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Catalyst 3750-E and 3560-E Switch Software Configuration Guide
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Chapter42 Configuring IP Multicast Routing
Understanding Cisco’s Implementation of IP Multicast Routing
passive interfaces. Only the nonredundant access router topology is supported by the PIM stub featu re.
By using a nonredundant topology, the PIM passive interface assumes that it is the only interface and
designated router on that access domain.
The PIM stub feature is enforced in the IP base image. If you upgrade to a higher software version, the
PIM stub configuration remains until you reconfigure the interfaces.
In Figure 42-2, switch A is configured as a PIM stub router connected to the routed net work. Switch A
routes PIM traffic to Host A, B, and C, which are in VLAN 20.
Figure42-2 PIM Stub Router Configuration
Auto-RP
This proprietary feature eliminates the need to manually configure the RP information in every router
and multilayer switch in the network. For auto-RP to work, you configure a Cisco router or multilayer
switch as the mapping agent. It uses IP multicast to learn which routers or switches in the network are
possible candidate RPs to receive candidate RP announcements. Candidate RPs periodically send
multicast RP-announce messages to a particular group or group range to announce their availability.
Mapping agents listen to these candidate RP announcements and use the information to create entries in
their Group-to-RP mapping caches. Only one mapping cache entry is created for any Group-to-RP range
received, even if multiple candidate RPs are sending RP announcements for the same range. As the
RP-announce messages arrive, the mapping agent selects the router or switch with the highest IP address
as the active RP and stores this RP address in the Group-to-RP mapping cache.
Mapping agents periodically multicast the contents of their Group-to-RP mapping caches. Thus, all
routers and switches automatically discover which RP to use for the groups that they support. If a router
or switch fails to receive RP-discovery messages and the Group-to-RP mapping information expires, it
changes to a statically configured RP that was defined with the ip pim rp-address global configuration
command. If no statically configured RP exists, the router or switch changes the group to dense-mode
operation.
Multiple RPs serve different group ranges or serve as hot backups of each other.
Host A Host B
VLAN 20
Switch A
Routed network
Host C
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