On 30/07/2026 08:45, Matthew Petach via NANOG wrote:
The point I was trying to make is that just because a routing protocol supports doing that doesn't mean it's a good idea operationally. There are other issues that come into network design that often suggest a single flat IGP might not give you enough knobs to allow you to control traffic flows and impact radius from unusual externalities.
An obvious case against multi-level IS-IS is that to create end-to-end MPLS LSP's, you need each node to have every other node in its LFIB. If you run a tiered L1 and L2 IS-IS network, you necessarily need to leak L2 routes into L1 for that full LFIB visibility. That automatically negates the need to have separate IS-IS levels. Might as well run the whole network in L2. Mark.