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Chapter 9

Multicast

A CBT router may become part of the tree once a host sends an IGMP

Membership Record to the directly connected router. The router then sends
a join tree request to the core router. If the request reaches a CBT tree mem-
ber first, that router will add the leaf router to the tree and begin forwarding
multicast data.

Pruning the tree is done much the same way. Once there are no more

active members on a router's interfaces, the router will send a prune request
to the upstream router. The answering router will remove the interface from
the forwarding cache if it is on a point-to-point circuit, or it will wait for a
timer to expire it if is on a shared access network. The timer gives enough
time for other CBT routers on the segment to override the prune request.

PIM SM

PIM sparse mode (PIM SM) also uses the architecture of shared tree distri-
bution. There is an RP (rendezvous point) router that acts as the root of the
shared tree. Unlike CBT, however, PIM SM uses the unidirectional shared
tree distribution mechanism. Because PIM SM uses the unidirectional
method, all multicast sources for any group must register with the RP of the
shared tree. This enables the RP and other routers to establish the RPT, or
RP tree (synonymous with SPT in source tree distribution).

Just as with CBT, PIM SM routers join the shared tree when they are noti-

fied via IGMP that a host requests membership of a multicast group. If the
existing group entry (*, G) does not already exist in the router's table, it is
created and the join tree request is sent to the next hop toward the RP. The
next router receives the request. Based on whether or not it has an exiting
entry for (*, G), two things can happen:

If an entry for (*, G) exists, the router simply adds the interface to the
shared tree and no further join requests are sent toward the RP.

If an entry for (*, G) does not exist, the router creates an entry for the
(*, G) group and adds the link to the forwarding cache. In addition to
doing this, the router sends its own join request toward the RP.

This happens until the join request reaches a router that already has the

(*, G) entry or a join request reaches the RP.

The next facet of PIM SM is the shared tree pruning. With PIM SM, prun-

ing turns out to be just the opposite of the explicit Join mechanism used to
construct the shared tree.

When a member leaves a group, it does so via IGMP. When it happens to

be the last member on a segment, the router removes the interface from the
forwarding cache entry and then sends a prune request toward the RP of the

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