Hi Acee:

    In the scenario mentioned by Robert, the section 6.1 solution doesn't work. 
When BGP needs to detect the reachability of the next hop to trigger BGP 
convergence.

Thanks
Zhibo Hu

-----Original Message-----
From: Acee Lindem (acee) [mailto:a...@cisco.com] 
Sent: Tuesday, July 28, 2020 9:24 PM
To: Huzhibo <huzh...@huawei.com>; Aijun Wang <wang...@chinatelecom.cn>; 
lsr@ietf.org
Cc: Xiaoyaqun <xiaoya...@huawei.com>
Subject: Re: [Lsr] New Version Notification for 
draft-wang-lsr-prefix-unreachable-annoucement-03.txt

See my response to Aijun... Please provide topologies where the section 6.1 
solution doesn't work. 
Acee

´╗┐On 7/27/20, 10:03 PM, "Huzhibo" <huzh...@huawei.com> wrote:

    Hi Acee:

    In fact, we have meet some scenarios where redundant paths cannot be 
deployed within a area. Especially when the access area uses the nearby access 
principle, ABRs form disordered combinations, which makes it difficult to 
deploy physical or tunnel connections. In addition, advertising unreachable 
prefixes can prevent traffic detours. Of course, This draft also has some other 
attempts to use Segment Routing to automatically establish connections between 
ABRs.

    Thanks

    Zhibo Hu

    -----Original Message-----
    From: Acee Lindem (acee) [mailto:a...@cisco.com] 
    Sent: Tuesday, July 28, 2020 1:51 AM
    To: Aijun Wang <wang...@chinatelecom.cn>; lsr@ietf.org
    Cc: Huzhibo <huzh...@huawei.com>; Xiaoyaqun <xiaoya...@huawei.com>
    Subject: Re: [Lsr] New Version Notification for 
draft-wang-lsr-prefix-unreachable-annoucement-03.txt

    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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