Brocade TurboIron 24X Series Configuration Guide - Manual (Page 866)

832 Brocade TurboIron 24X Series Configuration Guide
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Multicast Source Discovery Protocol (MSDP)
The RP sends the source information to each of its peers by sending 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 interface with its peer. By default, the IP address included in the RP
address field of the SA message is the IP address of the originating RP. However, if MSDP is
instructed to use a specific address as the IP address of the RP in a Source Address message (CLI
command originator-id <type> <number>), the Source Active message can be the IP address of any
interface on the originating RP. The interface is usually a loopback interface.
In this example, the Source Active message contains the following information:
• Source address: 10.251.14.22
• Group address: 232.1.0.95
• RP address: 10.251.17.41
Figure 108 shows only one peer for the MSDP router (which is also the RP here) in domain 1, so the
Source Active message goes to only that peer. When an MSDP router has multiple peers, it sends a
Source Active message to each of those peers. Each peer sends the Source Advertisement to its
other MSDP peers. The RP that receives the Source Active message also sends a Join message for
the group if the RP that received the message has receivers for the group.
Peer Reverse Path Forwarding (RPF) flooding
When the MSDP router (also the RP) in domain 2 receives the Source Active message from its peer
in domain 1, the MSDP router in domain 2 forwards the message to all its other peers. The
propagation process is sometimes called “peer Reverse Path Forwarding (RPF) flooding”. This term
refers to the fact that the MSDP router uses its PIM Sparse RPF tree to send the message to its
peers within the tree. In
Figure 108 , the MSDP router floods the Source Active message it receives
from its peer in domain 1 to its other peers, in domains 3 and 4.
Note that the MSDP router in domain 2 does not forward the Source Active back to its peer in
domain 1, because that is the peer from which the router received the message. An MSDP router
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 router 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 interdomain operations.
The MSDP routers in domains 3 and 4 also forward the Source Active message to all their peers
except the ones that sent them the message.
Figure 108 does not show additional peers.
Source active caching
When an MSDP router that is also an RP receives a Source Active message, the RP checks its PIM
Sparse multicast group table for receivers for the group. If the DR has a receiver for the group
being advertised in the Source Active message, the DR sends a Join message for that receiver back
to the DR in the domain from which the Source Active message came. Usually, the DR is also the
MSDP router that sent the Source Active message.
In Figure 108 , if the MSDP router 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 RP for the source,
in this case the RP in domain 1.