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. 


´╗┐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 
    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 

    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 

    Name:               draft-wang-lsr-prefix-unreachable-annoucement
    Revision:   03
    Title:              Prefix Unreachable Announcement
    Document date:      2020-07-27
    Group:              Individual Submission
    Pages:              11

       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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