Multiple Routers on a Network

The possibility was raised in the preceding section that multiple routers might be attached to a subnet, all of which need to know whether group members are present Figure 5-10 shows an example Two routers are attached to the subnet, both of which receive the same multicast stream from the same source over different routes If one router or route fails, the group members can continue to receive their multicast session from the other router Under normal circumstances, however, it is inefficient for both routers to forward the same data stream onto the subnet

Figure 5-10 Two Routers Receive the Same Multicast Session, but Only One Forwards It onto the Subnet

Figure 5-10 Two Routers Receive the Same Multicast Session, but Only One Forwards It onto the Subnet

The routers are aware of each other because of their routing protocols So one way to ensure that only one router forwards the session onto the subnet is to add a designated router, or querier, function to the multicast routing protocol The querier is responsible for forwarding the multicast stream The other router or routers only listen, and they begin forwarding the stream only if the querier fails

The problem with allowing the routing protocol to elect a querier is that multiple IP multicast routing protocols are available If the two routers in Figure 5-10 are running incompatible protocols, their respective querier election processes will not detect each other, each will decide that it is the querier, and both will forward the data stream

The local group management protocol, however, is independent of the routing protocols The routers have to run this common protocol to query group members, so it makes sense to give the querier function to the group management protocol This guarantees that the routers are speaking a common language on the subnet and can agree on which is responsible for forwarding the session

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