BGP Split horizon vs IP Split horizon & NEXT_HOP attribute

[Pradeep H A]

Hi all,

I have got 2 doubts - one about Split horizon & second about the usage of BGP NEXT_HOP Attribute.

I know BGP uses BGP split horizon rule which says "Routes learned from one iBGP peer cannot be advertised to another iBGP peer"

But does BGP also follow the IP Split horizon rule that says "Routes learned from one iBGP peer cannot be advertised back to the same iBGP peer?"

Suppose 3 routers R1, R2 & R3 are connected like this:

Network-Y |----------(Fa0) R1 (S0) ---------iBGP---------(S0/0) R2 (Fa0/1)-------OSPF----------(S1) R3 (Fa0)------| Network-X

R2 & R3 are OSPF neighbors

R2 & R1 are iBGP neighbors

Suppose Net-X is redistributed at R2 to BGP & advertised to R1, what will be the advertised NEXT_HOP for Net-X?

What NEXT_HOP IP will R1 use to tell R2 about Network-Y?

will R2 send an update about Network-Y back to R1 through iBGP though it learned about it from R1 itself? If yes then what NEXT_HOP will R2 advertise to R1?

Hope my questions are clear

Thanks a lot in advance

[cadet alain]

Hi,

the BGP split-horizon rule is there to prevent the iBGP peer from receiving back the update from another iBGP peer because as in iBGP the AS_PATH is not modified then the loop prevention of discarding an update with its own AS number in the AS-PATH is not possible anymore as it is in eBGP.

Regards.

Alain.

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[ameya_oke · Accepted solution]

Hi Pradeep,

Q1:

But does BGP also follow the IP Split horizon rule that says "Routes learned from one iBGP peer cannot be advertised back to the same iBGP peer?"

-> Yes you are right but there is no rule in BGP called "IP Split Horizon", Its just that AS_Path attribut wont allow network to be advertised back to same IBGP Peer.

Q2:

Suppose Net-X is redistributed at R2 to BGP & advertised to R1, what will be the advertised NEXT_HOP for Net-X?

-> Next hop would be the update source IP configured in BGP [By default its the outgoing interface]. In this case S0/0 of R2.

Q3:

What NEXT_HOP IP will R1 use to tell R2 about Network-Y?

-> Same Analogy as in Q2, here it would be S0 of R1.

Q4:

will R2 send an update about Network-Y back to R1 through iBGP though it learned about it from R1 itself? If yes then what NEXT_HOP will R2 advertise to R1?

-> No it wont be readvertised as AS_Path attribute will stop this advertisment.

Please rate if helpful

Ameya Oke

[milan.kulik]

Hi,

I'm afraid your answers to Q1 and Q4 are not correct.

As the AS number is not added to the original AS_PATH when sent to an iBGP neighbour, the AS_PATH loop check can't apply here.

Regarding Q2 and Q3:

RFC 4271

http://www.rfc-archive.org/getrfc.php?rfc=4271&tag=A-Border-Gateway-Protocol-4-%28BGP-4%29

says:

"When a BGP speaker receives an UPDATE message from an internal peer,

   the receiving BGP speaker SHALL NOT re-distribute the routing

   information contained in that UPDATE message to other internal peers

   (unless the speaker acts as a BGP Route Reflector [RFC 2796])."

According to the RFC wording, it could sent it back to the iBGP neighbour from which it was recieved?

But I don't believe Cisco routers behave that way.

The Next-hop value assignement is explained in all details in that RFC, too.

BR,

Milan