Configuring IP Multicast
How to enable IP multicast

By default, a Layer 3 device will isolate multicast traffic to the segment on which it was generated, not forwarding it across the router to other network segments. Enabling IP multicast routing allows a Layer 3 device to forward multicast packets based upon the configuration of the Multicast routing protocol. The general steps to enabling and verifying multicast routing are outlined:

1. Enable IP Multicast Routing
A single command is used from global configuration to enable multicast routing:

Switch(config)#ip multicast-routing

2. Enable a Multicast Routing Protocol
The multicast routing protocol, which is PIM on a campus network, establishes the rules by which multicast traffic will be forwarded onto various network segments by the Layer 3 device. An interface can be configured to operate in PIM dense mode, sparse mode, or sparse-dense mode. The mode determines how the Layer 3 device populates its multicast routing table and how it forwards multicast packets received from directly connected segments. Enabling PIM on an interface also enables IGMP operation on that interface.

At interface configuration mode, configure the PIM mode of operation for the interface. 

When the switch populates the multicast routing table, dense-mode interfaces are always added to the table. Sparse-mode interfaces are added to the table only when periodic join messages are received from downstream routers, or when there is a directly connected member on the interface. If configured for sparse or sparse-dense mode, multicast sparse mode operation will occur if there is a RP known for the group. If so, the packets are encapsulated and sent toward the RP. When no RP is known, the packet is flooded in a dense-mode fashion. By default, multicast routing is disabled on an interface.

3. Configure the RP for Sparse Mode Operation
Routers with group members on its direction-connected interfaces as the RP address to send PIM register messages up the tree. On routers in the tree that are closest to the RP, the RP address is used to send PIM join and prune messages to the RP, and to inform it about group membership down the tree from that point. The RP address must be configured on all routers anywhere in the tree that will carry traffic from a source to its member (including the RP router). 

4. Verify IP Multicast Operations
These commands are available to verify and monitor IP multicast operations such as; operational statistics, resource utilization, multicast database content, troubleshooting information, node reachability and Multicast routing paths. Some of the commands are further delineated in the lesson.

5. Verify PIM
Figure shows output from the show ip pim interface command.

Figure shows output from the show ip pim interface command with a count.

Figure shows sample output from the show ip pim interface command with a count when IP multicast is enabled. The example lists the PIM interfaces that are fast switched and process switched, and lists the packet counts for these. H indicates that multicast enabled interfaces.

6. Verify Multicast Routing & Clearing the Routing Table

Figure shows the output from several iterations of the show ip mroute command.

NOTE:

Output interface timers are not updated for hardware-forwarded packets. Entry timers are updated approximately every 5 seconds.

Figure  shows sample output from the show ip mroute command with the summary keyword.

Figure  shows sample output from the show ip mroute command with the active keyword.

Figure  shows sample output from the show ip mroute command with the count keyword:

Clearing the Multicast Routing Table
After configuration changes are made, the IP multicast tables may need to be cleared before accurate table information will display. This is particularly true when changes are made at several contiguous routers. Use the following EXEC mode command to clear the tables:

Switch#clear ip mroute

After executing this command, use the show ip mroute command to display new multicast routing table information built after the clear command was executed.