From: Anthony Sequeira (asequeira@internetworkexpert.com)
Date: Fri Jan 09 2009 - 23:15:32 ARST
Yeah - there is something commonly referred to as "BGP Split-Horizon"
but that specifically references a BGP speaker taking an iBGP update
and not passing it to another iBGP peer by default (using any
interface). It is not the typical use of the term split horizon that
we are used to with Distance Vector routing protocols.
Anthony J. Sequeira, CCIE #15626, CCSI #23251
Senior CCIE Instructor
asequeira@internetworkexpert.com
Internetwork Expert, Inc.
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On Jan 9, 2009, at 7:47 PM, Huan Pham wrote:
> Bgp does not use the concept of split horizon. Bgp can peer over
> more than one hop away, so it does not really care how (I.e. via
> what interface) routes are exchanged.
>
> I cannot read your topology properly so I cannot tell exactly where
> problem is. Few general suggestions you can check:
>
> - make sure bgp peerings are up
> - double check RR config
> - check what is next hop advertized via bgp. You have to have
> reachability to the next hop or bgp route is not installed. Use next
> hop self on the "edge" router for the iBGP session to change next
> hop advertised to iBGP peer if required
> - check bgp community attributes. No-advertise may stop RR from
> advertise routes received from one client to others
>
> - post full bgp config , I will have a look at it when I have access
> to my pc
>
>
> Sent from my iPhone
>
> On 10/01/2009, at 10:03 AM, Nitro Drops <nitrodrops@hotmail.com>
> wrote:
>
> Hi Guys,
>
> Can BGP prefixes be learned from a IBGP peer neighbor and resend out
> to
> another IBGP peer neighbor from the SAME interface, with route-
> reflector
> client configured?
>
> Understand BGP doesnt use 'split-horizon', for IBGP, it uses loop
> prevention
> via route suppression which requires the use of RR, Confederation,
> Full Mesh.
>
> R3 F0/0 (AS3) > EBGP > (AS1) F0/0 R4 F0/1 > F0/0 R6 F0/1 > Vlan30
> SW1 Vlan 30
> F0/0 R5
> AS3 advertising BGP prefixes to AS1, and full L3 reachability
> R3 - AS3
> R4, R6, SW1, R5 - AS1
> R4 - RR client of R6
> R5 - RR client of SW1
>
> In the above setup, SW1 is learning the the BGP prefixes from IBGP
> neighbor
> R6, which is non-RR-client of SW1, SW1 being a RR should reflect the
> BGP
> prefixes to R5, learning and sending via SVI Vlan 30.
>
> What i noticed, R5 cant learn the BGP prefixes at all, even though
> R5 has an
> underlying L3 routes to R3.
>
> I even made both SW1 a RR-client of R6, and R6 a RR-client of SW1,
> still R5
> cant learn the prefixes.
>
> Next, i created another new SVI Vlan 20 for the subnet between SW1
> and R5
>
> R3 F0/0 (AS3) > EBGP > (AS1) F0/0 R4 F0/1 > F0/0 R6 F0/1 > Vlan30
> SW1 Vlan 20
> F0/0 R5
>
> Only, then R5 is able to learn the BGP prefixes.
>
> So my question is, can a single SAME interface be used for learning
> BGP
> prefixes from one IBGP neighbor and sending them to another IBGP
> neighbor?
> Some kind of split-horizon concept here even though BGP doesnt use
> that.
> Anyway BGP commands to disable this?
>
> Cheers
> Nit
>
>
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