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Configuring IP Multicast Routing
Information About Cisco’s Implementation of IP Multicast Routing
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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.
PIM Stub Routing
The PIM stub routing feature reduces resource usage by moving routed traffic closer to the end user.
In a network using PIM stub routing, the only allowable route for IP traffic to the user is through a switch that is configured
with PIM stub routing. PIM passive interfaces are connected to Layer 2 access domains, such as VLANs, or to interfaces
that are connected to other Layer 2 devices. Only directly connected multicast (IGMP) receivers and sources are allowed
in the Layer 2 access domains. The PIM passive interfaces do not send or process any received PIM control packets.
When using PIM stub routing, you should configure the distribution and remote routers to use IP multicast routing and
configure only the switch as a PIM stub router. The switch does not route transit traffic between distribution routers. You
also need to configure a routed uplink port on the switch. The switch uplink port cannot be used with SVIs. If you need
PIM for an SVI uplink port, you should upgrade to the IP services feature set.
You must also configure EIGRP stub routing when configuring PIM stub routing on the switch.
The redundant PIM stub router topology is not supported. The redundant topology exists when there is more than one
PIM router forwarding multicast traffic to a single access domain. PIM messages are blocked, and the PIM assert and
designated router election mechanisms are not supported on the PIM 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.