Msdp Operation - Cisco IE-4000 Software Configuration Manual

Configuring MSDP
Information About MSDP

MSDP Operation

Figure 92 on page 732
to register a source with the RP of a domain. When MSDP is configured, this sequence occurs.
When a source sends its first multicast packet, the first-hop router (designated router or RP) directly connected to the
source sends a PIM register message to the RP. The RP uses the register message to register the active source and to
forward the multicast packet down the shared tree in the local domain. With MSDP configured, the RP also forwards a
source-active (SA) message to all MSDP peers. The SA message identifies the source, the group the source is sending
to, and the address of the RP or the originator ID (the IP address of the interface used as the RP address), if configured.
Each MSDP peer receives and forwards the SA message away from the originating RP to achieve peer reverse-path
flooding (RPF). The MSDP device examines the BGP or MBGP routing table to discover 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 MSDP
device forwards the message to all MSDP peers other than the RPF peer. For information on how to configure an MSDP
peer when BGP and MBGP are not supported, see
Figure 92 MSDP Running Between RP Peers
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 to all its MSDP peers.
The RP for a domain receives the SA message from an MSDP peer. If the RP has any join requests for the group the SA
message describes and 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. After the (S,G) join reaches the source's DR, a branch of the
source tree has been built from the source to the RP in the remote domain. Multicast traffic can now flow from the source
across the source tree to the RP and then down the shared tree in the remote domain to the receiver.
shows MSDP operating between two MSDP peers. PIM uses MSDP as the standard mechanism
RP + MSDP peer
MSDP SA
Register
Multicast
PIM
Source
DR
PIM sparse-mode
domain
Configuring a Default MSDP Peer, page
MSDP SA
TCP connection
BGP
MSDP peer
(S,G) Join
732
734.
MSDP peer
Peer RPF flooding
Receiver