would this one-arm router caused multicast failure?

[ewang]

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 and RP is WAN2 across the WAN links.

1. Normally, multist 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

[paolo bevilacqua]

Ewang,

I'm not good at mcast enough to give you an answer about PIM, but I can tell you that if the concern is packets coming in and out the same interface, you should set the redundant link. It does not need to be physical, you can use .1q across switches or directly, just configure the interfaces for tagging.