3Com 5500-SI Configuration Manual page 586

586 C 31: MSDP C
HAPTER ONFIGURATION
MSDP peers are interconnected over TCP connections (using port 639). A TCP
connection can be established between RPs in different PIM-SM domains, between
RPs in the same PIM-SM domain, between an RP and a common router, or between
common routers. Figure 169 shows the MSDP peering relationship between RPs.
Unless otherwise specified, examples in the following descriptions are based on MSDP
peering relationship between RPs.
An active multicast source S exists in the PIM-SM1 domain. RP1 in this domain learns
the specific location of the multicast source S through multicast source register
messages, and then sends source active (SA) messages periodically to MSDP peers (RP
nodes) in other PIM-SM domains. An SA message contains the IP address of the
multicast source S, the multicast group address G, the address of the RP that has
generated the SA message, and the first multicast data received by the RP in the
PIM-SM1 domain. The SA message is forwarded by peers. Finally, the SA message
reaches all the MSDP peers. In this way, the information of multicast source S in the
PIM-SM domain is delivered to all PIM-SM domains.
By performing reverse path forwarding (RPF) check, MSDP peers accept SA messages
only from the correct paths and forward the SA messages, thus avoiding SA message
loop. In addition, you can configure a mesh group among MSDP peers to avoid SA
flooding among MSDP peers.
Assume that RP4 in the PIM-SM4 domain receives the SA message. RP4 checks
whether receivers exist in the corresponding multicast group. If so, RP4 sends an (S,
G) join message hop by hop to the multicast source S, thus creating a shortest path
tree (SPT) based on the multicast source S. A rendezvous point tree (RPT) exists
between RP4 and receivers in the PIM-SM4 domain.
Through MSDP, a PIM-SM domain receiving information from the multicast source S
does not rely on RPs in other PIM-SM domains, that is, receivers can directly join the
SPT tree based on the multicast source without passing RPs in other PIM-SM domains.
MSDP application
You can also implement Anycast RP through MSDP. Anycast RP refers to such an
application that an MSDP peering relationship can be established between two RPs
with the same IP address in the same PIM-SM domain, to enable load balancing and
redundancy backup between the two RPs in the same domain. The candidate RP
(C-RP) function is enabled on an interface (typically the loopback interface) of each of
multiple routers in the same PIM-SM domain, and these interfaces have the same IP
address. An MSDP peering relationship is formed among these interfaces, as shown in
Figure 170