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

EIGRP split horizon

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 ?

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: EIGRP split horizon

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

1 REPLY
Cisco Employee

Re: EIGRP split horizon

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

217
Views
0
Helpful
1
Replies
CreatePlease to create content