EIGRP route selection process

EIGRP follows three general steps to select best routes:

  1. Neighbor Discovery
  2. Topology Exchange
  3. Choosing Routes

 

  1. Neighbor Discovery:

Neighbor discovery is done by sending multicast message using multicast address 224.0.0.10 called Hello Messages. This hello message contains EIGRP parameters such as the K-value, Hold Time and AS numbers. These are checked by the router receiving the hello message before forming neighbor ship. The hello message are send out every 5 seconds by default on a high bandwidth and every 60 seconds on a low bandwidth links. Hello message sent by stub routers also have “stub” parameters like connection, summary, redistributions, receiving-only and static. Hello message are multicast by default but if neighbors are configured statically on a NBMA network such as Frame Relay they are unicast. After analyzing the Hello Message if the parameters are matching with the receiving router, then the neighbor ship is formed.

  1. Topology Exchange:

Once the neighbors are formed on the basis of Hello Messages, the adjacent routers start exchanging topology table reliably by using RTP (Reliable Transport Protocol). This table contains all the possible routes to other EIGRP subnets having same EIGRP AS number. The routes are either marking as FD (Feasible Distance) or FS (Feasible Successor) on the basis of EIGRP composite matric calculation. The command to see the content of topology table is Show ip eigrp topology

 

R1#sh ip eigrp topology

IP-EIGRP Topology Table for AS(100)/ID(192.168.70.1)

Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply, r – reply Status, s – sia Status

 

P 192.168.10.0/24, 1 successors, FD is 11151872

        via 172.1.34.2 (11151872/2297856), Serial0/0

        via 172.1.36.1 (20640000/128256), Serial0/1

P 192.168.1.0/24, 1 successors, FD is 10639872

        via 172.1.34.2 (10639872/128256), Serial0/0

        via 172.1.36.1 (21152000/2297856), Serial0/1

  1. Choosing Routes

After exchanging the topology tables the routers will analyze it and choose the lowest-matric route to reach each subnet. The lowest-matric routes are then kept in the routing table. If you analyze the above output the routes that goes in the routing table are marked with FD these are 11151872 and 10639872. The other routes with the matric 20640000 and 21152000 are called as Feasible successor and these are backup routes to the same destinations and will remain in the topology table until any things happens with the primary routes.  Should anything happens with the primary routes, these feasible successor will take the place and will be in the routing table. The command to see the content the of routing table is sh ip route

R1#sh ip route

Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP

       D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area

       N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2

       E1 – OSPF external type 1, E2 – OSPF external type 2

       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2

       ia – IS-IS inter area, * – candidate default, U – per-user static route

       o – ODR, P – periodic downloaded static route

 

Gateway of last resort is not set

 

D    192.168.30.0/24 [90/11151872] via 172.1.34.2, 00:46:24, Serial0/0

C    192.168.60.0/24 is directly connected, Loopback1

D    192.168.10.0/24 [90/11151872] via 172.1.34.2, 00:46:24, Serial0/0

D    192.168.20.0/24 [90/11151872] via 172.1.34.2, 00:46:24, Serial0/0

C    192.168.50.0/24 is directly connected, Loopback0

D    192.168.1.0/24 [90/10639872] via 172.1.34.2, 00:46:25, Serial0/0

D    192.168.2.0/24 [90/10639872] via 172.1.34.2, 00:46:25, Serial0/0

C    192.168.70.0/24 is directly connected, Loopback2

D    192.168.3.0/24 [90/10639872] via 172.1.34.2, 00:46:25, Serial0/0

R1#

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