Hi,
Thanks for your response.
>> - Hence if the primary link fails, only "L1" will fail and L2 will not
L1 _/may/_ fail, with high probability, but it may also not fail. If
it does
not fail, there is a second transitioning of the post-primary-failure
link from FRR-backup (L2) to post-convergence link (L1), because L1
has a smaller metric.
By “ambiguity”, I meant that backup calculation taking SRLG into
account is based on speculated topology, whereas computation of
post-convergence path, ie, SPF, is based on actual topology. This
seems needs reconciling since in TI-LFA the backup is by definition
the post-convergence path, with a single path-transition after
link-failure as the intended outcome. Do I understand correctly that
the draft prefers to relax that expectation for SRLG?
Thanks,
Sikhi
*From:*Ahmed Bashandy (bashandy) [mailto:[email protected]]
*Sent:* 05 August 2017 01:19
*To:* Sikhivahan Gundu <[email protected]>; [email protected]
*Cc:* [email protected]; [email protected]; Stewart Bryant
<[email protected]>
*Subject:* Re: I-D Action:
draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt
HI,
All members of the same SRLG group are assumed to fail if one of them
fails.
Going back to you example
- L1 is in the same SRLG group as the primary link while L2 is
belongs a different group
- Hence if the primary link fails, only "L1" will fail and L2 will not
- Hence only L2 is candidate to become a backup path while L1 is not
- Hence there is no ambiguity
Thanks
Ahmed
On 8/1/2017 12:42 AM, Sikhivahan Gundu wrote:
Hi,
The draft mandates using “post-convergence path” as the backup path.
It states one advantage, among others, of doing so as follows:
“This .. helps to reduce the amount of path changes and hence
service
transients: one transition (pre-convergence to post-convergence)
instead
of two (pre-convergence to FRR and then post-convergence)”.
This suggests to me that the assumption here is that the
post-convergence
path can be uniquely determined in advance.
However, SRLG introduces ambiguity. To illustrate the point, let
us say a
loop-free alternative has two options: one link (L1) is of the
same metric
value as the primary link and is also in the same SRLG as the
primary; the
second option (L2) is in a different SRLG and has higher metric.
The actual post-convergence path would depend on whether or not L1
also failed along with the primary, so is not uniquely computed
in advance.
If TI-LFA picks L1, there might not be a guaranteed backup. If it
picks L2,
there’d be two link transitions because L2 would not be in a
(strict) SPF-
computed post-convergence path. A third option, of course, is to
give up
declaring that there is no TI-LFA backup, but it’d be preferable
to have
some backup than have none at all.
What do the authors suggest for this situation?
Thanks,
Sikhi
*From:*rtgwg [mailto:[email protected]] *On Behalf Of *Ahmed
Bashandy (bashandy)
*Sent:* 17 July 2017 12:56
*To:* [email protected] <mailto:[email protected]>
*Cc:* [email protected] <mailto:[email protected]>;
[email protected] <mailto:[email protected]>; Stewart Bryant
<[email protected]> <mailto:[email protected]>
*Subject:* Fwd: I-D Action:
draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt
Hi,
A new version of the ti-lfa draft has been posted to address
Stewart Bryant's comments
Thanks
Ahmed
-------- Original Message --------
*Subject: *
I-D Action: draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt
*Date: *
Mon, 17 Jul 2017 00:19:37 -0700
*From: *
[email protected] <mailto:[email protected]>
*Reply-To: *
[email protected] <mailto:[email protected]>
*To: *
<[email protected]> <mailto:[email protected]>
A New Internet-Draft is available from the on-line Internet-Drafts
directories.
Title : Topology Independent Fast Reroute using Segment
Routing
Authors : Ahmed Bashandy
Clarence Filsfils
Bruno Decraene
Stephane Litkowski
Pierre Francois
Filename :
draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt
Pages : 12
Date : 2017-07-17
Abstract:
This document presents Topology Independent Loop-free Alternate Fast
Re-route (TI-LFA), aimed at providing protection of node and
adjacency segments within the Segment Routing (SR) framework. This
Fast Re-route (FRR) behavior builds on proven IP-FRR concepts being
LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding
(DLFA). It extends these concepts to provide guaranteed coverage in
any IGP network. A key aspect of TI-LFA is the FRR path selection
approach establishing protection over post-convergence paths from
the point of local repair, dramatically reducing the operational
need to control the tie-breaks among various FRR options.
The IETF datatracker status page for this draft is:
https://datatracker.ietf.org/doc/draft-bashandy-rtgwg-segment-routing-ti-lfa/
There are also htmlized versions available at:
https://tools.ietf.org/html/draft-bashandy-rtgwg-segment-routing-ti-lfa-01
https://datatracker.ietf.org/doc/html/draft-bashandy-rtgwg-segment-routing-ti-lfa-01
A diff from the previous version is available at:
https://www.ietf.org/rfcdiff?url2=draft-bashandy-rtgwg-segment-routing-ti-lfa-01
Please note that it may take a couple of minutes from the time of submission
until the htmlized version and diff are available at tools.ietf.org.
Internet-Drafts are also available by anonymous FTP at:
ftp://ftp.ietf.org/internet-drafts/
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