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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 24 Configuring IP Multicast Routing
Cisco Implementation of IP Multicast Routing
Prune messages are sent up the distribution tree to prune multicast group traffic. This action permits
branches of the shared tree or SPT that were created with explicit join messages to be torn down when
they are no longer needed. For example, if a leaf router (a router without any downstream connections)
detects that it no longer has any directly connected hosts (or downstream multicast routers) for a
particular multicast group, it sends a prune message up the distribution tree to stop the flow of unwanted
multicast traffic.
Auto-RP
This proprietary feature eliminates the need to manually configure the rendezvous point (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 by joining the well-known Cisco-RP-announce
multicast group (224.0.1.39) to receive candidate RP announcements. Candidate RPs send multicast
RP-announce messages to a particular group or group range every 60 seconds (default) to announce their
availability. Each RP-announce message contains a holdtime that tells the mapping agent how long the
candidate RP announcement is valid. The default is 180 seconds.
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 multicast the contents of their Group-to-RP mapping cache in RP-discovery messages
every 60 seconds (default) to the Cisco-RP-discovery multicast group (224.0.1.40), which all Cisco PIM
routers and multilayer switches join to receive Group-to-RP mapping information. Thus, all routers and
switches automatically discover which RP to use for the groups they support. The discovery messages
also contain a holdtime, which defines how long the Group-to-RP mapping is valid. If a router or switch
fails to receive RP-discovery messages and the Group-to-RP mapping information expires, it switches
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.
Bootstrap Router
PIMv2 BSR is another method to distribute group-to-RP mapping information to all PIM routers and
multilayer switches in the network. It eliminates the need to manually configure RP information in every
router and switch in the network. However, instead of using IP multicast to distribute group-to-RP
mapping information, BSR uses hop-by-hop flooding of special BSR messages to distribute the mapping
information.
The BSR is elected from a set of candidate routers and switches in the domain that have been configured
to function as BSRs. The election mechanism is similar to the root-bridge election mechanism used in
bridged LANs. The BSR election is based on the BSR priority of the device contained in the BSR
messages that are sent hop-by-hop through the network. Each BSR device examines the message and
forwards out all interfaces only the message that has either a higher BSR priority than its BSR priority
or the same BSR priority, but with a higher BSR IP address. Using this method, the BSR is elected.
The elected BSR sends BSR messages to the all-PIM-routers multicast group (224.0.0.13) with a TTL
of 1. Neighboring PIMv2 routers or multilayer switches receive the BSR message and multicast it out
all other interfaces (except the one on which it was received) with a TTL of 1. In this way, BSR messages
