Subject: Mail regarding draft-ietf-rtgwg-lfa-applicability

From: Sriganesh Kini <[email-masked]>
Date: Mon, 4 Apr 2011 02:03:44 -0700

Hello Clarence and other authors,

A couple comments

1. Section 2 Terminology - "A per prefix LFA at destination D for a
node S ..." should read "A per prefix LFA at node S for destination
D..."

2. The equation for link/node protecting assumes that all downstream
paths are not used. Given that ISIS (which allows such paths) is used
as a reference, it is necessary to state this assumption. Otherwise
the equations must be modified.

Thanks
-- 
- Sri

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From: mike shand <[email-masked]>
Date: Tue, 05 Apr 2011 13:45:33 +0100

Regarding 2, yes, of course, it is not possible to use downstream paths 
for "normal" routing in conjunction with LFAs, since they would loop. If 
you have downstream paths deployed at all, then it is only possible to 
use downstream paths as repairs. This is equivalent to dealing with 
multiple simultaneous failures, where, of course LFAs have the potential 
for looping, but downstream paths do not.

While this is well known, it has not been considered to be a problem to 
date because as far as I know, there are no implementations of 
downstream path support in IS-IS, despite it having been permitted by 
ISO/IEC 10589 (clause 7.2.6.2) for the last 20 years.

However, you are right that it would be useful to draw attention to this 
in the draft.

Note that while IS-IS specifically permits such paths, the concept of 
downstream paths could be applied to any hop by hop forwarding routing 
paradigm.

Mike