information with other PIM Sparse domains by communicating with RPs in other domains that are
running MSDP.
The RP sends the source information to each peer through a Source Active message. The message
contains the IP address of the source, the group address to which the source is sending, and the IP
address of the RP.
In this example, the Source Active message contains the following information:
• Source address: 206.251.14.22
• Group address: 232.1.0.95
• RP address: 206.251.17.41
Figure 6 shows only one peer for the MSDP device (which is also the RP here) in domain 1, so the
Source Active message goes to only that peer. When an MSDP device has multiple peers, it sends a
Source Active message to each of those peers. Each peer sends the Source Advertisement to other
MSDP peers. The RP that receives the Source Active message also sends a Join message to the
source if the RP that received the message has receivers for the group and source.
Peer Reverse Path Forwarding (RPF) flooding
When the MSDP device (also the RP) in domain 2 receives the Source Active message from the peer
in domain 1, the MSDP device in domain 2 forwards the message to all other peers. This propagation
process is sometimes called "peer Reverse Path Forwarding (RPF) flooding". In Figure 6 on page 121,
the MSDP device floods the Source Active message it receives from the peer in domain 1 to peers in
domains 3 and 4.
The MSDP device in domain 2 does not forward the Source Active back to the peer in domain 1,
because that is the peer from which the device received the message. An MSDP device never sends a
Source Active message back to the peer that sent it. The peer that sent the message is sometimes
called the "RPF peer". The MSDP device uses the unicast routing table for its Exterior Gateway
Protocol (EGP) to identify the RPF peer by looking for the route entry that is the next hop toward the
source. Often, the EGP protocol is Border Gateway Protocol (BGP) version 4.
NOTE
MSDP depends on BGP for inter-domain operations.
The MSDP routers in domains 3 and 4 also forward the Source Active message to all peers except the
ones that sent them the message. Figure 6 on page 121 does not show additional peers.
Source Active caching
When an MSDP device that is also an RP receives a Source Active message, it checks the PIM
sparse multicast group table for receivers for that group. If there are receivers for that group being
advertised in the Source Active message, the RP sends a Join message towards the source.
In Figure 6 on page 121, if the MSDP device and RP in domain 4 has a table entry for the receiver, the
RP sends a Join message on behalf of the receiver back through the RPF tree to the source, in this
case the source in domain 1.
Source Active caching is enabled in MSDP on Brocade devices. The RP caches the Source Active
messages it receives even if the RP does not have a receiver for the group. Once a receiver arrives,
the RP can then send a Join to the cached source immediately.
The size of the cache used to store MSDP Source Active messages is 4K. MSDP SA cache size can
be configured using the system-max msdp-sa-cache command. The default value is 4K; the range is
1K to 8K.

Peer Reverse Path Forwarding (RPF) flooding

122 FastIron Ethernet Switch IP Multicast Configuration Guide
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