Subject: [spring] question regarding draft-hegde-spring-node-protection-for-sr-te-paths

From: Chris Bowers <[email-masked]>
Date: Tue, 17 Jul 2018 14:55:41 -0500

Stewart,

I wanted to follow up on the question you asked following the presentation
in
SPRING on Monday on draft-hegde-spring-node-protection-for-sr-te-paths.

After thinking about your question more, I think I understand it better.
If I understand your question, I think you are concerned that with the
proposed
solution, a transit router acting as a PLR would need to maintain state
for each SR-TE path that passes through the PLR.  The presentation doesn't
make this clear, but the draft itself goes into more detail explaining what
state is needed.  The short answer is that the PLR doesn't need per-path
state.  The long answer is below.

For just link protection, the PLR needs a backup forwarding entry for every
valid
incoming first label that the PLR might receive. For link protection, this
corresponds to needing a backup forwarding entry for every prefix-SID
in the network and every adj-SID label advertised by the PLR.

To provide protection against potential node failure, the PLR needs
additional
backup forwarding entries for a subset of valid incoming (first-label,
second-label)
pairs.  Backup forwarding entries for label pairs are needed where
the first-label is an adj-SID label advertised by the PLR or the
first-label corresponds
to a node-SID which is a direct neighbor of the PLR, and the second-label
is a valid prefix-SID or adj-SID at that direct neighbor.  These
node-protecting backup forwarding entries can be created based on segment
routing
advertisements, independent of whatever SR-TE LSPs that have actually been
instantiated in the network.

Hopefully this answers the question.  And hopefully that was actually
your question in the first place.

Thanks,
Chris