with DR/BDR
At the end of this stage DR/BDR election occurs for
broadcast and non-broadcast multi-access networks.
 EXSTART
 routers and their DR/BDR establish a master/slave relationship and choose the initial sequence number
for adjacency formation
 the router with the highest router ID becomes the master and starts the exchange (it also is the only
router that can increment the sequence number)
 master/slave election takes place on a per-neighbor basis
 EXCHANGE
 routers exchange DBD (Database Description) packets in this state
 each DBD packet has a sequence number which can be only incremented by master (slave explicitly
acknowledges it)

 LOADING
 the actual exchange of link-state information occurs
 based on the information received in DBD routers send link-state request packets (which are provided in
LSUs)
 FULL
 routers are fully synchronized with each other (all the router and network LSAs are exchanged and the
routers’ databases are fully synced)
 ready to run SPF (Shortest Path First) algorithm and individually figure out the best routes to networks
from their own perspective
Considered a normal state for an OSPF router (if routers
are stuck in other states it may indicate problems with
forming adjacencies - with the exception of 2-WAY state
which is a desired state between DROTHERS).

OSPF NETWORKS
NETWORK OVERVIEW COMMENTS
 MULTI-ACCESS BROADCAST
 a multi-access broadcast network e.g. Ethernet
 DB/DBD election for each segment
 1 x mode of operation
The DR/BDR concept is at the link level i.e. router can have different interface
belonging to different areas acting as DR, BDR or DROTHER
 POINT-TO-POINT
 a network that joins a single pair of routers e.g. PPP, HDLC
 mode auto-detected by OSPF
 OSPF packets are sent using multicast 224.0.0.5
 no DB/DBD election
 default timers: 10 HELLO / 40 DEAD
 1 x mode of operation
 may also be a sub-interface running FR or ATM
 the IP source address of a packet is set to the address of the outgoing
interface
 NON-BROADCAST MULTI-
ACCESS
 a network that interconnects more than two routers but has no
broadcast capabilities e.g. FR, X.25
 5 x modes of operation
 LOOPBACK
 the default OSPF network type for a loopback interface, causing the
OSPF to advertise host routers instead of actual network masks
 the LOOPBACK network type is a CISCO proprietary extension that is not
configurable but present on a loopback interface by default
 ip ospf network point-to-point - on an loopback interfaces ensures that
the whole subnet is advertised (the interface is treated as a stub host)
 VIRTUAL LINK  act as a tunnel formed to join two areas across an intermediate area

OSPF OVER NBMA
MODE OVERVIEW COMMENTS
 BROADCAST
 CISCO proprietary
 HELLO / DEAD = 10/40 sec.
 DR/BDR elected
 single subnet
 neighbors are automatically discovered
 acts like a LAN environment
 preferred topology: full mesh
CONFIGURATIONS:
 Setting network type:
<Router(config-if)#ip ospf network broadcast>
 Enabling pseudo broadcast over Frame Relay cloud:
<Router(config-if)#frame-relay map ip A.A.A.A (DLCI) broadcast>
 Cisco proprietary in a sense that CISCO defined the style over
NBMA
 works best with fully meshed topologies (implementing this
mode in hub and spoke + partial mesh topologies will result in
connectivity issues)
 broadcast - enables pseudo broadcast (sends broadcast style
unicast packet)

 NON-BROADCAST
 RFC compliant mode
 DR/BDR elected
 single subnet
 neighbors are statically configured
 acts like a LAN environment with broadcast disabled
 preferred topology: full mesh
 default OSPF mode for all NBMA networks
CONFIGURATIONS:
<Router(config-if)#ip ospf network non-broadcast>
 Adding neighbors:
<Router(config-router)#neighbor (A.A.A.A) (*priority (0-255)) (*cost (1-65535))>
In hub and spoke topology, DR must be manually hardcoded on the
hub so that the spokes can form full adjacencies with it
Also, the spokes should never become BDR because they have no full
connectivity with the rest of the networks
In full mesh it’s acceptable for the DR/BDR election to automatically
elect DR/BDR
 neighbor A.A.A.A - manually hardcodes the OSPF neighbor
 priority - hardcodes the priority of the neighbor (good practice to
configure priority value on both ends to avoid errors)
 cost - hardcodes the cost to reach the neighbor
In hub and spoke topologies the neighbors only need to be hardcoded
on the hub (reason: it is the hub that initiates the HELLO exchange
process; the spokes only respond to it) - however it is still a good
practice to hardcode all neighbors
 POINT-TO-MULTIPOINT
BROADCAST
 RFC compliant mode
 DR/BDR not elected
 single subnet
 neighbors are automatically formed
 treats the cloud like a series of point-to-point links
 preferred topology: partial | star
CONFIGURATIONS:
<Router(config-if)#ip ospf network point-to-multipoint>
 in this mode OSPF advertises host routes (not networks)
 can be mixed with point-to-point mode on the far ends as long as
timers are adjusted

