Overview
MSDP routers in a PIM-SM domain have a MSDP peering relationship with MSDP peers in another domain using a TCP connection. MSDP peering is the first step towards exchanging inter-domain multicast source information using MSDP SA (Source-Active) messages.
When an RP in a PIM-SM domain first learns of a new sender (via PIM register messages), it constructs an SA message and sends it to its MSDP peers.
All RPs which intend to originate or receive SA messages must establish MSDP peering with other RPs, either directly or via an intermediate MSDP peer.
An SA message contains these fields:
• Source address of the data source
• Group address the data source sends to
• IP address of the RP
Each SA message received from a MSDP peer goes through an RPF check. The peer-RPF check compares the RP address carried in the SA message with the MSDP peer from which the message was received:
• If the MSDP peer receives an SA from a non-RPF peer towards the originating RP, it drops the message.
• Otherwise, it forwards the message to all its MSDP peers (except the one from which it received the SA message).
When an RP receives a new SA message from a peer in another domain, it checks if there are any receivers interested in the traffic. An RP checks for a (*, G) entry with a non-empty outgoing list. If the outgoing list is non-empty, the RP sends a (S,G) join towards the source.
Last modified date: 08/29/2023