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Cisco ME 3800X and 3600X Switch Software Configuration Guide
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Chapter 33 Configuring IP Multicast Routing
Understanding Cisco’s Implementation of IP Multicast Routing
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Sparse mode and dense mode are properties of a group, as opposed to an interface. We strongly
recommend sparse-dense mode, as opposed to either sparse mode or dense mode only.
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PIM join and prune messages have more flexible encoding for multiple address families.
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A more flexible hello packet format replaces the query packet to encode current and future
capability options.
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Register messages to an RP specify whether they are sent by a border router or a designated router.
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PIM packets are no longer inside IGMP packets; they are standalone packets.
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.
