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| 7.3 |  | Routing Multicast Traffic |
| 7.3.14 |  | Optional multicast routing tasks |

Define Scope of Delivery of Multicast Packets
The function of the TTL parameter on a multicast-enabled interface controls whether packets are forwarded out of that interface.

Only multicast packets with a TTL greater than the interface TTL threshold are forwarded on the interface. Those packets with a TTL equal to or less than the interface threshold are discarded. The packet TTL is compared with the interface threshold first. The router then decrements the packet TTL upon sending the packet out the interface.

The default value for the TTL threshold on an interface is zero (0). A value of zero means that all multicast packets are forwarded on the interface. To change the default TTL threshold value, enter the following command in interface configuration mode.

> Router(config-if)#ip multicast ttl-threshold ttl

Where ttl signifies the TTL value in router hops. This value can range from 0 to 255 hops.

In Figure , the TTL threshold is set to 16 router hops.

To restore the default value of zero (0), enter the no ip multicast ttl-threshold command in interface configuration mode.

> Note: Because the TTL field is used to limit the range, or scope, of a multicast transmission, configure the TTL threshold only on border routers. Routers on which a TTL threshold value is configured automatically become border routers.

Join a Multicast Group
Cisco routers can be configured to be members of a multicast group. Being a member of a multicast group is useful in determining multicast reachability in a network. If a router is configured to be a member of a specific multicast group, that router can respond to commands, such as ping and Internet Control Message Protocol (ICMP) echo requests, addressed to that group. A group member router can also participate in multicast Cisco IOS traceroute actions.

To have the router join a multicast group, enter the following command in interface configuration mode.

> Router(config-if)#ip igmp join-group group-address

> (group-address is the address of the multicast group)

The configuration example in Figure places the router in multicast Group 224.1.1.1.

Issuing a ping command specifying that multicast group address will cause all routers in that group to respond.

To cancel the membership of a router in a multicast group, enter the no ip igmp join-group group-address command.

> Note: The term interface describes the primary interface on an attached network. If a router has multiple physical interfaces on a single network, this protocol needs to run on only one of the interfaces. Hosts, on the other hand, need to perform their actions on all interfaces that have memberships associated with them.

Change IGMP Versions
Cisco routers do not automatically detect the IGMP version of systems on the subnet and dynamically switch between versions.

IGMP Version 2 mode is the default for all systems using Cisco IOS Release 11.3(2)T or later. IGMP Version 3 is available with release 12.0(15)S or later. To determine the current version setting for an interface, enter the following command in privileged EXEC mode.

> Router#show ip igmp interface type-number

> (type-number represents the name of the interface on which the IGMP protocol is configured)

An example of this command is shown in Figure .

To enable a specific IGMP Version enter the following command in interface configuration mode.

> Router(config-if)#ip igmp version {3 | 2 | 1}

To restore the default value of Version 2, enter the no ip igmp version command.

Because of its unique capability to send out a single data stream to multiple clients, IP multicast technologies have become the transmission method of choice for most multimedia applications.