Specifications

1-27
Cisco MWR 2941 Mobile Wireless Edge Router Software Configuration Guide, Release 12.2(33)MRB
OL-21227-02
Chapter 1 Cisco MWR 2941 Router Overview
Features
domains. A useful feature of MSDP is that it allows each domain to maintain an independent RP that
does not rely on other domains, but it does enable RPs to forward traffic between domains. PIM-SM is
used to forward the traffic between the multicast domains.
The RP in each domain establishes an MSDP peering session using a TCP connection with the RPs in
other domains or with border routers leading to the other domains. When the RP learns about a new
multicast source within its own domain (through the normal PIM register mechanism), the RP
encapsulates the first data packet in a Source-Active (SA) message and sends the SA to all MSDP peers.
Each receiving peer uses a modified Reverse Path Forwarding (RPF) check to forward the SA, until the
SA reaches every MSDP router in the interconnected networks—theoretically the entire multicast
internet. If the receiving MSDP peer is an RP, and the RP has a (*, G) entry for the group in the SA (there
is an interested receiver), the RP creates (S,G) state for the source and joins to the shortest path tree for
the source. The encapsulated data is decapsulated and forwarded down the shared tree of that RP. When
the last hop router (the router closest to the receiver) receives the multicast packet, it may join the
shortest path tree to the source. The MSDP speaker periodically sends SAs that include all sources within
the domain of the RP.
MSDP was developed for peering between Internet service providers (ISPs). ISPs did not want to rely
on an RP maintained by a competing ISP to provide service to their customers. MSDP allows each ISP
to have its own local RP and still forward and receive multicast traffic to the Internet.
Anycast RP
Anycast RP is a useful application of MSDP. Originally developed for interdomain multicast
applications, MSDP used for Anycast RP is an intradomain feature that provides redundancy and
load-sharing capabilities. Enterprise customers typically use Anycast RP for configuring a Protocol
Independent Multicast sparse mode (PIM-SM) network to meet fault tolerance requirements within a
single multicast domain.
In Anycast RP, two or more RPs are configured with the same IP address on loopback interfaces. The
Anycast RP loopback address should be configured with a 32-bit mask, making it a host address. All the
downstream routers should be configured to “know” that the Anycast RP loopback address is the IP
address of their local RP. IP routing automatically will select the topologically closest RP for each
source and receiver. Assuming that the sources are evenly spaced around the network, an equal number
of sources will register with each RP. That is, the process of registering the sources will be shared
equally by all the RPs in the network.
Because a source may register with one RP and receivers may join to a different RP, a method is needed
for the RPs to exchange information about active sources. This information exchange is done with
MSDP.
In Anycast RP, all the RPs are configured to be MSDP peers of each other. When a source registers with
one RP, an SA message will be sent to the other RPs informing them that there is an active source for a
particular multicast group. The result is that each RP will know about the active sources in the area of
the other RPs. If any of the RPs were to fail, IP routing would converge and one of the RPs would
become the active RP in more than one area. New sources would register with the backup RP. Receivers
would join toward the new RP and connectivity would be maintained.
Note The RP is normally needed only to start new sessions with sources and receivers. The RP facilitates the
shared tree so that sources and receivers can directly establish a multicast data flow. If a multicast data
flow is already directly established between a source and the receiver, then an RP failure will not affect
that session. Anycast RP ensures that new sessions with sources and receivers can begin at any time.