EIGRP routes

Unanswered Question
Aug 7th, 2007

Another thing that i read is this example:

imagine a diamond shape with 4 routers - each at each end + 1 router that goes through the middle ans is connected to only 2 of the router(the ones that at the end of the more wide angles).

so it says that a router A(found at the end of one side of the less wide angles of the diamond) chooses his routes(Successor and FS)to the lan behind router C which is at the other end.so eventually router A chose E as his FS(E is in the middle and is NOT CONNECTED to A) - but how can that be? he has no physical link to it, only through other routers.(if he wants to go through the middle router he has to go over 3 routers(all the other routers basically) and that is not worth the effort because the metric is high).

i know that RIP and IGRP calculate their routes based on adjacent routers but if you think about it - how can anything else be possible?

I hope you understand my question, it is very hard to explain.

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Pavel Bykov Tue, 08/07/2007 - 14:44

Are all connections Layer 2 or Layer 3?

Router A cannot choose an FS that is not it's neighbor. In Eigrp neighbors are established based on layer 3 connectivity, using IP protocol number 88 and multicasts (to

One reason I can think of is that you have a VLAN spanned from router A to router E.

Other from that, router A can only see it's neighbors and based on the Reported Distance and Feasible Distance choose a Feasible Successor. But it cannot be any other router that a neighboring one.

Check for neighbor relationships on A using "show ip eigrp neighbors"

milkdroogy Tue, 08/07/2007 - 15:18

as i said - this is a scenario from a book and i cannot check anything.

Second, theres no vlans involved.

the connections are layer 3 offcourse, cos we are talking about EIGRP which is routing protocol and it operates at layer 3 .

Pavel Bykov Tue, 08/07/2007 - 15:48

What book is it?

I think they have either accidentally or otherwise omitted a link from A to E, and E to C.

Errors are not uncommon in the book.

All I can say is that router that is not a neighbor cannot be FS.

Pavel Bykov Wed, 08/08/2007 - 03:31

Unfortunately I don't have that book. I'd ask you to scan the page, but i am not sure if that's legal. You can probably redraw the diagram, and retype the question, so we would know for sure that there is no catch in the question/scenario.

So far it seems like a good candidate to submit to the author as a question. Wendell Odom responded to my questions and comments to his QoS book. When you are sure that there is an error in the book, you should definitely write the author.


This Discussion