Stub multicast routing is useful in situations where you have small remote sites connected to a centralized WAN cloud across low-bandwidth links that use low-end routers. Such remote sites never perform any transit function, and they may suffer from resource starvation caused by PIM DM’s flood-and-prune behavior, or PIM SM RP announcements, RP cache growth, and mroute state proliferation. The command to achieve this is ip igmp helper-address <Upstream-IP>.
Assume R1 is the central router and R2 is the stub router with directly connected receivers. Under a normal configuration, If this was a PIM-DM domain R2 would still be susceptible to the flood and prunce behavior of R1. If this were a PIM-SM domain, R8 would have to accept RP discovery message, build its own RP cache, and maintain states for all multicast groups joined by local receivers. Being a Stub exempts R2 from PIM and IGMP processing.
R1(config)# access-list 12 deny 155.1.1.2
R1(config)# access-list 12 permit any
R1(config)# interface GigabitEthernet0/1.12
R1(config-if)# ip pim sparse-mode
R1(config-if)# ip pim neighbor-filter 12
R1(config-if)# exit
R1(config)# interface GigabitEthernet0/1.12
R1(config-if)# ip pim dense-mode
R1(config-if)# ip igmp helper-address 155.1.1.1
R1(config-if)# exit
R1(config)# interface GigabitEthernet0/2
R1(config-if)# description TO_CLIENTS
R1(config-if)# ip pim dense-mode
Notice how R2 is configured using PIM-DM for both its upstream interface to the igmp helper and downstream to its client. This will allow the router to flood ANY multicast traffic received from upstream sources down to clients; this is based on default PIM DM behavior, because there is no downstream router to prune the tree.
Also notice how R1 is configured with PIM enabled on its downstream interface, but with a special neighbor filter applied, to make sure the upstream and the downstream routers never form a PIM adjacency. This allows the upstream router to flood multicast traffic downstream, because IOS will not send a multicast packet out of a non-PIM interface. At the same time, the downstream router will not be able to prune any group using PIM signaling.
The next effect is that the upstream router (R1) builds a multicast distribution tree on behalf of the downstream router (R2) by virtue of the IGMP proxy function performed by the downstream router. At the same time, no excessive load is being put on the stub router or the stub link, effectively saving bandwidth and router resources. Essentially, R1 performs all multicast routing functions for R2, and the latter is only used as a “dumb” packet forwarder.