Cisco Systems 78-11741-02 manual Configuring PIM Dense Mode State Refresh, IPC-405

Configuring IP Multicast Routing

Enabling PIM on an Interface

Command	Purpose
Router(config-if)#ip pim sparse-dense-mode	Enables PIM to operate in sparse or dense mode, depending on
	the group.

Configuring PIM Dense Mode State Refresh

If you have PIM dense mode (PIM-DM) enabled on a router interface, the PIM Dense Mode State Refresh feature is enabled by default.

PIM-DM builds source-based multicast distribution trees that operate on a “flood and prune” principle. Multicast packets from a source are flooded to all areas of a PIM-DM network. PIM routers that receive multicast packets and have no directly connected multicast group members or PIM neighbors send a prune message back up the source-based distribution tree toward the source of the packets. As a result, subsequent multicast packets are not flooded to pruned branches of the distribution tree. However, the pruned state in PIM-DM times out approximately every 3 minutes and the entire PIM-DM network is reflooded with multicast packets and prune messages. This reflooding of unwanted traffic throughout the PIM-DM network consumes network bandwidth.

The PIM Dense Mode State Refresh feature keeps the pruned state in PIM-DM from timing out by periodically forwarding a control message down the source-based distribution tree. The control message refreshes the prune state on the outgoing interfaces of each router in the distribution tree.

This feature also enables PIM routers in a PIM-DM multicast network to recognize topology changes (sources joining or leaving a multicast group) before the default 3-minute state refresh timeout period expires.

By default, all PIM routers that are running a Cisco IOS software release that supports the PIM Dense Mode State Refresh feature automatically process and forward state refresh control messages. To disable the processing and forwarding of state refresh control messages on a PIM router, use the ip pim state-refresh disable global configuration command.

To configure the origination of the control messages on a PIM router, use the following commands beginning in global configuration mode:

	Command	Purpose
Step 1
	Router(config)# interface type number	Specifies an interface and places the router in interface
		configuration mode.
Step 2
	Router(config-if)#ip pim state-refresh	Configures the origination of the PIM Dense Mode State Refresh
	origination-interval [interval]	control message. Optionally, you can configure the number of
		seconds between control messages by using the interval
		argument. The default interval is 60 seconds. The interval range
		is from 4 to 100 seconds.

Note The origination interval for the state refresh control message must be the same for all PIM routers on the same LAN. Specifically, the same origination interval must be configured on each router interface that is directly connected to the LAN.

See the “PIM Dense Mode State Refresh Example” section later in this chapter for an example of how to configure the PIM Dense Mode State Refresh feature.

Cisco IOS IP Configuration Guide

IPC-405