Re: [Lsr] Prefix Unreachable Announcement Use Cases

Gyan Mishra <> Tue, 17 November 2020 14:28 UTC

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From: Gyan Mishra <>
Date: Tue, 17 Nov 2020 09:28:44 -0500
Message-ID: <>
To: Robert Raszuk <>
Cc: "Acee Lindem (acee)" <>, Aijun Wang <>, Jeff Tantsura <>, lsr <>
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Subject: Re: [Lsr] Prefix Unreachable Announcement Use Cases
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On Tue, Nov 17, 2020 at 4:01 AM Gyan Mishra <> wrote:

> On Tue, Nov 17, 2020 at 3:36 AM Robert Raszuk <> wrote:
>>    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.
>    Gyan> Agreed.  The point of PUA in ABR use case is the ability to track
> the component prefixes and in case where component is down and traffic is
> still forwarded to the ABR and dropped.  The other more important use case
> is when links are down within the area and the area is partitioned and so
> one ABR has all component prefixes however other ABR is missing half the
> component prefixes.  So since the ABR will by default advertise the summary
> as long as their is one component UP the summary is still advertised.  So
> this use case is severely impacting as now you have an ECMP path to the
> other area for the summary via the two ABRs and you drop half your
> traffic.  So now with PUA the problem is fixed and the PUA is sent and now
> traffic is only sent to the ABR that has the component prefixes.
>> Please present us a picture indicating before and after ABRs behaviour.
>      Gyan> will do
>>    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 ?
>     Gyan> Sorry I am stating that PUA feature can also be used intra area
> where if a link or node goes down to improve data plane convergence.
>>> 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.
>     Gyan> Agreed which is why I mentioned the BGP next hop self use case
> if I could figure out how PUA could help there that would be a major
> benefit of PUA.

      Gyan>. We could use Aijun’s passive interface new top level TLV to
link the next hop rewrite loopback to the PE-CE links all being set to
passive. So if any PE-CE link goes down a PUA is sent and the next hop
converges PIC core PE-CE link which is now associated with the Loopback.
This would be a major benefit of PUA for PIC core convergence when
next-hop-self is used which applies to MPLS and SR and IP based core.

So the two main critical  use cases where this solution solves a problem is
partitioned area scenario and nest hop convergence when next-hop-self is
used scenario.

I will update the presentation deck and share.

>> Thx,
>> R.
>> --
> <>
> *Gyan Mishra*
> *Network Solutions A**rchitect *
> *M 301 502-134713101 Columbia Pike *Silver Spring, MD
> --


*Gyan Mishra*

*Network Solutions A**rchitect *

*M 301 502-134713101 Columbia Pike *Silver Spring, MD