Refer to the exhibit. Router E learned about the PIM RP (designated as 7.7.7.7) from four different sources. Routers A and D advertised the 7.0.0.0 network via EIGRP. Routers B and C advertised the 7.0.0.0 network via OSPF. Considering that all four Ethernet interfaces on

router E could potentially lead back to the PIM-RP, when router E receives the first multicast packet down the shared tree, which incoming interface will be used to successfully pass the RPF check?

A.
E0

B.
E1

C.
E2

D.
E3

E.
None of these interfaces will be used to successfully pass the RPF check.

F.
All of these interfaces would successfully pass the RPF check.

Explanation:
On a shared LAN segment, amongst the PIM enabled routers, a selected router must assume the responsibilty for i) sending any PIM register/prune messages to the RP and for ii) sending IGMP query messages.

I was until recently under the misunderstanding that the PIM DR router performed both of these functions – wrong!! These functions are completely decoupled and in fact they have a different election process and selection criteria.

First the Querier Election Process.
At start up each router sends a query message to the all systems group 224.0.0.1 from its own interface address. The router with the lowest ip address is elected IGMP querier.

Second the PIM DR Election Process
The router with the highest ip address is elected as PIM DR. This selection process can also be influenced by configuring a pim DR priority. By default all routers have priority 1, hence highest ip address wins by default. However if DR priority is used then highest DR priority wins.