 POINT-TO-MULTIPOINT
NONBROADCAST
 CISCO extension
 single subnet
 used when multi / broadcasts are not allowed on the virtual circuits
CONFIGURATIONS:
<Router(config-if)#ip ospf network point-to-multipoint non-broadcast>
 acts like point-to-multipoint mode with broadcast disabled
 can be mixed with point-to-point mode on the far ends as
long as timers are adjusted
 POINT-TO-POINT
 CISCO proprietary
 one subnet for each point-to-point link
CONFIGURATIONS:
<Router(config-if)#ip ospf network point-to-point>

EXAMPLES: OSPF OVER NBMA CONFIGURATION
CONSIDERATIONS:
 choosing appropriate OSPF mode over NBMA will depend on particular circumstances such as:
o support for broadcasts / multicasts
o topology used (fully meshed, partially meshed, hub-and-spoke (star))
o IP addresses availability
 DR/BDR have to have full connectivity with the rest of the nodes (unless the network is fully meshed this process cannot be automatic)
 for automatic neighbor discovery use broadcast parameter with FR mapping
 for static neighbor hardcoding use neighbor command under OSPR process sub-configuration mode
 there is no one right way to configure OSPF over NBMA - technically each mode can be configured over every topology
 the aim is to achieve fully network connectivity over the cloud as efficiently as possible - if a mode is working over a suboptimal topology then tuning is essential

SCENARIO 1: BROADCAST MODE
MAIN CHARACTERISTICS:
 HELLO/DEAD = 10/40 sec.
 DR/BDR elected
 single subnet
 neighbors are automatically discovered
 broadcasts / multicasts are allowed over the cloud

R1(config-if)#no shutdown
 frame-relay inverse arp - maps a known L2 address (DLCI) to
an unknown L3 address (IP)
 arp frame-relay - allows the router to answer to remote
router’s ARP query
Since broadcast capabilities have to be enabled while statically
adding FR mapping automatic FR neighbor discovery should be
disabled.
R2(config-if)#frame-relay map ip 10.1.123.3 201
To confirm FR mappings:
 broadcast - enables pseudo broadcast (forwards broadcast
style unicast packets to the specified node)

 HARDCODE OSPF PRIORITIES ---> R1(config)#interface s1/0
R1(config)#ip ospf priority 255
On fully meshed networks it is fine to let the routers automatically
elect the DR/BDR.
Because the network is not fully meshed, letting the DR/BDR be
automatically elected can lead to connectivity issues - the
DR/BDR/DROTHER assignment will take place on a link basis
(R1 <--> R2, R1 <--> R4) and not a segment basis.
Both DR/BDR rely on full connectivity with all the nodes on the
segment to work properly. Since only the hub (R1) meets this
requirement it has be hardcoded as DR. R1 and R4 only have direct
connection to R1 and not to each other. Therefore neither can
become BDR and none will be elected. Both routers need to be
hardcoded as DROTHERS.
 HARDCODE OSPF MODES R1(config-if)#ip ospf network broadcast
R2(config-if)#ip ospf network broadcast
R4(config-if)#ip ospf network broadcast
To confirm OSPF mode:
Router#show ip ospf interface
 ENABLE OSPF ---> R1(config)#router ospf 1
R1(config-router)#router-id 1.1.1.1
R1(config-router)#network 10.1.123.1 0.0.0.0 area 0
R1(config)#router ospf 1
At this stage the adjacencies will be formed and OSPF will be
operational.

