[MBONED] MBGP/MSDP path selection question

[Antonio Querubin <tony at lava.net>]

I'm trying to get a better handle on how PIM works with MSDP.

Assume in the diagram below the links denote MBGP peering links.  AS4 
wants to receive multicast traffic sourced by AS1.

AS1 ---- AS2 ---- AS3 --- AS4
 |                 |
 --- AS5 --- AS6 ---

AS1 announces its prefixes to AS2 and AS5 which in turn announce to AS3 
and AS6.  AS3 announces to AS4.  Further assume that the unicast 
connectivity/peering topology is similar.

Suppose however due to internal policy AS2 will not allow multicast 
transit between AS1 and AS3 and either

1) blocks MSDP SAs received from AS1 from reaching AS3 (and vice versa)

or

2) allows MSDP SAs to reach AS3 but blocks multicast traffic from AS1 to 
AS3.

In case #1, will MSDP at AS3 accept the SAs from AS6 or will it reject 
them because it still sees AS2 as the RPF peer due to the shorter MBGP 
path?  If the SAs are accepted from AS6, will PIM Joins/Prunes from a 
router in AS4 attempting to join the SPT for a source in AS1 follow the 
AS5/AS6 path back to AS1 or will they follow AS2?  Ie. will the SPT from 
AS1 to AS4 be built through AS2 or through AS5/AS6 or not at all?

In case #2, don't we have an outright blackhole situation?


Antonio Querubin
808-545-5282 x3003
e-mail/xmpp:  tony at lava.net