EIGRP split horizon

Answered Question
May 10th, 2010
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Hi experts.


When i disbale split horizon any interface, it starts sending the primary network in its updates. Consider the following scenario


R1 is connected to R2 via ethernet link


R1 ethernet ip is 11.0.0.1/8

R2 ethernet ip is 11.0.0.2/8


When i disable split horizon, i can see R1 advertising 11.0.0.0/8 out the same interface !.


Why is this so.


My assumption is, since eigrp doesnt check for subnet mask when forming adjacency, so its quite possible that masks of both neighbors might be different, hence R1 for safe side advertises its original network of the link to R2 for topology sake.


Is my assumption correct ?

Correct Answer by Peter Paluch about 6 years 10 months ago

Jonn,


I don't see anything unusual in what you have observed. The network 11.0.0.0/8 on R1's ethernet interface is learned by that interface itself. Thus, it is subject to split horizon. If you deactivate the split horizon, it will be advertised along with other routes. The RIP implementation in IOS, as far as I know, behaves in the same way.


There is no subnet mask checking in EIGRP, to my best knowledge - this is contrary to OSPF Hello packets that also carry the netmask of the interface that sourced them.


Best regards,

Peter

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Correct Answer
Peter Paluch Mon, 05/10/2010 - 04:22
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  • Cisco Employee,

Jonn,


I don't see anything unusual in what you have observed. The network 11.0.0.0/8 on R1's ethernet interface is learned by that interface itself. Thus, it is subject to split horizon. If you deactivate the split horizon, it will be advertised along with other routes. The RIP implementation in IOS, as far as I know, behaves in the same way.


There is no subnet mask checking in EIGRP, to my best knowledge - this is contrary to OSPF Hello packets that also carry the netmask of the interface that sourced them.


Best regards,

Peter

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