SCENARIO 2: NON-BROADCAST MODE
 DR/BDR elected
 single subnet
 broadcasts / multicasts are not allowed over the cloud

Since broadcasts are not allowed over the FR cloud, building the FR
map should rely on static entries with the dynamic mapping
disabled.
 STATICALLY ADD FR MAPS ---> R1(config-if)#frame-relay map ip 10.1.123.2 102
It is necessary to add maps in a way that both spokes can reach
other - otherwise the spoke won’t be able to reach networks
advertised by the other spoke.
Since the topology is not fully meshed the route to remote spoke
should be mapped through the hub.
DR/BDR election will occur during establishing adjacencies.
DR/BDR/DROTHER assignment will take place on a link basis
(R1 <--> R2, R1 <--> R4) and not a segment basis.

 HARDCODE OSPF MODES ---> R1(config-if)#ip ospf network non-broadcast
R2(config-if)#ip ospf network non-broadcast
R4(config-if)#ip ospf network non-broadcast
At this stage the adjacencies will not form since OSPF is working in
non-broadcast mode and will not multicast HELLOs.
Neighbors need to be statically configured under the OSPF
process.
 MANUALLY ADD OSPF NEIGHBORS ---> R1(config)#router ospf 1
R1(config-router)#neighbor 10.1.123.2 priority 0
Technically, the neighbors only need to be hardcoded on the hub -
it is the hub that initiates the HELLO exchange process; the spokes
only respond to it - however it is still a good practice to hardcode
all neighbors.
Same case with priority - it’s already configured on each router on
the FR interface but it’s a good practice to hardcode it again under
the neighbor statement.
No need for the spokes to become neighbors since all the traffic
has to go through the hub anyways.
The neighbor command causes the HELLOs to be unicasted instead
of multicasted.

SCENARIO 3: POINT-TO-MULTIPOINT BROADCAST
 single subnet

Since broadcast capabilities have to be enabled while statically
adding FR mapping automatic FR neighbor discovery should be
disabled.
 HARDCODE OSPF MODES ---> R1(config-if)#ip ospf network point-to-multipoint
R2(config-if)#ip ospf network point-to-multipoint
R4(config-if)#ip ospf network point-to-multipoint

At this stage the adjacencies will be formed and OSPF will be
operational.
(*note: take notice of how host routes are advertised not the
whole networks)

SCENARIO 4: POINT-TO-MULTIPOINT NON-BROADCAST
 single subnet

Since broadcasts are not allowed over the FR cloud, building the FR
map should rely on static entries with the dynamic mapping
disabled.
 STATICALLY ADD FR MAPS ---> R1(config-if)#frame-relay map ip 10.1.123.2 102
It is necessary to add maps in a way that both spokes can reach
other - otherwise the spoke won’t be able to reach networks
advertised by the other spoke.
Since the topology is not fully meshed the route to remote spoke
should be mapped through the hub.
DR/BDR election will occur during establishing adjacencies.
DR/BDR/DROTHER assignment will take place on a link basis (R1
<--> R2, R1 <--> R4) and not a segment basis.

 HARDCODE OSPF MODES ---> R1(config-if)#ip ospf network point-to-multipoint non-broadcast
R2(config-if)#ip ospf network point-to-multipoint non-broadcast
R4(config-if)#ip ospf network point-to-multipoint non-broadcast
At this stage the adjacencies will not form since OSPF is working in
non-broadcast mode and will not multicast HELLOs.
Neighbors need to be statically configured under the OSPF
process.
 MANUALLY ADD OSPF NEIGHBORS ---> R1(config)#router ospf 1
Technically, the neighbors only need to be hardcoded on the hub -
it is the hub that initiates the HELLO exchange process; the spokes
only respond to it - however it is still a good practice to hardcode
all neighbors.
Same case with priority - it’s already configured on each router on
the FR interface but it’s a good practice to hardcode it again under
the neighbor statement.
No need for the spokes to become neighbors since all the traffic
has to go through the hub anyways.
The neighbor command causes the HELLOs to be unicasted instead
of multicasted.

