HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 535

Each entry includes a list of outgoing interfaces. Unlike a unicast routing table
entry, a multicast table entry can include multiple forwarding, or outgoing,
interfaces. Because a multicast address applies to all hosts who have joined
the multicast group, and because these hosts may be in different networks,
the router may copy packets destined to a single multicast address and route
them out multiple interfaces.
For PIM-SM, the multicast routes form unidirectional trees; traffic for a
particular group and from a particular source must always arrive on the same
interface. Therefore, a multicast routing table entry also specifies the incom-
ing interface on which the router expects to receive the multicasts. If traffic
does not arrive on the correct incoming interface, the router drops it. A router
forwards join/prune messages through the corresponding group's incoming
interface.
Entries can also include the address of the group's RP and various flags which
will be discussed in more detail later.
PIM-SM categorizes entries in a multicast routing table into two types:
(*, G) entries, which are associated with an RP tree and apply to traffic
for a specific group, but from any source
(S, G) entries, which are primarily associated with an SP tree and apply
to traffic for a specific group and from a specific source
When this guide refers to entries, a star (*) indicates a wildcard; G, a multicast
group address; and S, the unicast address of the multicast source.
(*, G) entries are based on the RP tree and allow routers to receive multicast
packets as soon as any source begins to transmit them. For example, a (*,
239.255.1.1) entry instructs the router how to forward any traffic destined for
239.255.1.1. The outgoing interfaces listed in the entry are downstream inter-
faces in the RP tree. The incoming interface is the interface through which the
router connects to the RP.
(S, G) entries are source-specific and based on the SP tree. For example, a
(192.168.1.254, 239.255.1.1) entry applies only to multicast traffic for the group
239.255.1.1 from the multicast source currently streaming traffic from
192.168.1.254. When a router adds an (S, G) entry to its table, it determines the
incoming interface using RPF. That is, the incoming interface is the forwarding
interface listed for the source address in the unicast routing table. The router
copies the outgoing interfaces from the equivalent (*, G) group. The router
will later receive PIM join/prunes, with which it can tailor the outgoing
interface list to the SP tree.
Configuring Multicast Support with PIM-SM
Overview
11-7