Hi Sarah,
I have not looked at MSDP for a while but this is what I recall from my notes:
1) “The RP in each domain establishes an MSDP peering session using a TCP connection with the RPs in other domains or with border routers leading to the other domains.” https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/Phase_1/mcstmsdp/mcst_p1.html#wp1079524
From my understanding, the above statement means that an MSDP peering session can be established between all RPs in different domains that need to share (S,G) information, so in your example, MSDP peering can be established between R1, R2 and R3 in a full-mesh scenario in order to share SA messages. Alternatively, an MSDP session can established between border routers that lead to other domains, so in your example, R2 would be the border router and will establish an MSDP peering session to R1 and R3. When R2 receives SA messages from R1, it will send these message to R3.
2) I believe that MSDP can run without BGP as when you configure an MSDP peering session you don’t need to specify a remote-AS. Also MSDP is commonly used for Anycast RP which typically operates within the same domain which may or may not be using BGP for unicast routing.
As with regular intra-domain multicast, once the RPs have received the SA information from their MSDP peer(s), the SPT can then be built back to the source using the unicast routing for the RFP check. The unicast table can be populated using BGP, EIGRP, static routes etc.
I remember configuring MSDP with BGP and having to specify the ‘remote-as’ keyword but I don’t recall why. I will have to look into this again.
I hope that this helps
Will
