MBGP is not 'required' for MSDP, but the MBGP table, which I suppose is now
the MBONE, is the source of all MBONE Multicast. So, if you are not using
MBGP, your multicast RPF topology MUST be congruent to your unicast BGP
topology. For policy reasons, this may be undesirable. Thus, you can
create inet.2 and import M-NLRI into inet.2 with different policies.
The reverse-poisoning is used by MSDP to remove unnecessary flooding of SAs
across AS boundaries. Essentially, MSDP will accept a message from a peer
if the BGP route to the RP is via the same path as the MSDP peer. This is
the topological congruence condition. When determining to which external
peers to flood SA messages, MSDP checks to ensure that all peers are using
the local MSDP speaker as the next-hop to the RP. This cannot be done
across AS-boundaries in BGP without poisoned routes, since the only way a
downstream AS can inform an upstream AS which path it using is by
reannouncing the best route back to the upstream. However, the upstream
peer drops this route because of the AS-PATH loop condition, thus, poisoning
the route. In essence, this mechanism is an optimization, not a
requirement. If reverse-poisoning isn't done, then the downstream AS will
simply drop the SA if it is received from a peer that in't the next-hop to
the source RP.
HTH,
chris
> -----Original Message-----
> From: JunoGuy [mailto:junoguy@earthlink.net]
> Sent: Tuesday, February 05, 2002 9:00 AM
> To: Martin, Christian; 'andrew kevin'; je@juniper.net;
> juniper-nsp@puck.nether.net
> Cc: juniper@groupstudy.com
> Subject: RE: [j-nsp] BGP route
>
>
> Can you elaborate more on this because (correct me if I am
> wrong) is MBGP a requirement for MSDP? If not then only a
> simply BGP session is needed. Even if you use MBGP, routes
> received from a peer are placed into a routing table based on
> the SAFI and even in inet.0 and inet.2 you would still get
> normal Juniper BGP behavior meaning that the same BGP default
> policy applies to the 2 tables. So how is it that MSDP
> requires MBGP to poison routes received from a peer in a
> neighbor AS if the same default policy applies when the
> routes are received from a peer? Just trying to understand.
>
>
> JunoGuy
> --
>
>
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