Aug 18, 2009

Determining DR and BDR in OSPF Router

In OSPF Routing protocols, Shortest Path First Algorithm (SPF) is used to determine DR and BDR in OSPF Router. When link-state of a Router on the area change. The Router will be send this change (LSA packet) to multicast address 224.0.0.5 (DR and BDR will receive this information). Then DR in the area will flood this change to multicast address 224.0.0.6 to all other routers (DRother). This will help decrease bandwidth in the area which cause is LSA packet. DR and BDR is determined when Routers exchange Hello Packet. DR and BDR is determined by the following steps:

1. Router that have the heighest priority will become DR and the second is BDR. The following command can configure this parameter in the interface mode:
ip ospf priority
2. In multiaccess environments, all routers is connected together by fastethernet interface. By default, the value of Priority of the Routers is equal. In this case, OSPF Router determine the heighest Router ID of any routers as DR and the second as BDR.
To verify DR and BDR, you can execute the following command in EXEC mode:
show ip ospf neighbor
Note: When DR and BDR is defined, the Router do not automatically change DR and BDR regardless the current DR and BDR have lowest Priority or Router ID of value. You can change manually by execute the command in EXEC mode:
clear ip ospf process
type yes to comfirm
I hope these information are help you clearly understand DR and BDR in OSPF Router. Any question is welcome.


Author : Lâm Viết Thảo // 10:47 AM
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