SCENARIO 5: POINT-TO-POINT
 one subnet for each point-to-point link

R1(config-if)#interface s0/0.102 point-to-point
R1(config-if)#frame-relay interface-dlci 102
R1(config-if)#interface s1/0.103 point-to-point
R1(config-if)#ip add 10.1.1.5 255.255.255.252
R2(config-if)#interface s1/0.201
R4(config-if)#interface s1/0.301
Auto discovery can be left on - since there is only one node at each
end there is no risk of mapping to undesired / unknown networks.
When configuring FR sub-interfaces, the FR encapsulation and FR
parameters (LMI type etc.) only need to be configured on the main
interface.
Only the main interface needs to be turned on (no shutdown).
No need to hardcode OSPF mode on the interfaces - the point-to-
point mode is default for point-to-point interfaces.
At this state the adjacencies will be formed and OSPF will be
operational.

OSPF CONFIGURATIONS
ACTIVATION
 START OSPF PROCESS
<Router(config)#router ospf (process ID; 1-65535)>  process ID - a locally significant number that
does not affect the OSPF operation
 HARDCODE ROUTER ID <Router(config-router)#router-id A.A.A.A>
 ADD INTERFACES TO OSPF
PROCESS
<Router(config-router)#network (A.A.A.A) (M.M.M.M | W.W.W.W) area (area ID; 0-4294967295)>
Alternatively:
<Router(config-if)#ip ospf (process ID) area (OSPF area)>
To add every interface:
<Router(config-router)#network 0.0.0.0 255.255.255.255 area (area ID)>
To manually add neighbor:
<Router(config-router)#neighbor A.A.A.A>
 network - specifies what interfaces to add to
the OSPF process (added interface will send /
receive HELLO packets and advertise the
networks to which they belong)
The wildcard mask is used for matching prefix only.
The prefix-length is not matched.
A network command with the most specific
wildcard is revised first.
If a statement ends with subnet mask it will be
converted into appropriate wildcard mask and
saved in the running config. in this format
 PASSIVE INTERFACES
<Router(config-router)#passive-interface (default | (interface))>
To verify:
 passive-interface - no HELLOs are sent on the
interface (hence no relationship can be
formed) but the network is still advertised
 passive-interface default - sets all interfaces
as passive
A passive interface is still part of the OSPF process
and the network advertised but no HELLOs are sent
to that interface.
 HARDCODE AREA TYPE
<Router(config-router)#area (area ID) (stub (no-summary)) (nssa (default-information originate)
(no-summary))>

 VIRTUAL LINK
<Router(config-router)#area (transit area ID) virtual-link (router ID of the far end router) (*hello-
interval (seconds)) (*dead-interval (seconds))>
 hello-interval - specifies the HELLO timer
 dead-interval - specifies the time that must
pass without HELLO packets being seen
before the neighbor declares the router
down
 PROPAGATE DEFAULT
GATEWAY
<Router(config)#ip route 0.0.0.0 0.0.0.0 (exit interface)>
<Router(config-router)#default-information originate (*always) (*metric (0-16777214)) (*metric-
type (1-2))
 default-information originate - distributes a
default route if it exists in the routing table
 always - always advertises a default route
even if it doesn’t exist in the routing table
 metric - propagate default route with
hardcoded OSPF metric
 metric-type - type of OSPF metric

TUNING
FEATURE COMMANDS COMMENTS
 ADJUST AD
Globally:
<Router(config-router)#distance ospf (external (AD 1-255)) (inter-area (AD 1-255)) (intra-area (AD,
1-255))>
Per routes:
<Router(config-router)#distance (AD, 1-255) (source IP Address) (*1-99 | 1300-1999 | ACL name)>
 ADJUST TIMERS
o HELLO <Router(config-if)#ip ospf hello-interval (1-65535 sec)> To verify:
o HOLD
<Router(config-if)#ip ospf dead-interval (1-65535 sec)>
<Router(config-if)#ip ospf dead-interval minimal hello-multiplier (3-20)>
 ADJUST RETRANSMIT
INTERVAL
<Router(config-if)#ip ospf retransmit-interval (1-65535 sec)> ip ospf retransmit-interval – controls the time
interval between advertisement retransmission if
the previous packet was not acknowledged
 ADJUST REFERENCE
BANDWIDTH
<Route(config-router)#auto-cost reference bandwidth (1-4294967)>
To verify:
 ADJUST I-FACE COST <Router(config-if)#ip ospf cost (1-65535)>
 ADJUST I-FACE PRIORITY <Router(config-if)#ip ospf priority (0-255)> Default = 1
 HARDCODE NETWORK TYPE
<Router(config-if)#ip ospf network (broadcast | non-broadcast | point-to-multipoint | point-to-
multipoint non-broadcast | point-to-point>
To verify:

