Hello Acee,

I would like to question your assessment that signalling unreachable routes
is unnecessary.

Imagine hierarchical network with areas. Under no failures area 1
advertises to area 0 summary LSA with That block covers PE's
loopbacks which within the area are /32s. Those loopbacks are also BGP next

Now imagine PE1 with fails. Well till BGP reconverges all paths
advertised by this PE with are still valid as this next hop is
still reachable entire network wide. That means that traffic is being sent
to this failed PE1 for relatively long period of time.

It seems natural that without breaking benefits of summarization across
areas or domains in the above scenario we could continue to advertise - That is when I see most benefits of advertising
unreachability aka negative routing.

Of course said all of the above - if you search your employer's archives -
you will see a proposal where the above mechanism can be done within BGP
itself with no touch to the IGP - just using a bit of twisted next hop
validation steps and BGP native recursion. I am not going to make any
judgements here which method is better or easier - naturally I personally
like BGP one more :).

But I hope this is clear why at least discussion on the subject is
important. It also illustrates why the below statement is not
necessarily correct:

"Note that the unreachability of a given summarized prefix is only relevant
if it is reachable through another ABR. "

Kind regards,

On Mon, Jul 27, 2020 at 7:51 PM Acee Lindem (acee) <acee=
40cisco....@dmarc.ietf.org> wrote:

> Speaking as an LSR Working Group member:
> Asking the WG precisely how to advertise prefix unreachability is the
> wrong question - it is analogous to asking whether to use a car or truck to
> drive off the edge of a cliff. Rather than messing up OSPF and IS-IS with
> these complex and unnecessary mechanisms, it would be better to address the
> requirement in your network design. Note that the unreachability of a given
> summarized prefix is only relevant if it is reachable through another ABR..
> In this case, the network design should provide adequate intra-area
> redundancy to provide communications between the ABRs. If this cannot be
> accomplished, an intra-area adjacency should be established over a tunnel
> between the ABRs in the backbone. Contrary to section 6.1, Looping is
> normally not a problem as ABRs should add back hole routes for their
> advertised summaries.
> Acee
> ´╗┐On 7/26/20, 9:34 PM, "Lsr on behalf of Aijun Wang" <lsr-boun...@ietf.org
> on behalf of wang...@chinatelecom.cn> wrote:
>     Hi, LSR experts:
>     We have uploaded the new version of this PUA(Prefix Unreachable
> Announcement) draft. The main updates are the followings:
>     1) Describes the solution that using tunnel to redirect traffic among
> ABRs, when all ABRs reaches the PUA limit.
>     2) Describe fast rerouting to avoid routing black hole.
>     3) Defining PUA capabilities announcements for OSPFv2/OSPFv3 and ISIS..
>     There are also some arguments about the current solution for PUA, for
> example:
>     1) Is it suitable to set the "Prefix Originator" sub-TLV to NULL to
> indicate the prefix is unreachable?
>     2) if not, what's the consideration? What's the other convincible
> solution?
>     Wish to hear comments and suggestions on the above issues. We will
> also have the presentation on the coming IETF LSR meeting.
>     Best Regards
>     Aijun Wang
>     China Telecom
>     -----Original Message-----
>     From: internet-dra...@ietf.org [mailto:internet-dra...@ietf.org]
>     Sent: Monday, July 27, 2020 9:16 AM
>     To: Zhibo Hu <huzh...@huawei.com>; Aijun Wang <wang...@chinatelecom.cn>;
> Yaqun Xiao <xiaoya...@huawei.com>
>     Subject: New Version Notification for
> draft-wang-lsr-prefix-unreachable-annoucement-03.txt
>     A new version of I-D,
> draft-wang-lsr-prefix-unreachable-annoucement-03.txt
>     has been successfully submitted by Aijun Wang and posted to the IETF
> repository.
>     Name:               draft-wang-lsr-prefix-unreachable-annoucement
>     Revision:   03
>     Title:              Prefix Unreachable Announcement
>     Document date:      2020-07-27
>     Group:              Individual Submission
>     Pages:              11
>     URL:
> https://www.ietf.org/internet-drafts/draft-wang-lsr-prefix-unreachable-annoucement-03.txt
>     Status:
> https://datatracker.ietf.org/doc/draft-wang-lsr-prefix-unreachable-annoucement/
>     Htmlized:
> https://tools.ietf.org/html/draft-wang-lsr-prefix-unreachable-annoucement-03
>     Htmlized:
> https://datatracker.ietf.org/doc/html/draft-wang-lsr-prefix-unreachable-annoucement
>     Diff:
> https://www.ietf.org/rfcdiff?url2=draft-wang-lsr-prefix-unreachable-annoucement-03
>     Abstract:
>        This document describes the mechanism that can be used to announce
>        the unreachable prefixes for service fast convergence.
>     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.
>     The IETF Secretariat
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