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Monday, October 11, 2010

OSPF Designated router (DR) and Backup DR (BDR)

In an OSPF environment all routers floods hellos and LSA's in order to adjacent neighborhood and exchanging routing information.

This kind of behavior is a little problematic , on a multiaccess link, due to the fact that OSPF will form full mesh connectivity between every and each router and will cause waste of bandwidth (with LSA floods).

In order to solve this issue a Designated Router (DR) concept has been made, were an election is preformed, a DR and BDR (Backup DR) are elected, and every router (AllSPFRouter) will adjacent one instance with the DR instead of one instance with every router on a multiaccess link(which can be great sum of instances in a large scale network).

The DR and the BDR election occurs through the use of hello messages, were the router with the highest priority wins. The RID (Router ID) is used as a tie breaker. take note that no preemption can occur, once DR and BDR are elected, no election process is done until both DR and BDR are fails.

After selecting DR and BDR for the multiaccess link, All routers keeps sending hellos to ALLSPFRouters at in order to inform all neighbors about their existence and updates messages to ALLDRouters at, which only the DR and BDR are listen to, and update the rest of the routers (DR Others) through ALLSPFRouters (

the DR/BDR concept doesn't work for every type of topology, for example LAN interfaces default to use an OSPF network type of broadcast. OSPF broadcast networks elect a DR, use Hellos to dynamically find neighbors, and allow more than two routers to be in the same subnet on that LAN.

For HDLC and PPP links, OSPF uses a network type of point-to-point, meaning that no DR is elected, only two IP addresses are in the subnet, and neighbors can be found through Hellos.

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