Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

Silver

EIGRP question

In CCO docs much is made of EIGRP's ability to provide "sub-second" convergence via feasable successors. Are feasable successors required at both ends of a route in order for this to work? For example, if router A can get to D through B or C, and C is a feasable successor in A's topology table, then C will immediately be used in A should B fail. My question is: Does D require that C be a feasable successor to A for this to work end to end? If D routes through B and does not have a feasable successor to A then will D not be required to query for a route to A regardless of the fact that A is ready to go?

4 REPLIES
Hall of Fame Super Silver

Re: EIGRP question

Mike

EIGRP does not require feasible successor at both ends. Each router makes its own determination of the feasibility test. A can determine that C does qualify as a feasible successor. And that has no impact - and no dependency on whether D considers C as a feasible successor or not.

It is possible that if B fails that A will use the feasible successor and immediately start to route through C while D recognizes the lost route and must initiate a query.

HTH

Rick

Silver

Re: EIGRP question

So if I ping D from A under these conditions, then no echo requests get dropped, but replies will have to wait until D converges a route back to A?

Re: EIGRP question

hi...

yes here D must have route A...to send packet back...

regards

Devang

Silver

Re: EIGRP question

So my orginal assertion stands? Bidirectional sub-second convergence requires that a both A and D have feasable successors? The exception obviously being that if D happens to use C as it's only route to begin with. Then there would be no issue. But if A and D both prefer B and only A has C as a feasable successor then the "sub-second" convergence is unidirectional and requires that D also have C as a feasable successor to provide bidirectional "sub-second" convergence.

124
Views
6
Helpful
4
Replies
CreatePlease to create content