Source-Specific Multicast; Pim Sm Over Frame Relay - Enterasys Security Router X-PeditionTM User Manual

Describing the XSR's PIM-SM v2 Features
Assert messages are used to negotiate which router will forward the multicast packets. The rule
for the assert winner is the router with the lower preference (usually a unicast routing protocol
preference) and a metric learned from that protocol. If the preference is the same between the two
parallel routers, then whichever router has the lower metric toward the source of the data packet
will win out. If the metric is the same, the interface with the highest IP address wins the assert.
The XSR conducts assert processing automatically at the lowest topological level to service the end
system network.

Source-Specific Multicast

Source-specific Multicast (SSM) was designed specifically for one-to-many multicast traffic. It
collects subscriber data in a manner that it is much simpler than PIM-SM and can be implemented
as a subset of the protocol. Both the regular IP multicast service and SSM service can coexist on the
same router implemented by the protocol.
In SSM, delivery of datagrams is based on source specific (S, G) channels. Traffic for one (S, G)
channel consists of datagrams with an IP unicast source address S and the multicast group
address G as the IP destination address. Systems will receive this traffic by becoming members of
the (S, G) channel. No signaling is required to become a source. But, in SSM, receivers must
subscribe or unsubscribe to (S, G) channels to receive or not receive traffic from specific sources. In
other words, receivers can receive traffic only from (S, G) channels that they are subscribed to. The
proposed standard approach for channel subscription signaling utilizes IGMP INCLUDE mode
membership reports, which are only supported in Version 3 of IGMP. The IP address range from
232.0.0.0 to 232.255.255.255 is reserved especially for SSM service. A receiver is also allowed to
issue an (S,G) join request in the non-SSM address range; but, in that case there is no guarantee
that it will receive service according to the SSM model.

PIM SM over Frame Relay

Frame Relay networks are not normally fully meshed but they appear to the IP layer as a logical
LAN network. This is a challenge to the XSR's implementation of multicast functionality because
when a prune message is sent from one remote node to a central node, other remote nodes will not
receive multicast prune packets and, therefore, none of the remote nodes will override the prune
message. The central site will remove the Frame Relay interface from the outgoing list for the
specified group when its timer expires. Also, when a multicast data packet is sent from a remote
node, other nodes will not receive the packet either. The XSR addresses this problem with a Point-
to-Point solution.
For each remote node, the central site router configures a point-to-point sub-interface with its own
IP address, thus mirrorings the behavior of multiple point to point interfaces. From the PIM
perspective, no special processing is needed.
7-12 Configuring PIM-SM and IGMP