Cisco Catalyst 3850 series Configuration Manual page 220

• Introduces a more manageable approach for building multicast distribution trees between multiple
domains.
MSDP is a mechanism to connect multiple PIM-SM domains. The purpose of MSDP is to discover multicast
sources in other PIM domains. The main advantage of MSDP is that it reduces the complexity of interconnecting
multiple PIM-SM domains by allowing PIM-SM domains to use an interdomain source tree (rather than a
common shared tree). When MSDP is configured in a network, RPs exchange source information with RPs
in other domains. An RP can join the interdomain source tree for sources that are sending to groups for which
it has receivers. The RP can do that because it is the root of the shared tree within its domain, which has
branches to all points in the domain where there are active receivers. When a last-hop device learns of a new
source outside the PIM-SM domain (through the arrival of a multicast packet from the source down the shared
tree), it then can send a join toward the source and join the interdomain source tree.
If the RP either has no shared tree for a particular group or a shared tree whose outgoing interface list is
Note
null, it does not send a join to the source in another domain.
When MSDP is enabled, an RP in a PIM-SM domain maintains MSDP peering relationships with
MSDP-enabled devices in other domains. This peering relationship occurs over a TCP connection, where
primarily a list of sources sending to multicast groups is exchanged. MSDP uses TCP (port 639) for its peering
connections. As with BGP, using point-to-point TCP peering means that each peer must be explicitly configured.
The TCP connections between RPs, moreover, are achieved by the underlying routing system. The receiving
RP uses the source lists to establish a source path. If the multicast sources are of interest to a domain that has
receivers, multicast data is delivered over the normal, source-tree building mechanism provided by PIM-SM.
MSDP is also used to announce sources sending to a group. These announcements must originate at the RP
of the domain.
The figure illustrates MSDP operating between two MSDP peers. PIM uses MSDP as the standard mechanism
to register a source with the RP of a domain.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
196
Configuring MSDP
OL-32598-01