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OSPF (Router ID)

ohitzjohn
Level 1
Level 1

OSPF the router ID is assigned base on what?

I remember that it was based on first loopback address.

if there're two loopback address than the first one is picked or the one with the highest IP address get picked?

Also what the point of loopback address for OSPF?

1 Accepted Solution

Accepted Solutions

blau grana
Level 7
Level 7

Hello John,

As Matthew said, this is order in which router choose OSPF ID.

1. configured router ID

2. IP of highest up/up loopback

3. IP of highest up/up physical interface

As you know, OSPF is link state routing protocol, it knows entire topology which belongs to same area. This topology is created based on router ID. So router ID for each router has to be stable. You can imagine that if router has to change router ID (physical interface went down) entire topology on all routers has to be rebuilt.

Best Regards

Please rate all helpful posts and close solved questions

Best Regards Please rate all helpful posts and close solved questions

View solution in original post

2 Replies 2

matthewathom
Level 1
Level 1

The router ID is based on the highest IP address configured on the router unless there is a loopback configured. Then it is the highest loopback address.

Physical interfaces can go down though. A logical interface like a loopback is always up. That's why you want your router ID attached to a loopback interface.

Sent from Cisco Technical Support iPhone App

blau grana
Level 7
Level 7

Hello John,

As Matthew said, this is order in which router choose OSPF ID.

1. configured router ID

2. IP of highest up/up loopback

3. IP of highest up/up physical interface

As you know, OSPF is link state routing protocol, it knows entire topology which belongs to same area. This topology is created based on router ID. So router ID for each router has to be stable. You can imagine that if router has to change router ID (physical interface went down) entire topology on all routers has to be rebuilt.

Best Regards

Please rate all helpful posts and close solved questions

Best Regards Please rate all helpful posts and close solved questions
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