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can this one-armed router cause multicast failure?

Unanswered Question

pls look at diagram attached.

Router A and B are branch routers. B is PIM DR on the LAN becuase it has higher IP 10.36.200.3. There is only sparse-mode running and RP is WAN2 across the WAN links.


1. Normally, multicast tree is built from router B to WAN2(RP)

2. But if link B goes down, router B must build tree from B, A, WAN1, core-rtr1 then WAN2

3. however would this ever work? because when link B goes down, router B is one-armed with only interface F1/0 connected. But I was told interface F1/0 can't be incoming and outgoing interfaces at the same time. As a result, the tree can never be set up.

is it true?


3. if true, does it mean this design is flawed for multicast?


4. remedy?

a backup link between router A and B via interface F1/1?

Thus router B can build tree via interface F1/1 which also becomes incoming int while F1/0 is outgoing interface




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Harold Ritter Fri, 12/15/2006 - 18:13
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This design is perfectly valid even without the additional link between router A and B (F1/1 to F1/1).


If you loose link B, router B remains the DR and sends a PIM join towards router A as it receives the IGMP report message from the receiver. Router A puts F1/0 in the OIList for the specific group and therefore the receiver gets the multicast stream directly from Router A. As you stated router B doesn't put F1/0 in its OIList as this interface is also the incoming interface (rpf interface).


Hope this helps,

Harold Ritter Sun, 12/17/2006 - 16:53
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  • Cisco Employee,

1. Router B doesn't need to build the mcast tree since it is not in the forwarding path. It simply needs to send the PIM join to router A when it gets the IGMP report message from the mcast listener (aka mcast receiver).


2. As I already stated, this design is perfectly valid even when link B goes down.


Hope this helps,

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