PIM Sparse Mode – Designated Routers

Where there are multiple PIM-SM routers on a subnet, an election is called to determine a designated router (DR). The designated router helps prevent duplicates of multicast traffic from being sent to to the rendezvous point.

In an election between PIM sparse mode routers, the highest priority wins the election. By default the priority of PIM routers is 1. If all routers in a subnet have the same priority, the election tiebreaker is the highest IP address in the subnet.

On the first hop router, the designated router is responsible for encapsulating register messages (in unicast) that are originated by a source, with a destination of the rendezvous point.

On a last hop router, the designated router is responsible for sending PIM join and prune messages towards the rendezvous point to inform it about group membership. The last hop router is also responsible for a shortest tree path switch-over.

If there are no designated routers, all the of the last hop routers on the same subnet would send PIM joins upstream. This may result in duplicate multicast traffic arriving on the LAN.

For the first hop routers, if there are no elected designated routers, multiple register messages to the rendezvous point can occur.

If there are no hello messages sent in 105 seconds, 3.5 times the hello interval, a new designated router will be elected.




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