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Understanding Cisco's Implementation of IP Multicast Routing
PIM Modes
PIM can operate in dense mode (DM), sparse mode (SM), or in sparse-dense mode (PIM DM-SM),
which handles both sparse groups and dense groups at the same time.
PIM DM
PIM DM builds source-based multicast distribution trees. In dense mode, a PIM DM router or multilayer
switch assumes that all other routers or multilayer switches forward multicast packets for a group. If a
PIM DM device receives a multicast packet and has no directly connected members or PIM neighbors
present, a prune message is sent back to the source to stop unwanted multicast traffic. Subsequent
multicast packets are not flooded to this router or switch on this pruned branch because branches without
receivers are pruned from the distribution tree, leaving only branches that contain receivers.
When a new receiver on a previously pruned branch of the tree joins a multicast group, the PIM DM
device detects the new receiver and immediately sends a graft message up the distribution tree toward
the source. When the upstream PIM DM device receives the graft message, it immediately puts the
interface on which the graft was received into the forwarding state so that the multicast traffic begins
flowing to the receiver.
PIM SM
PIM SM uses shared trees and shortest-path-trees (SPTs) to distribute multicast traffic to multicast
receivers in the network. In PIM SM, a router or multilayer switch assumes that other routers or switches
do not forward multicast packets for a group, unless there is an explicit request for the traffic (join
message). When a host joins a multicast group using IGMP, its directly connected PIM SM device sends
PIM join messages toward the root, also known as the RP. This join message travels router-by-router
toward the root, constructing a branch of the shared tree as it goes.
The RP keeps track of multicast receivers. It also registers sources through register messages received
from the source's first-hop router (designated router [DR]) to complete the shared tree path from the
source to the receiver. When using a shared tree, sources must send their traffic to the RP so that the
traffic reaches all receivers.
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.
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 cache. Thus, all
routers and switches automatically discover which RP to use for the groups they support. If a router or
switch fails to receive RP-discovery messages and the Group-to-RP mapping information expires, it
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
34-4
Chapter 34
Configuring IP Multicast Routing
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