configuring ipv6 ospf6 cost on loopback interface does not work

[pguibert6WIND]

FRR upstream
ubuntu-22.04

Describe the bug

Configuring ipv6 cost over loopback interface is useless.
The value seems to be reset when pursuing the configuration.

To Reproduce

ubuntu2204(config)# interface lo
ubuntu2204(config-if)# ipv6 ospf6 cost 44
ubuntu2204(config-if)# do show running-config
Building configuration...

Current configuration:
!
frr version 9.2-dev-my-manual-build
frr defaults traditional
hostname ubuntu2204
log stdout
no ip forwarding
no ipv6 forwarding
!
interface lo
ipv6 ospf6 cost 44
exit
!
end
ubuntu2204(config-if)# ipv6 ospf6 network point-to-point
2023/11/13 12:18:25 OSPF6: [RB0PM-VM0Y3] interface_up: Not scheduling Hello for lo as there is no area assigned yet
ubuntu2204(config-if)# do show running-config
Building configuration...

Current configuration:
!
frr version 9.2-dev-my-manual-build
frr defaults traditional
hostname ubuntu2204
log stdout
no ip forwarding
no ipv6 forwarding
!
interface lo
ipv6 ospf6 cost 0
ipv6 ospf6 network point-to-point
exit

Expected behavior

The ospf6 cost 44 is kept.

Additional context
("80dfa9e012eb") - ospf6d: Set loopback interface cost to 0

breaks that behaviour

[ton31337]

@pguibert6WIND just a question: does it make sense to set network type to point-to-point for the loopback interface?

[pguibert6WIND]

@pguibert6WIND just a question: does it make sense to set network type to point-to-point for the loopback interface?

I am not an ospf6 specialist. Personally, I think it does not make sense. I just want to point out the fact that there are multiple places in the code where the ospf cost is overridden (mostly when interface_up() is called).

About last patch (Set loopback interface cost to 0) , I wonder if it was not better to increase the range by 0 for loopback interfaces only ?

[ton31337]

I'd say, we shouldn't allow changing network type for a loopback interface at all, but might be wrong. Maybe @aceelindem or @choppsv1 could advice here?

[pguibert6WIND]

I'd say, we shouldn't allow changing network type for a loopback interface at all, but might be wrong. Maybe @aceelindem or @choppsv1 could advice here?

network-type change is not the problem.
If you just turn down the lo interface, you will even lose the cost configuration.

There is a problem with keeping the cost per interface configuration !

[ton31337]

A fixup here #14787.