User Manual

Protocol Independent Multicast (PIM)
280 Enterasys X-Pedition User Reference Manual
2. Sending “hello” messages to determine neighbor presence and configuration.
3. Sending “join/prune” messages to determine the need to retain multicast route information for
a particular group on an interface.
4. Sending “assert” messages to resolve conflicts that occur regarding inbound interfaces.
PIM-SM message types
PIM-SM defines the following message types:
• Hello messages announce the sender's presence to other PIM-SM routers. The hello packet
includes options such as hold time (the length of time to keep the sender reachable) and
designated router (DR) priority (used to designate which PIM-SM router will act on behalf of
sources and receivers in the PIM-SM domain).
• Register messages (sparse only) are used by a source's DR to encapsulate (i.e., register)
multicast data and send it to the rendezvous point (RP)—a PIM sparse mode router designated
as the root of a shared tree.
• Register-Stop messages (sparse only) are used by the RP to tell the source's DR to stop
registering traffic for a particular source.
• Join/Prune (J/P) messages contain information on group membership received from
downstream routers. J/P messages contain one or more group entries which, in turn, contain one
or more source entries called joins and prunes. J/P messages allow a PIM-SM router to build or
modify the distribution tree—join messages maintain existing links or create new ones as part
of the tree; prune messages remove links from the tree. Routers always send Join/Prune (J/P)
messages upstream, toward the root of the tree—the interface that receives the join is the
outbound interface (OIF) and the interface that sends the join is the inbound interface (IIF).
Because routers build the tree opposite the flow of traffic, this process is called reverse-path
forwarding.
Joins and prunes employ two important bits—the RPT (R) bit indicates that the join or prune
applies to the shared tree and the WC (W) bit indicates that the join or prune applies to the
group-only (*,G) forwarding state. If the router does not set the WC bit, the join or prune will
apply to source—group pairs (S,G). If the router sets the W bit, it must also set the R bit.
If a source entry contains the address for the group's RP instead of a source, the entry is a (*,G)
join or prune and has its WC (W) and RPT (R) bits set. PIM-SM routers forward (*,G) joins and
prunes toward the RP—not toward a source. Similarly, if the source entry contains the address
of a multicast source that is sending packets to the group, the entry is an (S,G) join or prune. A
PIM-SM router forwards (S,G) joins and prunes toward the source's DR. If a PIM-SM router
sets the R bit in an (S,G) entry, the join or prune becomes an (S,G,RPT) join or prune and
indicates an interest or a lack of interest in a particular source on the shared tree. Like (*,G) joins
and prunes, a PIM-SM router forwards (S,G,RPT) joins and prunes toward the RP for the group.
• Assert messages indicate that the router received a data packet on its OIF for the group. Assert
messages report the metric or distance to the source or RP to help the router identify the most
direct path to the root of the tree. If multiple routers claim to have the most direct path to the