The TTL value controls whether packets are forwarded out of an interface. You specify the TTL value in hops. Only multicast packets with a TTL greater than the interface TTL threshold are forwarded on the interface. The default value is 0, which means that all multicast packets are forwarded on the interface. To change the default TTL threshold value, use the following interface configuration command.
R1(config)# interface GigabitEthernet0/1
R1(config-if)# ip multicast ttl-threshold 6
R1(config-if)# exi
When setting up Auto-RP, you can also limit the scope with TTL. To set up a router as an RP and limit the scope of the RP announcements to 4 hops you would use the following:
R1(config)# ip pim send-rp-announce GigabitEthernet0/1 scope 4
R1(config)# exit
Also within Auto-RP you can set the scope on the RP Mapping AGent. All routers within the ttl defines will receive the Auto-RP discobery messages.
R1(config)# ip pim send-rp-discovery scope 12
R1(config)# exit