Configuration Guide User guide

FastIron Configuration Guide 1561
53-1002494-02
Multicast Source Discovery Protocol (MSDP)
Figure 176 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 Multicast Source Discovery Protocol (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 176, 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.
NOTE
MSDP is supported only on FSX devices.
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 176 does not show additional peers.
Source active caching
When the Multicast Source Discovery Protocol (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 176, 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.
Some MSDP routers that are also RPs can cache Source Active messages. If the RP is not caching
Source Active messages, the RP does not send a Join message unless it already has a receiver that
wants to join the group. Otherwise, the RP does not send a Join message and does not remember
the information in the Source Active message after forwarding it. If the RP receives a request from
a receiver for the group, the RP and receiver must wait for the next Source Active message for that
group before the RP can send a Join message for the receiver.