In OSPF over NBMA, why do you choose the Hub as DR in a hub & spoke topology? Also, why do you set the priority to 0 on the spoke routers that do not join them in elections?

Is there any reason?


In a nutshell, the designated router is the one responsible for synchronizing the LS databases of all routers on the network and flooding LS updates to all other routers. If one of the spokes is the designated router, then all database synchronization and LS update traffic must flow down the spoke link to the DR and then back to the hub and then to the other spoke routers. This is very inefficient and will likely overload the spoke link. In addition, often spoke routers are small with limited memory and processing ability, and may not be able to process all the LS traffic. We set the spokes' priority to 0 to prevent them from accidentally becoming the DR.

I recommend you read OSPF and IS-IS by Jeff Doyle. He goes into much more detail on the role of designated routers and their operation.


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