| 9.6 | Optional IP Multicast Routing Tasks | ||
| 9.6.2 |
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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.
Where ttl signifies the TTL value in router hops. This value can range from 0 to 255 hops. In the 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.
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