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Cisco IOS Cookbook, 2nd Edition by Ian Brown, Kevin Dooley

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Manipulating DR Selection

Problem

You want to manipulate the Designated Router (DR) selection process on a particular subnet.

Solution

The ip ospf priority configuration command allows you to weight the Designated Router (DR) selection process on a network segment. The following configuration examples are for three different routers that all share the same Ethernet segment. Router5 has the highest OSPF priority, so it will become the DR. Router1 has the second highest priority because we want it to be the Backup Designated Router (BDR).

Router1 is connected to this network segment through a VLAN trunk:

Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface FastEthernet0/0.1
Router1(config-subif)#ip ospf priority 2
Router1(config-subif)#exit
Router1(config)#end
Router1#

We will configure Router3 with a priority of 0. The default priority is 1. A router with priority 0 will never become the DR or BDR:

Router3#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router3(config)#interface FastEthernet0/0.1
Router3(config-subif)#ip ospf priority 0
Router3(config-subif)#exit
Router3(config)#end
Router3#

Router5 has the highest priority, so it will become the DR for the segment:

Router5#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router5(config)#interface Ethernet0
Router5(config-if)#ip ospf priority 10
Router5(config-if)#exit
Router5(config)#end
Router5#

Discussion

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