Cisco Systems 78-11741-02 manual IPC-478, Msdp Running Between RP Peers

Configuring Multicast Source Discovery Protocol

How MSDP Works

and the address or the originator ID of the RP, if configured. If the peer is an RP and has a member of that multicast group, the data packet is decapsulated and forwarded down the shared-tree in the remote domain.

The PIM designated router (DR) directly connected to the source sends the data encapsulated in a PIM register message to the RP in the domain.

Note The DR sends the encapsulated data to the RP only once per source, when the source goes active. If the source times out, this process happens again when it goes active again. This situation is different from the periodic SA message that contains all sources that are registered to the originating RP. These messages have no data.

Each MSDP peer receives and forwards the SA message away from the originating RP to achieve peer-RPF flooding. The concept of peer-RPF flooding is with respect to forwarding SA messages. The router examines the BGP or MBGP routing table to determine which peer is the next hop toward the originating RP of the SA message. Such a peer is called an “RPF peer” (Reverse Path Forwarding peer). The router forwards the message to all MSDP peers other than the RPF peer.

If the MSDP peer receives the same SA message from a non-RPF peer toward the originating RP, it drops the message. Otherwise, it forwards the message on to all its MSDP peers.

When an RP for a domain receives an SA message from an MSDP peer, it determines if it has any group members interested in the group the SA message describes. If the (*, G) entry exists with a nonempty outgoing interface list, the domain is interested in the group, and the RP triggers an (S, G) join toward the source.

Figure 78 MSDP Running Between RP Peers

RP + MSDP peer MSDP peer

MSDP SA

Peer RPF flooding

					MSDP SA
					TCP connection
							Router B
						BGP
			Register					MSDP peer
		Multicast
					(S, G) Join
				PIM
Source				DR
	PIM sparse mode
		domain

Receiver

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Cisco IOS IP Configuration Guide

IPC-478