Re: [Lsr] Prefix Unreachable Announcement Use Cases

Aijun Wang <> Tue, 17 November 2020 06:21 UTC

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From: "Aijun Wang" <>
To: "'Jeff Tantsura'" <>, "'Robert Raszuk'" <>
Cc: "'lsr'" <>, "'Acee Lindem \(acee\)'" <>
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Date: Tue, 17 Nov 2020 14:20:56 +0800
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Subject: Re: [Lsr] Prefix Unreachable Announcement Use Cases
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Hi, Jeff and Robert:


Please see the response inlines.


From: Jeff Tantsura <> 
Sent: Monday, November 16, 2020 6:36 PM
To: Robert Raszuk <>
Cc: Aijun Wang <>cn>; lsr <>rg>; Acee Lindem (acee) <>
Subject: Re: [Lsr] Prefix Unreachable Announcement Use Cases


+1 with Robert.


So you expect the following RIB state after PUA has been advertised: - drop

10/24 - forward 

[WAJ] Yes, this is the expected result.


Unless there’s a recursively discarded next-hop (ala RTBH )  - how do you envision it?

[WAJ] This is the action that the node receives the PUA should do. Existing router may not support it, so we define the PUA capabilities announcement in section-6.3 of this draft <>  



On Nov 16, 2020, at 00:25, Robert Raszuk < <> > wrote:

I was not bringing RIFT's negative routies example as something inherently negative. I was just pointing it out to illustrate that today's data plane lookup does not really support "if does not match" checks.

[WAJ] In data plane, the device do still the “match” check, not “does not match” check.  When the router receives the PUA information, it will install one black hole route for a short time.


So your idea is that you install route for unreachable prefix to /dev/null ? 

[WAJ] Yes.


And how would that help connectivity restoration ? 

[WAJ] This action will trigger the path protection procedures, which will divert the traffic to other backup path.


Moreover it seems that it will just also prevent any local protection to locally bypass the failed destination. 

[WAJ] No, It will trigger the local protection instead, not prevent.


Bottom line is that I agree with one problem statement. However IMHO described actions upon reception of PUA are questionable at best.