Why EIGRP 2nd route not in routing table here?

[jmaxwellUSAF]

Hello.

...

4431#sh ip eigrp topo

P 172.16.39.0/24, 1 successors, FD is 1541632

      Number of secondary paths: 0,

        via 192.168.30.1 (1664512/261632), Tunnel30

       192.168.102.135 via 192.168.102.1 (1920512/1285632), Tunnel20

--

4431# sh ip route

D EX     172.16.38.0/24 [170/665856] via 192.168.102.1, 01:08:06, Tunnel20

D EX     172.16.39.0/24 [170/1664512] via 192.168.30.22, 00:34:17, Tunnel30

D        172.16.40.0/27 [90/1664512] via 192.168.30.22, 00:34:18, Tunnel30

QUESTION: Because the EIGRP feasibility condition is satisfied with the secondary route through tu20, shouldn't the 2nd EIGRP route be present in the routing table?

Thank you.

[MHM Cisco World]

It be in eigrp topolgy not in RIB

And router use it as backup when primary failed then router immediately add it to RIB.

What you see correct.

[jmaxwellUSAF]

Why are not two routes in the routing table?

[MHM Cisco World]

It not equal metric so eigrp use equal cost multi path (ecmp).

[M02@rt37]

@jmaxwellUSAF 

In EIGRP, when there are multiple paths to the same destination with unequal costs, the router can install multiple routes in the routing table, each associated with its respective metric. This is known as unequal-cost load balancing.

However, this is different from the scenario you presented earlier with feasible successors and successors. Unequal-cost load balancing typically occurs when there are multiple paths to the same destination with different metrics, and all these paths meet the feasibility condition.

In your original output, you had one successor (best path) and one feasible successor (backup path) for the destination 172.16.39.0/24. The EIGRP feasibility condition was satisfied for the backup path, but only the successor was installed in the routing table.

If unequal-cost paths were present, EIGRP could install multiple routes in the routing table, each associated with its respective metric. However, for the feasibility condition to be applied, the metrics must be within a certain range (the reported distance must be less than the feasible distance of the successor). If this condition is not met, unequal-cost load balancing won't occur, and only the successor will be installed in the routing table.

[MHM Cisco World]

https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13677-19.html

Read this for more info

[Joseph W. Doherty]

BTW, in @MHM Cisco World's reference, it's suggested to enable per-packet load sharing.  Generally this is a bad thing to do.

[MHM Cisco World]

To be honest, because of these many confusing points of eigrp it not so common to use in network. 

Many prefer and I am one of them prefer using ospf. 

The only case that I see you must use eigrp is dmvpn other I dont prefer. 

That my opinion. 

MHM

[M02@rt37 · Accepted solution]

Hello @jmaxwellUSAF,

The EIGRP feasibility condition is a mechanism used by EIGRP to determine a loop-free backup path to a destination in case the primary path goes down. However, the feasibility condition alone does not automatically install the secondary route in the routing table. The EIGRP topology table may contain feasible successors, but the routing table only displays the best path.

If the primary path were to go down, the secondary path might become the primary path, provided it meets the feasibility condition. However, as long as the primary path is available and satisfies the feasibility condition, it will be the one installed in the routing table.

The EIGRP feasibility condition influences the selection of backup paths, but the actual installation of routes in the routing table is based on the best path, which may or may not be a feasible successor.

[David Ruess · Accepted solution]

Hello,

By default EIGRP only installs ECMP (Equal cost Multi-paths) in the routing table from EIGRP. If the routes meet the Feasibility condition but are un-equal you need to add the "Variance" command. By default Variance in EIGRP is 1. This number is multiplied against the current Feasible Distance of the Successor Route to determine what other paths to install. If you change the Variance to 2 you can see that it should install both routes into the routing table.

router eigrp <#>

variance 2

Hope this helps

-David