Joining A Shared Or Rp Tree - HP 7102dl - ProCurve Secure Router Configuration Manual

Configuring Multicast Support with PIM-SM
Overview
13-8
Although (S, G) entries relate to SP trees, routers that are only part of an RP
tree can also store special (S, G) entries with the RP-bit set. These entries
prune downstream neighbors from the RP tree for multicasts from a specific
source, but allow the neighbors to remain in the RP tree for traffic from other
sources for the group.
A router always matches packets to the most specific entry first. In effect, it
first processes (S, G) entries and then (*, G) entries. Therefore, by default, the
router will always prefer an SP tree when such a tree exists for the source.
After a router has not received packets from a source for a certain interval,
the (S, G) entry times out, and the router reverts to using the (*, G) entry.

Joining a Shared or RP Tree

A router joins the shared RP tree when it receives:
an IGMP join from a directly connected host
a (*, G) join from a PIM neighbor
The router creates a (*, G) entry, completing these steps:
1. It selects the RP for the group. (See "RP Selection" on page 13-17 to learn
how the router decides on the RP.)
2. The router uses RPF to determine the best connection to the RP. It sets
the incoming interface and RPF neighbor for the entry accordingly.
3. The router includes the interface on which it received the join in the
entry's outgoing interface list.
4. The router sends a (*, G) join to the RPF neighbor.
The upstream router then follows the same steps and sends a (*, G) join to its
upstream neighbor. (If the upstream router already has an entry for the group,
it simply adds the interface on which it received the join to the entry's outgoing
interface list.)
The process continues until the join reaches the RP.
When the RP creates its own (*, G) entry, it sets the incoming interface to null
and the RPF neighbor to 0.0.0.0; the RP is the root of the tree and does not
need to forward joins to any upstream neighbor.
The section of the tree from the RP to the local router has now been estab-
lished. Figure 13-2 illustrates this process.