EIGRP Feasibility Condition

Problem:

R1 has both a successor and a Feasible successor for R3 L3 (3.3.3.0/27) and R2 L2 (2.2.2.0/24)

But, when looking at the EIGRP Topology Table for R2 and R3, there isn't a Feasible successor for R3 L3 (3.3.3.0) nor for R2 L2 (2.2.2.0) - what's the deal?!

It's probably due to the feasibility condition

Network:

• IP Proto: EIGRP - network converged (all devices can ping each other)
• Frame-relay setup w/R2 and R3 as spokes
• R1 has split horizon disabled on s0/0

R1(config-if)#no ip split-horizon eigrp 100
R1(config-if)#

Lets look at the eigrp topo tables for all three devices but focus on the topology table 3.3.3.0/24 entry for R1 & R2:

R1

R2

R3

One would think there should be a second entry (feasible successor) for R2 since 3.3.3.3 can be reached from R2 via s0/0 or fa0/0; but only the fa0/0 is showing. Lets shut int fa0/0 on R2 and see what the table shows:

R2

Look at that; R2 queried its neighbor R1 who provided an alternate route. The table on R2 changed from:

P 3.3.3.0/24, 1 successors, FD is 409600
via 172.12.23.3 (409600/128256), FastEthernet0/0

To:

P 3.3.3.0/24, 1 successors, FD is 2809856
via 172.12.123.1 (2809856/2297856), Serial0/0

So, we know there's an alternate route but it's not being selected as a feasible successor.

From a cisco doc:
"A feasible successor is a path whose reported distance is less than the feasible distance (current best path)." *1 

So, the AD (reported distance) going through s0/0 is 2297856 which is more than 5X the FD going through fa0/0 409600, hence it's not going to show up as a feasible successor. Blah!

Yeah, this is probably CCNA stuff but still good to have as a blog for myself… to share with others :) And I needed to make a post - it's been soooooooo long since my last one.

Cheers!

*1 - Pasted from <http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/16406-eigrp-toc.html#feasibleandreported>

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