OSPF - Split Horizon on OSPF

[dave.dong0203]

Hi all,

   intra-area OSPF does not have split horizon concept, but If R1------R2  point-to-point connected, and R2 receive a new LSA from R1, R2 will not flood this new LSA back to R1, ican this be considered as a kind of split horizon?

Thansk,

Qixin

[Vasilii Mikhailovskii · Accepted solution]

Hello, Qixin.

Split horizon means: we received some route via interface, accepted it (put into RIB) and, then, announcement process is tweaked not to use that same interface. OSPF floods LSA not as a result of RIB update, but as a requirement for consistent LSDB.

So, LSA would be forwarded further even if you filtered the prefix out of RIB.

You need to remember that within single area LSDB should be consistent through all the participants.

So, you can't name it "split horizon".

PS: OSPF process floods LSDB every 30 minutes.

[JohnTylerPearce · Accepted solution]

In this scenario, when R2 receives this new LSU Update from R1, it will obviously examine the contents such as, does the new information in this LSA already exist, if it does, does this LSA informed R2 of a higher sequence number etc etc. It will however send an LSA ACK back to R1.

Think of it like Classic 802.1D spanning tree, where BPDUs flow downstream not upstream.

Feel free to lab it up, or use GNS3, and run 'debug ip ospf ?' and you should be able to see the above actually happen.

[dave.dong0203]

Thank you very much John.

[dave.dong0203]

Thanks for the clarification Mikhailovsky.