Sp Tree - HP 7102dl - ProCurve Secure Router Configuration Manual

Configuring Multicast Support with PIM-SM
Overview
these sources may change. In addition, when hosts join a multicast group, they
do not know the address of the source. Sources and receivers need a common
point at which to discover each other, and the RP provides this point.
The DR of each subnet forwards join/prunes toward the RP so that the RP can
begin forwarding multicasts to the appropriate routers as soon as a source
begins transmitting. When a source does begin sending multicasts, the
source's DR encapsulates and unicasts the multicasts to the RP, the predeter-
mined distribution point.
On ProCurve Secure Routers, which use static RPs, the network administrator
specifies the router or routers that can act as an RP.

SP Tree

An RP tree applies to multicast traffic from any source. Once a specific source
begins to transmit a multicast stream, PIM-SM allows routers to generate a
more efficient tree to that source. This tree is called the SP tree.
When a router receives a multicast packet, it learns the address of the
multicast source from the source address in the packet's IP header. From its
unicast routing table, the router may know of a better connection (one with
a lower metric or administrative distance) to the source address than that
through the RP. The router can use the better connection as an SP connection
to the source. The process of using a packet's source address to find the
upstream interface for a multicast tree is called reverse path forwarding
(RPF).
Only edge routers can initiate the generation of an SP tree. Intervening routers,
which transit multicasts but do not directly connect to multicast hosts, cannot
join an SP tree in response to multicast traffic from a source. However, they
can join an SP tree when they receive a PIM-SM join from a downstream router.
A group's RP can also establish an SP tree to the source, even if it does not
also act as an edge router for that group. The RP uses its SP tree to receive
multicasts from a source that has already registered with it. The RP can
forward multicasts it receives on the SP tree to directly connected hosts,
downstream routers in its SP tree, or routers that have not switched from the
RP to the SP tree.
Like the RP tree, the SP tree is unidirectional. The root of the tree is the DR
for the multicast source, and edge routers that connect to hosts in the
multicast group are the leaves. Traffic always flows from the source to the
edge routers.
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