Overview

A router must use a unique OSPFv3 Router ID (RID) to properly communicate with other OSPFv3 routers.

The RID is 32 bits and has the same format as an IPv4 address.

The priority of RID assignment, in order, is as follows:

  1. Manual Assignment
  2. If the RID is not manually assigned, the highest IPv4 address assigned to a loopback interface is automatically assigned
  3. If the RID isn't assigned in step 1 or 2 above, the highest IPv4 address assigned to a physical interface is automatically assigned

It’s a best practice to manually assign the RID. If the router automatically assigns the RID from an interface, big problems could occur in the future if the addressing changes.

Pro Tip:
When merging networks, be on the lookout for duplicate RIDs. Every router in an OSPF domain should have a unique RID.

Configuration and Configuration

First and foremost, ipv6 unicast-routing command must be issued:

R-1(config)#ipv6 unicast-routing

Now that ipv6 unicast-routing has been enabled, we can create an OSPFv3 process:

R-1(config)#ipv6 router ospf 6
R-1(config-rtr)#
*Jul  9 03:39:59.422: %OSPFv3-4-NORTRID: Process OSPFv3-6-IPv6 could not pick a router-id, please configure manually

After creating the OSPFv3 process, we enter OSPFv3 router configuration mode (see prompt).

Notice the level 4 syslog message that tells us the router couldn't pick a RID value from an interface for auto-assignment. It then requests to manually assign a RID.

Pro Tip: Don't forget, it's a best practice to manually configure the RID even if there are IPv4 addresses on an interface that can be used for auto-assignment.

The OSPFv3 router ID has the same format as a 32 bit IPv4 address:

R-1(config-rtr)#router-id 1.1.1.1
R-1(config-rtr)#end

That's all there is to it!

Let's verify:

R-1#show ipv6 ospf database

            OSPFv3 Router with ID (1.1.1.1) (Process ID 6)

The show ipv6 ospf database command cleanly shows us the OSPFv3 RID is 1.1.1.1 is active within PID 6.

We can also view the RID under the OSPFv3 PID 6 in the running-configuration:

R-1#show run | section ospf
ipv6 router ospf 6
 router-id 1.1.1.1

Notice the RID is under the OSPFv3 router process and not an IPv4 OSPF process.

Summary

Every router in an OSPFv3 domain needs a Router ID (RID) and it should be unique.

These RIDs have the same format as an IPv4 address.

It is a best practice to manually assign the RID, even if the router can automatically assign a RID from an interface.

The RID should be assigned immediately after creating the OSPFv3 process. Neighborships can't be formed without it.

RFC 7503: OSPFv3 Autoconfiguration
Cisco: Sample Configuration for OSPFv3

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