AUTHENTICATION
TYPE COMMANDS COMMENTS
 PLAIN TEXT
<Router(config-if)#ip ospf authentication>
<Router(config-if)#ip ospf authentication-key (authentication key)>
 authentication Type 1 (default = 0, disabled)
 ip ospf authentication - enables plain text
authentication
 ip ospf authentication-key - OSPF password
 MD5
For the entire area:
<Router(config-router)#area (area ID) authentication message-digest>
<Router(config-if)#ip ospf message-digest-key (1-255) md5 (authentication key)>
For an interface:
<Router(config-if)#ip ospf authentication message-digest>
<Router(config-if)#ip ospf message-digest-key (1-255) md5 (authentication key)>
 authentication Type 2
 ip ospf authentication message-digest -
enables md5 authentication
 ip ospf message-digest-key (1-255) md5 -
MD5 OSPF password
Routers must use the same key ID to authenticate
each other.
The router uses the most recently added key for
authenticating sent packages.
SUMMARIZATION
 by default, the metric of the summary route is equal to the highest (worst) metric of the component subnet
TYPE COMMANDS COMMENTS
 INTERNAL ROUTES
<Router(config-router)#area (area ID) range A.A.A.A M.M.M.M> Configured on and performed by an ABR.
The ABR advertises only the summary route if at
least one subordinate subnets exists as an (IA)
inter-area route.
Also creates a summary route pointing toward
Null0 for the same range - (behavior known as
sending unknown traffic to bit bucket - if the router
advertising the summary route receives a packet
destined for something covered by the summary
route but not in the routing table, it drops it)
 EXTERNAL ROUTES <Router(config-router)#summary-address A.A.A.A M.M.M.M> Configured on and performed by an ASBR.

REDISTRIBUTION
ROUTING PROTOCOLS
To set a default-metric (NOTE: this command does not affect the metric of directly connected networks):
<Router(config-router)#default-metric (1-16777214)>
PULL ROUTES FROM: COMMANDS COMMENTS
 RIP
<Router(config)#router ospf (process ID)>
<Router(config-router)#redistribute rip (*metric (0-16777214)) (*metric-type (1-2)) (*nssa-only) (*route-map
(route map name)) (*subnets)>
 metric - redistribute router with the specified
metric (by default it is set to 20) (overridden
by a route-map if used)
 metric-type - External Type 1 (increment the
seed metric by adding the internal cost) or
Type 2 (do not increment metric)
 nssa-only - redistribute only NSSA external
routes
 route-map - apply a route map for filtering of
redistributed routes
 subnets - prevents automatic summarization
of the redistributed routes
Defaults:
 when redistributing BGP the metric = 1
 when redistributing another OSPF process
take the source route’s metric
 when redistributing all other sources, use a
default metric = 20
 creates a Type 5 LSA for each redistributed
route if not inside an NSSA area
 creates a Type 7 LSA for each redistributed
route if inside an NSSA area
 uses External Type 2 metric
 redistribute only classful networks (ignores
subnets)
 EIGRP
<Router(config)#router ospf (process ID)>
<Router(config-router)#redistribute eigrp (AS 1-65535) (*metric (0-16777214)) (*metric-type (1-2)) (*nssa-
only) (*route-map (route map name)) (*subnets)>
Example:
 BGP
<Router(config)#router router (process ID)>
<Router(config-router)#redistribute bgp (AS #) (*metric (0-16777214)) (*metric-type (1-2)) (*nssa-only)
(*route-map (route map name)) (*subnets)>

