Multicast Trees - HP 7102dl - ProCurve Secure Router Configuration Manual

Configuring Multicast Support with PIM-SM
Overview
13-4
An entry in the multicast routing table lists connections to downstream
routers and networks as outgoing interfaces and the connection to the
upstream router as the incoming interface. A router only accepts a multicast
packet if it arrives on the appropriate incoming interface. If accepted, the
router matches the packet to an entry in its multicast routing table according
to its destination (and, optionally, source) address. The router then forwards
the multicast packet through the outgoing interfaces indicated.
PIM-SM builds the multicast routing table conservatively. Interfaces only
become outgoing interfaces for a multicast group if they specifically receive
a PIM or IGMP join for that group. Because PIM-SM minimizes floods, it is
well-suited for a WAN environment, in which bandwidth is often limited.

Multicast Trees

As hosts join and leave a group, the networks and routers that need the
multicast traffic change. A multicast tree directs the flow of multicast traffic
to a group's edge routers, which are the routers that connect to hosts active
in the group.
A multicast tree indicates the upstream neighbor from which a router expects
to receive multicasts for a specific group and the downstream neighbors to
which the router must forward these multicasts.
PIM-SM defines two different types of tree:
a shared, or RP tree
a shortest path (SP) tree
RP Tree
Edge routers use the RP tree to reach the RP, which is the router that initially
connects the routers that need multicasts for a group to sources for that group.
Using PIM-SM, routers generate a shared, RP tree for each multicast group.
The leaves of the tree are networks that contain at least one host in that group.
The root of the tree is the RP for the group. The tree is unidirectional: traffic
must flow from the RP to the multicast hosts.
It might seem most logical that the root of the tree would be the designated
router (DR) for the source of the traffic. (Indeed, the source's DR is the root
of the SP tree.) However, multicast sources and receivers do not initially know
where to find each other. A multicast group may have multiple sources, and