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 rendezvous point (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.
When the number of PIM-enabled interfaces exceeds the hardware capacity and PIM-SM is enabled with the
SPT threshold is set to infinity, the switch does not create (source, group (S, G) ) entries in the multicast
routing table for the some directly connected interfaces if they are not already in the table. The switch might
not correctly forward traffic from these interfaces.
PIM Stub Routing
The PIM stub routing feature, available in all software images, reduces resource usage by moving routed
traffic closer to the end user.
The IP Base image contains only PIM stub routing. The IP Services image contains complete multicast
routing. On a switch running the IP Base image, if you try to configure a VLAN interface with PIM
dense-mode, sparse-mode, or dense-sparse-mode, the configuration is not allowed.
Note
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.Note
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 asset and designated router election mechanisms are not supported on the PIM passive interfaces.
Catalyst 2960-XR Switch IP Multicast Routing Configuration Guide, Cisco IOS Release 15.0(2)EX1
OL-29426-01 39
Configuring PIM
PIM Stub Routing
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