Friday, November 1, 2013

Cisco: Reflecting the Change of Router-ID

It's rate to change the OSPF router ID but nice to know how to change a configured router ID.

OSPF's router ID is chosen based on the following criteria:

  1. Manually configured Router ID
  2. The highest IPv4 address on a loopback interface
  3. The highest IPv4 address on an active (up/up) interface

The router ID is elected when the ospf process starts. Once elected, a manual intervention in the OSPF process is necessary to reflect the change of the router ID. A text book says you need to issue "# clear ip ospf process" after changing the router ID, but in some cases, the command is not sufficient.

When explicitly configured with "(config-router)# router-id ROUTER_ID", "# clear ip ospf process" will work for you.

When implicitly configured with "a loopback interface" or "an active interface", you need to "#reload" the router to make the change effective.

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