Multicast stub router

If we have R1>R2>R3, where R1 is an HQ main router and R2 a remote office router, there is little point in asking R2 to spend resources on multicast states, since all it does is forward packets to and from the HQ.

  1. We should therefore configure R8 to ask R5 to hold its IGMP group states with the command:

ip igmp helper-address <address of the link of R5 towards R8>

2. Then let’s make R8 as ”stupid” as possible by making its PIM dense towards R5 and R10 (where hosts are connected).

int eth0/0.58

ip pim dense-mode

int eth0/0.108

ip pim dense-mode

3. Next, let’s prevent R8 from establishing a neighbor relationship with R5 or R10 (it should only forward multicast packets!) with:


ip pim neighbor-filter 58

eth 0/0.108

ip pim neighbor filter 108


access-list 58 deny

access-list 58 permit any

access-list 108 deny

access-list 108 permit any

4. On R10, PIM DM must also be enabled on the link towards R8. Let’s also join a group.


ip pim dense-mode

ip igmp join-group


Now R5 will see the hosts as if they were directly connected to R5. R5 will use sparse mode to create the tree, R8 will use dense mode, and R10 will use dense mode to process received multicast traffic.


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