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
(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 the
Configuring a Default MSDP 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 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.
Figure 1. MSDP Running Between RP Peers. This figure shows MSDP operating between two MSDP peers. PIM uses
MSDP as the standard mechanism to register a source with the RP of a domain.
When MSDP is configured, this sequence occurs.
By default, the switch
does not cache source or group pairs from received SA messages. When the switch
forwards the MSDP SA information, it does not store it in memory. Therefore, if
a member joins a group soon after an SA message is received by the local RP,
that member needs to wait until the next SA message to hear about the source.
This delay is known as join latency.
Local RPs can send SA requests and get immediate responses for all
active sources for a given group. By default, the switch does not send any SA
request messages to its MSDP peers when a new member joins a group and wants to
receive multicast traffic. The new member waits to receive the next periodic SA
If you want a new member of a group to learn the active multicast
sources in a connected PIM sparse-mode domain that are sending to a group,
configure the switch to send SA request messages to the specified MSDP peer
when a new member joins a group.