Subject: Re: [Lsr] Review of draft-ietf-lsr-isis-ttz-00.txt

From: Christian Hopps <[email-masked]>
Date: Fri, 2 Oct 2020 06:03:48 -0400

> On Sep 22, 2020, at 1:21 AM, Donald Eastlake <[email-masked]> wrote:
> 
> 
> One final question: I may be confused but my understanding of IS-IS is
> that there are Level 1 links, Level 2 links, and links that are both
> Level 1 and 2. A border router between Level 1 and Level 2 is a router
> that has both types of links attached to it. Such a router is a part
> of each Level 1 Area to which it is linked and a part of Level 2. So,
> the Level 1 / Level 2 boundary is, in a real sense, internal to such a
> border router.

Actually IS-IS is different than this (your last 2 sentences). This is a key conceptual difference between OSPF and IS-IS. In OSPF an OSPF router (instance) can be attached to several areas. In IS-IS an L1 or L1L2 router (instance) resides belongs to only a single L1 area. Thus in OSPF area boundaries exist within the OSPF instance, while in IS-IS the L1 area boundaries exists on the wire between L2 adjacencies.

Thanks,
Chris.