OS10 Enterprise Edition User Guide Release 10.4.0E(R3)

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Designated and backup designated routers

OSPF elects a designated router (DR) and a backup designated router (BDR). The DR is responsible for generating LSAs for the entire multiaccess network. Designated routers allow a reduction in network traffic and in the size of the topological database.

Designated router
Maintains a complete topology table of the network and sends updates to the other routers via multicast. All routers in an area form a slave/master relationship with the DR. Every time a router sends an update, the router sends it to the DR and BDR. The DR sends the update out to all other routers in the area.
Backup designated router
Router that takes over if the DR fails.

Each router exchanges information with the DR and BDR. The DR and BDR relay information to other routers. On broadcast network segments, the number of OSPF packets reduces by the DR sending OSPF updates to a multicast IP address that all OSPF routers on the network segment are listening on.

The DRs and BDRs are configurable. If you do not define DR or BDR, OS10 assigns them per the protocol. To determine which routers are the DR and BDR, the OSPF looks at the priority of the routers on the segment —default router priority is 1. The router with the highest priority is elected the DR. If there is a tie, the router with the higher router ID takes precedence. After the DR is elected, the BDR is elected the same way. A router with a router priority set to zero cannot become the DR or BDR.


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