OSPF DR BDR Election

OSPF DR and BDR

The Designated Router concept is a link-specific concept, not for the whole router. A router can be a DR on one link, a BDR on second link and a DROTHER on a third link.

DR and BDR election

The router with the highest OSPF priority becomes the DR. The router with the second highest OSPF priority becomes the BDR.

If there is a tie, then the election is based on router IDs: the router which has the higher router ID wins and becomes the DR. The second router in this election becomes the BDR.

All routers, including the BDR, form full adjacency with the DR.

The BDR stays in standby. When it does not hear LSAs from the DR for the duration of the wait timer, it assumes the current DR has failed and promotes itself as a new DR.

All routers, except the DR and the BDR, form two-way adjacency with each other.

A DROTHER is a router that is neither DR nor BDR.

DR Operations

ospf-tutorial-broadcast-multiaccess-networks-without-drothers

LSA exchange without DROTHERS

When a DR is elected, all routers send LSUs to it using multicast address 224.0.0.6. The DR sends back to each one of them an LSAck and assumes responsibility for multicasting the LSAs to all other routers. All non-DROTHER routers form a two-way adjacency with each other. In the diagram below, we concentrate only on router 3.

Router 3 sends its LSAs to the multicast address of 224.0.0.6. Router 5, which is the DR, is listening on that multicast address. Router 5 sends back a LSAck to Router 3, then multicasts Router 3’s LSA information at IP address 224.0.0.5.

ospf-tutorial-broadcast-multiaccess-networks-with-drothers

LSA exchange with DROTHERS

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