DIRECTLY CONNECTED NETWORKS
COMMANDS COMMENTS
<Router(config-router)#redistribute connected (*metric (0-16777214)) (*metric-type (1-2)) (*nssa-only) (*route-map (route map name)) (*subnets)>
STATIC ROUTES
COMMANDS COMMENTS
<Router(config-router)#redistribute static (*metric (0-16777214)) (*metric-type (1-2)) (*nssa-only) (*route-map (route map name)) (*subnets)>

OSPF VERIFICATION AND TSHOOTING
 show ip ospf neighbors
 show ip ospf neighbors detail
 show ip ospf interface
 show ip ospf interface brief
 show ip ospf
 show ip ospf border-routers
 show ip route ospf
 debug ip ospf adjacencies
 clear ip ospf process
COMMAND VERIFIES EXAMPLE
 show ip ospf neighbor
 neighbor ID
 neighbor priority
 adjacency state
 neighbor IP address
 local interface through which the neighbor is accessible
 show ip ospf neighbor detail
 detailed neighbor related information

 show ip ospf interface (*interface)
 local interface(s) that participate in OSPF processes
 show ip ospf interface brief
 local interface(s) that participate in OSPF processes
 the areas the interface belongs to
 interface IP address
 interface COST
 interface network type
 the number of neighbors

 show ip ospf
 OSPF processes
 router ID
 OSPF areas
 various LSAs in the OSPF database organized by area and type

 show ip ospf border-routers
 lists boundary routers information
 show ip route ospf
 network in the routing table learnt via OSPF processes
 router ID
 networks OSPF is routing for
 reference bandwidth
 administrative distance
 show ip ospf virtual-link
Information about virtual links created on the local router

 debug ip ospf adj
Debugs OSPF adjacency events
 clear ip ospf process Restarts OSPF processes

1. THE ROUTERS DETERMINE THEIR OWN ROUTER-IDs
router’s name in the OSPF process
Determined in the following order:
1 ID configured with <Router(config-router)#router-id A.A.A.A>
2 Highest IP of a Loopback interface
3 Highest IP of an active interface (doesn’t have to be OSPF enabled but has to be UP:UP)
* the router-id doesn’t change until:
the router is rebooted
the OSPF process is restarted <Router#clear ip opsf process>
2. ADD INTERFACES TO THE LINK STATE DATABASE
Use the network command:
Router(config)#router ospf 1
Router(config-router)#network A.A.A.A W.W.W.W
OR
Router(config-router)#network A.A.A.A M.M.M.M
HELLO packet contains:
router ID
* HELLO/DEAD timers
* network mask
* area ID
known neighbors
interface priority
DB/DBD IP address
* authentication password
* these need to match on both routers in order
to successfully form an adjacency
3. THE ROUTERS SEND HELLO PACKETS ON OSPF
ENABLED INTERFACES
*** DOWN STATE ***
the router sent a HELLO message
but haven’t received a reply yet
RouterA RouterB
RouterA initiates OSPF process:
RouterA(config)#router ospf 1
RouterB initiates OSPF process:
RouterA(config)#router ospf 1
HELLOs are multicasted on 224.0.0.5

4. THE ROUTERS RECEIVE HELLO PACKET
*** INIT STATE ***
Routers received a HELLO and sent
one back
Read the HELLO packet:
Check if all the conditions required to form a
relationship are met. If so:
add the router-id of the router that originated
the HELLO packet to the neighbor list
send a unicast HELLO packet back
If routers go from DOWN STATE INIT DOWN
it is most likely because some of the conditions
required to form a neighbour relationship are not
met
5. BIDIRECTIONAL COMMUNICATION HAS BEEN ESTABLISHED
*** 2 WAY STATE ***
Both routers check the HELLOs received
and check the neighbor field:
Router finds itself in the
neighbor field?
Not a neighbor yet:
Go to Step 6
Already a neighbor:
reset the DEAD timer
await another HELLO
YES
Routers have exchanged the HELLO
packets and are in bidirectional
communicatoin
Is this a broadcast
network?
NO
Elect DR/BDR
DROTHERS remain in 2WAY
relationship
BD/BDR form FULL
relationship with itself and
with DROTHERS (Go to Step
6)
YES
NO

250601416-Advanced-Routing-Reference-Manual-Ver-0-9

250601416-Advanced-Routing-Reference-Manual-Ver-0-9

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