Cisco Systems 3750E manual Auto-RP, 42-6

Chapter 42 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 feature. 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 network. Switch A routes PIM traffic to Host A, B, and C, which are in VLAN 20.

Figure 42-2 PIM Stub Router Configuration

Routed network

Switch A

Host A Host B Host C

VLAN 20

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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-addressglobal 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.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

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