Multicast – Designated Router (DR)

The role of the designated router has been bothering me every now and again. I always end up forgetting its role, where its present, and how its elected. So  i thought i would post some info on it, maybe to keep my own mind fresh 🙂

In any lan environment, a designated router will be elected by PIM (Protocol Independent Multicast). Its role is to handle joins in a sparse-mode environment, and if IGMP version 1 is used, to elect the querier of the lan. If version 2 (or 3) is being used, the lowest IP address will be the lan querier.

The DR will also be the one that sends a source-register message to the rendevouz point, on behalf of a source.

By default, your DR will be the one with the highest IP address. In newer IOS releases, a priority can be set on the interface level to change which one will be the DR. The command is “ip pim dr-priority ”. Default priority is 1.

I hope this small post will provide some useful information, at least for reference. I certainly hope it will for me.