EIGRP No backup route

[Mokhalil82]

Hi

I am using the following topology to ask my question are on the same site with R1 on a different site and are spoke routers, whereas R1 is on a different site being the Hub

R1-------------------------R2

        |                      |

              |         |

                 R3

My question is, Why would R3 have a successor route to R1, but no feasible successors.

R2 has a route to R1 both successor (direct) and feasible successor (via R3). All routers have established a neighbours.

I have checked "Show ip eigrp topology all-links" on R3 and even there it shows just the 1 (direct) route to R1 but no secondary route via R2

Hope someone can help

Thanks

[samarjit dutta]

supoose R2's best route to R1 ( via R2- R1 link) has a metric m1.
and

R3's best route to R1 ( via R3- R1 link) has a metric m2.

now m1>m2

so if R2 advertise a route to R3 it will have a reported distance of m2 which is higher than the feasible distance for R1's network from R3.... Does not satisfying the feasibility condition.
Such route may create routing loop.
Check show ip eigrp topology all-link in R3. you can find the alternative route which is not a successor.

one line answer is the alternative route does not satisfying the feasibility condition so it is not selected as feasible successor

[Jon Marshall · Accepted solution]

If R2 sees R3 as path to R1 it cannot then advertise the same route back to R2.

This is the split horizon rule ie. you cannot advertise a route to a destination subnet out of a next hop interface if that next hop interface is the one you are using to get to the destination.

Jon

[Mokhalil82]

Thanks for the responses. Yes it turned out it was Split horizon, I disabled it and tested and now both routes were being seen.

I did already check to see if the the backup route met the feasible successor criteria by runnning the command "Show ip eigrp topology all-links" but the backup route was not even in there. Usually even if a route does not meet the criteria as a feasible successor, I would expect it to be in the topology all-links output.

Thanks