24-10
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-03
Chapter24 Configuring IP Multicast Routing
Cisco Implementation of IP Multicast Routing
Figure 24-6 RPF Check
PIM uses both source trees and RP-rooted shared trees to forward datagrams (described in the “PIM
DM” section on page 24-5 and the “PIM SM” section on page 24-7); the RPF check is performed
differently for each:
•If a PIM router or multilayer switch has a source-tree state (that is, an (S,G) entry is present in the
multicast routing table), it performs the RPF check against the IP address of the source of the
multicast packet.
•If a PIM router or multilayer switch has a shared-tree state (and no explicit source-tree state), it
performs the RPF check on the rendezvous point (RP) address (which is known when members join
the group).
Sparse-mode PIM uses the RPF lookup function to determine where it needs t o sen d joi n s and prune s:
•(S,G) joins (which are source-tree states) are sent toward the source.
•(*,G) joins (which are shared-tree states) are sent toward the RP.
DVMRP and dense-mode PIM use only source trees and use RPF as pre viou sl y desc rib ed.
Neighbor Discovery
PIM uses a neighbor discovery mechanism to establish PIM neigh bor a dja cenc ies . To establish
adjacencies, a PIM router or multilayer switch sends PIM hello messages to the all-PIM-routers
multicast group (224.0.0.13) on each of its multicast-enabled interfaces. The hello me ssage conta ins a
holdtime, which tells the receiver when the neighbor adjacency associated w ith the sender expires if no
more PIM hello messages are received. (Keeping track of adjacencies is important for PIM DM
operation for building the source distribution tree.)
PIM hello messages are also used to elect the DR for multi-access networks (Ethernet). The router or
multilayer switch on the network with the highest IP address is the DR. With PIM DM operation, the DR
has meaning only if IGMPv1 is in use; IGMPv1 does not have an IGMP querier ele ction process, so the
elected DR functions as the IGMP querier. In PIM SM operation, the DR is the router or switch that is
directly connected to the multicast source. It sends PIM register messages to notify the RP that multica st
traffic from a source needs to be forwarded down the shared tree.
Multicast
packet from
source 151.10.3.21
is forwarded.
Multicast
packet from
source 151.10.3.21
packet is discarded.
Routing Table
Network Interface
151.10.0.0/16 Gigabit Ethernet 0/1
Gigabit Ethernet 0/1
198.14.32.0/32 Gigabit Ethernet 0/3
Gigabit Ethernet 0/3
204.1.16.0/24 Gigabit Ethernet 0/4
Gigabit Ethernet 0/4
Gigabit Ethernet 0/2
45149
Si