Four ways to affect DRBDR election

1. Manually configure IP OSPF priority

2. Manually configure router ID

3. Highest IP address on a loopback interface

4. Highest IP address on a physical interface

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Cisco CCIE Prep v1.0—Module 7-18

OSPF uses the highest Internet Protocol (IP) address configured on an interface as its router ID. If the interface associated with this IP address is ever brought down, or if the address is removed, the OSPF process must recalculate a new router ID and resend all of its routing information back out.

If a loopback interface is configured with an IP address, the Cisco IOS software will use this IP address as its router ID, even if other physical interfaces have higher IP addresses. Due to the fact that loopback interfaces never go down, greater stability in the routing table is achieved using a loopback interface as the router ID.

OSPF automatically prefers a loopback interface over a physical interface of any kind, and it chooses the highest IP address among all loopback interfaces if multiple loopbacks exist. If no loopback interfaces are present, the highest IP address on a physical interface is chosen as the router ID. You cannot tell OSPF to use any particular interface as the router ID; however, you can manually set the router ID with the router-id command.

The best way to control the DR/BDR election is to manually change the priority on the interface. The default priority on an interface is 1. Higher values have higher priority. This value can be set to any number from 0 - 255. A setting of 0 prevents the router from participating in the DR/BDR election.

Table 6-2: DR/BDR Election Commands

Command

Description

interface loopback 0

Creates a virtual loopback interface on the router

ip address ip-address mask

Assigns an IP address to this interface

router-id A.B.C.D

Allows you to manually set the router ID on a router. The router ID does not have to be an IP address that exists on the router, but must be in dotted decimal format, similar to an IP address. Performed from router configuration mode

ip ospf priority <0255>

Interface configuration command that controls the likelihood of the router becoming the DR or BDR for a network segment

Note Once the OSPF process is started on a router and neighbor adjacencies have been formed, the router ID will not change. For example, if an IP address is added to the router that is higher than the current router ID, the router ID will not change. However, the router ID will change to the new IP address when the OSPF process is restarted or the router is reloaded. This will invalidate items such as virtual-links that were configured with the old router ID. Virtual-links will be covered in the next lesson.

Configuring Optional Commands

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