EIGRP on F/R ?

Unanswered Question
Nov 7th, 2008
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Hi,

Lab Scenario:


R2, R5 & R6 are connected in a Partial Mesh F/R topology with a F/R switch at the centre.


R2 is multi-point sub-int whereas R5 & R6 are using physical ints.


EIGRP is configured on each router advertising network 150.50.0.0 via 'network' command.


F/R map statements are configured on each such that R2 maps to R5 & R6 'broadcast'. R5 maps to R2 & R6 'broadcast' and R6 maps to R2 and R5 'broadcast'.


My Question: R2 has formed neighbors with both R5 & R6 and therefore R2 is the hub. How did EIGRP decide that R2 would become the hub ? I am comparing in my mind to the DR/BDR rules in OSPF.


Cheers,


Phil.

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Richard Burts Fri, 11/07/2008 - 11:07
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Phil


When you describe this as a partial mesh, am I correct in assuming that R2 has 2 PVCs (one to each remote router) and that R5 and R6 have a single PVC (to R2)? If so that is the reason why EIGRP is building the neighbor relationships this way. R5 receives a hello from R2, sends a hello and they become neighbors. Similarly R6 receives a hello from R2, sends a hello and they become neighbors. The hello sent from R5 gets to R2 but does not get to R6 so R5 and R6 do not become neighbors.


Perhaps what is a bit confusing to the issue is what it means when your frame relay map includes broadcast. This means that the router will send broadcasts (by making a copy for each PVC) but it does not mean that a router will forward broadcasts (or multicasts).


Perhaps another way of looking at it is that R5 can see directly only R2. R5 has access to R6 (via the frame relay map) but its access is indirect (using R2 as a relay). EIGRP only forms neighbor relationships on direct connections and not on indirect connections.


HTH


Rick

CriscoSystems Fri, 11/07/2008 - 15:05
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Ricky-baby


Hey is it true that a FR router with "broadcast" configured in its mappings won't forward a received broadcast packet to another FR neighbor, even if that neighbor is in the same subnet?


Richard Burts Sat, 11/08/2008 - 08:57
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Seth


Yes it is true that a FR router with "broadcast" configured in its mappings won't forward a received broadcast packet to another FR neighbor, even if that neighbor is in the same subnet. The broadcast parameter in the map is about how the router handles its own traffic. It does not enable forwarding of received broadcasts.


HTH


Rick

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