All things related to multicast

["Spurling, Shannon" <>]

I am going to tell you of a solution I have used with Cisco routers with 
reasonable success. You have to be careful about this. I use it mainly with 
peers with their own PIM domains and no BGP, or the MSDP peer/PIM-RP being 
deep inside their network. 

Use MSDP default-peer and filter the heck out of the SA's, so that only SA's 
with their prefixes come from the site, and SA's without their prefixes go to 
the site. 


Shannon Spurling
WAN Engineer



-----Original Message-----
From: Zenon Mousmoulas 
[mailto:]
 
Sent: Thursday, June 10, 2010 12:57 PM
To: wg-multicast
Subject: inter-domain MSDP peering without BGP FIRT

Dear all,

We (GRNET) have a downstream network (let's call them U) who receive a  
partial routing table from us. U runs BGP and MSDP peerings on a  
single border router (RP in their PIM domain) with two distinct GRNET  
routers (anycast RPs in the GRNET domain, running full-mesh IBGP and  
MSDP). The BGP peerings are setup in a primary/backup scheme (using  
communities, local pref etc). BGP peerings carry both unicast and  
multicast NLRIs, but we use separate sessions for IPv4 and IPv6.  
Pretty standard stuff.

Since U receive only a partial routing table, they rely on the  
"default network" mechanism for internet routing. This gateway of last  
resort is configured once and, AFAIK, can't be set separately per  
NLRI. Since IOS (12.2SXF in their case) performs RPF lookups across  
both the MRIB and the unicast RIB, it seems to work out fine: "show ip  
rpf" points to the default network when no prefix matches the source,  
with a "unicast" RPF type. However MSDP peer-RPF check fails: SA  
announcements are rejected when no prefix matches the RP/Originator,  
so the default network doesn't apply in this case, or so it seems.

MSDP debug also logs messages like this:

Jun  7 18:15:33: MSDP(0): <grnet-msdp-peer>: Peer RPF check failed for  
w.x.y.z, used IBGP route's peer 0.0.0.0

which don't make much sense.

Judging from the above, it looks like we effectively can not MSDP peer  
with a downstream network without announcing to them (the equivalent  
of) FIRT in MBGP. Of course this is not a big issue while the number  
of prefixes is low, however there are still practical reasons against  
this, namely avoiding incongruent routing.

Have you ever noticed this? I have a feeling that enforcing compliance  
to RFC3618 (ip msdp rpf rfc3618) would affect this behavior, but I'm  
not even sure if their border router supports it.

Thanks in advance,
Z.