Just an update, turns out one has to enable IS-IS on the Loopback interface in FRR, even if it will be a passive interface. So the below fixes this issue: interface lo0  ip router isis 1  ipv6 router isis 1  isis passive Now just to fix authentication, and I think we are golden :-). Mark. On 7/Apr/20 16:40, Mark Tinka wrote:

Hi all.

When I add my Loopback interface to isisd.conf and restart it, it throws the below:

2020/04/07 14:35:44 errors: ISIS: libyang: Missing required element "area-tag" in "isis". (/frr-interface:lib/interface[name='lo0'][vrf='default']/frr-isisd:isis) 2020/04/07 14:35:44 warnings: ISIS: [EC 100663337] nb_candidate_commit_prepare: failed to validate candidate configuration % Configuration failed: validation error.

A bit of Googling indicated that "router isis 1" would need to come first in the configuration, since some previous versions of FRR processed the files line by line, and not atomically. But even after doing that, I still get the same error; but get this, ONLY when I add "interface lo0" to the configuration. Without that, and with just the physical interface in the configuration, IS-IS starts up fine.

Any thoughts?

System FRR 7.3 on FreeBSD-12.1. Thanks.

Mark.