OSPFv3 question regarding the Router ID

[erkange005]

This OSPF is mind boggling.  Explanation on this question please:

An engineer wants to set the OSPFv3 router ID for router R1. Which of the following answers could affect R1’s choice of OSPFv3 router ID?

a. The ipv6 address command on interface Gigabit0/0

b. The ip address command on interface Serial0/0/1

c. The ospf router-id command in OSPFv3 configuration mode

d. The ipv6 address command on interface loopback2

The answer is B.  Why not the addresses for A or D?  I am confused on the correlation between the serial interface possibly affecting the choice for the Router ID.  

Thank you in advance

[Jon Marshall]

It's not the serial interface part of the question it is the ip address part.

So the other questions refer to an IPv6 address whereas that question refers to an ip address ie. an IPv4 address.

OSPFv3 is backwards compatible with OSPFv2 so the router id is chosen -

1) if the interface on which OSPFv3 is enabled has an IPv4 IP address then it is used

2) if there are multiple IPv4 IPs available then the same rules apply as for OSPFv2 in choosing which one to use.

Note that is from the documentation and it does not state clearly whether that means multiple IPs under the interface or just multiple IPs ie. they can be assigned to other interface.

I am assuming it means any IPv4 IP address on the router.

Jon