PIM SM

PIM SM
PIM sparse mode (PIM SM) also uses the architecture of shared tree distribution. There is
an RP 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 notified 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. Depending on whether it has an existing
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, pruning 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 shared tree. If there is another router with active
members connected to the router requesting the prune, it is removed from the outgoing
interface list and no additional prune messages are sent to the RP. See Figure 19.21 for a
visual description.