Designated Router & Backup Designated Router in OSPF

Designated Router & Backup Designated Router

Before you start

Objectives : You will be able to understand how OSPF DR & BDR elects, and what are different tasks of DR & BDR routers.

Prerequisites: You have to know about OSPF protocol.

Tags : OSPF , Routing Protocol, Designated Router , Backup Designated Router.

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Designated Router

Every broadcast and NBMA network has a Designated Router. The Designated Router performs two main functions for the routing protocol:
1.The Designated Router originates a network-LSA on behalf of the network. This LSA lists the set of routers (including the Designated Router itself) currently attached to the network. The Link State ID for this LSA is the IP interface address of the Designated Router. The IP network number can then be obtained by using the network's subnet/network mask.
2. The Designated Router becomes adjacent to all other routers on the network. Since the link state databases are synchronized across adjacencies (through adjacency bring-up and then the flooding procedure), the Designated Router plays a central part in the synchronization process.

Backup Designated Router

In order to make the transition to a new Designated Router smoother, there is a Backup Designated Router for each broadcast and NBMA network. The Backup Designated Router is also adjacent to all routers on the network, and becomes Designated Router when the previous Designated Router fails. If there were no Backup Designated Router, when a new Designated Router became necessary, new adjacencies would have to be formed between the new Designated Router and all other routers attached to the network. Part of the adjacency forming process is the synchronizing of link-state databases, which can potentially take quite a long time. During this time, the network would not be available for transit data traffic. The Backup Designated obviates the need to form these adjacencies, since they already exist. This means the period of disruption in transit traffic lasts only as long as it takes to flood the new LSAs (which announce the new Designated Router).

DR & BDR election process:

1. Highest Priority. [1-255].
2. If routers have equal priority, then it looks for highest router ID.
Note: BDR is elects via a next highest priority, again if priority equals then it looks for next highest router ID.

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Reference : https://www.ietf.org/rfc/rfc2328.txt