EIGRP - Number of successors

[vlad.vlaicu]

Hi,

I would like to ask some questions related to the following output listed below.

1. Why does sh ip eigrp topology lists 1 successor for 4.4.4.0 network when there are actually 2 (even a feasable successor). Variance 2 is used.

2. When can I actually see in the output 2 successors?

Thanks,

Vlad

R1#sh ip eigrp top

IP-EIGRP Topology Table for AS(1)/ID(1.1.1.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

       r - reply Status, s - sia Status

P 1.1.1.0/24, 1 successors, FD is 256

        via Connected, Loopback0

P 4.4.4.0/24, 1 successors, FD is 1536

        via 10.12.0.2 (1536/512), FastEthernet0/0

        via 172.13.0.3 (1792/768), FastEthernet0/1

P 10.12.0.0/24, 1 successors, FD is 1024

        via Connected, FastEthernet0/0

P 10.24.0.0/24, 1 successors, FD is 1280

        via 10.12.0.2 (1280/256), FastEthernet0/0

        via 172.13.0.3 (1792/768), FastEthernet0/1

P 172.34.0.0/24, 1 successors, FD is 1536

        via 172.13.0.3 (1536/512), FastEthernet0/1

        via 10.12.0.2 (1792/768), FastEthernet0/0

P 172.13.0.0/24, 1 successors, FD is 1024

        via Connected, FastEthernet0/1

P 192.15.0.0/24, 1 successors, FD is 1024

        via Connected, FastEthernet1/0

P 192.54.0.0/24, 1 successors, FD is 1792

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

     1.0.0.0/24 is subnetted, 1 subnets

C       1.1.1.0 is directly connected, Loopback0

     4.0.0.0/24 is subnetted, 1 subnets

D      4.4.4.0 [90/1792] via 172.13.0.3, 00:00:52, FastEthernet0/1

                [90/1536] via 10.12.0.2, 00:00:52, FastEthernet0/0

     172.13.0.0/24 is subnetted, 1 subnets

C       172.13.0.0 is directly connected, FastEthernet0/1

     172.34.0.0/24 is subnetted, 1 subnets

D       172.34.0.0 [90/1536] via 172.13.0.3, 00:00:52, FastEthernet0/1

                   [90/1792] via 10.12.0.2, 00:00:52, FastEthernet0/0

     10.0.0.0/24 is subnetted, 2 subnets

C       10.12.0.0 is directly connected, FastEthernet0/0

D       10.24.0.0 [90/1792] via 172.13.0.3, 00:00:54, FastEthernet0/1

                  [90/1280] via 10.12.0.2, 00:00:54, FastEthernet0/0

C    192.15.0.0/24 is directly connected, FastEthernet1/0

D    192.54.0.0/24 [90/2560] via 192.15.0.5, 00:01:02, FastEthernet1/0

                   [90/3072] via 172.13.0.3, 00:01:02, FastEthernet0/1

                   [90/2816] via 10.12.0.2, 00:01:02, FastEthernet0/0

[skarthic · Accepted solution]

Hi Vlad,

AFAIK, EIGRP successors refer to the path having the least metric. Refer

http://en.wikipedia.org/wiki/Enhanced_Interior_Gateway_Routing_Protocol#Successor

So you will see more than 1 successor if you have 2 or more best cost paths to the prefix. Routing table chooses more than one route if variance conditions and the input is the topology table entries.

So remember this way.

- Changing the variance will not affect the topology table entries. Irrespective of how much you change the variance, the number of successors will be equal to number of equal cost best paths to a prefix as per the topology table

- Variance affects the routing table by choosing paths for routing table which pass the feasibility criteria and after applying the variance conditions on topology table entries.

[cadet alain]

Hi,

there are only one successor and one feasible successor but as the FD of the feasible successor* variance is <= FD of successor then EIGRP will install both paths in the RIB and CEF will take care of the unequal cost  load-sharing.

Regards

Alain

Don't forget to rate helpful posts.

[vlad.vlaicu]

Makes sense. Thank you!