Re: [Lsr] Prefix Unreachable Announcement Use Cases

Robert Raszuk <> Tue, 17 November 2020 08:36 UTC

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From: Robert Raszuk <>
Date: Tue, 17 Nov 2020 09:36:24 +0100
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To: Gyan Mishra <>
Cc: Aijun Wang <>, lsr <>, Jeff Tantsura <>, "Acee Lindem (acee)" <>
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Subject: Re: [Lsr] Prefix Unreachable Announcement Use Cases
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   Robert, I believe the original intention was related to having the data
> plane converge quickly when summarization is used and flip so traffic
> converges from the Active ABR to the Backup ABR.

I do not buy this use case. Flooding within the area is fast such that both
ABRs will get the same info. As mentioned before there is no practical use
of PUA for making any routing or fwd decision on which ABR to use. If your
ABRs are not connected with min redundancy this draft is a worst patch ever
to work around such a design.

Please present us a picture indicating before and after ABRs behaviour.

   However PUA can be used in the absence of area segmentation within a
> single area when a link or node fails to converge the data plane quickly by
> sending PUA for the backup path so the active path.

If there is no area segmentation then there is no summaries. So what are we
missing in the first place ?

> With the IGP tuned with BFD fast detection on ISIS or OSPF links and LFA &
> RLFA for MPLS or TI-LFA for SR local protection - with those tweaks the
> convergence is well into sub second.  So for Intra area convergence with
> all the optimizations mentioned I am not sure how much faster the data
> plane will converge with PUA.

Even without any of the above listed chain of acronymous things will
generally work well intra-area without PUAs.