Split-horizon is on by default on EIGRP ?

[sanjaynadarajah]

Hi,

I just want to confirm if split-horizon is turned on by default on for EIGRP ? This is what I got from the doc for IOS 12.4

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Defaults

The behavior of this command is enabled by default.

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The reason for this q is that I understand for Frame-Relay using Hub-and-spoke, the hub needs to have split-horizon turned off so that it can send data between both the spokes.

So therefore is it correct that I would need to turn off split-horizon 'no ip split-horizon eigrp as-number' for both the spokes to talk ?

Pls advice,

-sn-

[Jon Marshall]

Hi

Yes, split horizon is enabled by default in EIGRP.

Because split horizon blocks route information from being advertised out an interface from which the route was learned you may well need to disable it in a frame-relay setup where the hub is using subinterfaces to connect to the spokes.

HTH

Jon

[Richard Burts]

Jon

I believe that some clarification is in order:

- if the hub router is using point to point subinterfaces for each spoke router, then there is no need to turn off split horizon for EIGRP. The hub will communicate with each spoke just fine and will easily advertise to a spoke the routes from the other spoke.

- if the hub router is not using point to point subinterfaces (is either using the physical interface or is using a multipoint subinterface) then there is need to disable split horizon for EIGRP on the hub router. This will allow it to advertise to a spoke the routes from the other spoke.

HTH

Rick

[Jon Marshall]

Rick

Thanks for the clarification. I should have explained the whole thing a lot more clearly instead of just saying you "may" have to disable split-horizon.

Jon

[flashsplash]

I want to clarify s'thing and thats split-horizon is only an issue if ur using an hub- and-spoke network with distance vector protocols which are Rip, Rip v2 and IGRP. Eigrp is an hybrid protocol so split-horizon shouldn't be an issue here.

And it's better to use subinterfaces instead of turning spilt-horizon off.

[Richard Burts]

Laurens

You are not correct about EIGRP and split horizon. It is true that EIGRP is sometimes described as a hybrid protocol, but that does not mean that split horizon is not an issue. I know from experience that split horizon is very much an issue in EIGRP.

I agree that it is best practice to use point to point subinterface for Frame Relay. But if you are in a situation where you are using a multipoint interface for hub and spoke and are using EIGRP then split horizon will be an issue to deal with.

HTH

Rick