Pim Stub Routing - Cisco 3032 Software Configuration Manual

Chapter 44 Configuring IP Multicast Routing
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.
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 (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
Note
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.
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. For more
information, see the
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. 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.
The PIM stub feature is enforced in the IP base image. If you upgrade to a higher software version, the
PIM stub configuration remains until you reconfigure the interfaces.
OL-12247-04
"EIGRP Stub Routing" section on page
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
Understanding Cisco's Implementation of IP Multicast Routing
38-43.
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