Re: [Lsr] New Version Notification for draft-wang-lsr-prefix-unreachable-annoucement-03.txt

Hi, Robert:

Aijun Wang
China Telecom

> On Jul 31, 2020, at 00:23, Robert Raszuk <robert@raszuk.net> wrote:
> 
> 
> Hi,
> 
> Imagine I have two ABRs connecting area 1 to area 0. One is signalling transition to down for subset of summary and the other does not .. maybe it is slow ... maybe it does not support this new feature. 
> 
> So all routers in the area 0 are receiving a full summary from one ABR and a summary with hole from the other one. Should that be logical AND or OR ? 

[WAJ] It should be OR. All routers in the area 0 should prefer to the ABR that not advertising PUA to forward traffic.
> 
> Then on the other side there is area 2. Should the transition to down be leaked ?

[WAJ] The PUA should be leaked.

> If so in a nicely stable network we may see instead of few /20 summaries jump to 1M transitions to down yet summary continues - would that not have a bit negative effect on the entire network ? Where would ABR remove summary itself - when all atomic routes subsumed by the summary transition to down ? 

[WAJ] https://datatracker.ietf.org/doc/html/draft-wang-lsr-prefix-unreachable-annoucement-03#section-6 has described the extreme conditions for advertising PUA+Summary, Detail Reachable Prefixes only, or Summary Address with Max metric.
Is there any other graceful advertising in these conditions? 

> 
> - - - 
> 
> I am supportive of the idea to signal unreachable network elements. But I am not sure if we should do it in IGP and BGP or only in BGP. 

[WAJ] Such information is for underlay link state and should be flooded via IGP? The ambiguity arises from IGP summary behavior and should be solved by itself?
> 
> Thx,
> R.
> 
> 
>> On Thu, Jul 30, 2020 at 6:08 PM Huzhibo <huzhibo@huawei.com> wrote:
>> HI acee：
>> 
>> PUA does not advertise reachable or unreachable details, it advertise events with prefix from up to down.
>> 
>> 
>> 
>> thanks
>> 
>> Zhibo
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> --------------------------------------------------
>> 胡志波 Hu Zhibo
>> Mobile: +86-18618192287
>> Email: huzhibo@huawei.com
>> 
>> 发件人：Acee Lindem (acee) <acee@cisco.com>
>> 收件人：Peter Psenak <ppsenak=40cisco.com@dmarc.ietf.org>;Robert Raszuk <robert@raszuk.net>
>> 抄 送：Aijun Wang <wangaj3@chinatelecom.cn>;Xiaoyaqun <xiaoyaqun@huawei.com>;Huzhibo <huzhibo@huawei.com>;Aijun Wang <wangaijun@tsinghua.org.cn>;lsr <lsr@ietf.org>
>> 时 间：2020-07-31 00:04:02
>> 主 题：Re: [Lsr] New Version Notification for draft-wang-lsr-prefix-unreachable-annoucement-03.txt
>> 
>> So, how do we define a reachable route - is it any route subsumed by the summary LSA that we knew about in the past that becomes unreachable? When the PUA is withdrawn, how do we know whether it is because of expiration of the interval or the route becoming reachable again? This is a slippery slope. 
>> Thanks,
>> Acee
>> 
>> On 7/30/20, 10:34 AM, "Lsr on behalf of Peter Psenak" <lsr-bounces@ietf.org on behalf of ppsenak=40cisco.com@dmarc.ietf.org> wrote:
>> 
>>     On 30/07/2020 16:30, Robert Raszuk wrote:
>>     > Hey Peter,
>>     > 
>>     > Not sure how smart you really want to be here but keep in mind that BGP 
>>     > say option C may never hear about it all the way to the egress PE in 
>>     > other domain or area ... It is almost always incongruent with IGP.
>>     > 
>>     > So if the BGP path is installed it will indeed be at risk to resolve via 
>>     > less specific when it is still active BGP path and you too quickly 
>>     > remove info about unreachability.
>> 
>>     again, if you are smart you can use this info to your advantage, even 
>>     without putting it in the forwarding and leaving the less specific stuff 
>>     intact.
>> 
>>     Peter
>> 
>> 
>>     > 
>>     > Thx
>>     > R.
>>     > 
>>     > On Thu, Jul 30, 2020 at 4:21 PM Peter Psenak <ppsenak@cisco.com 
>>     > <mailto:ppsenak@cisco.com>> wrote:
>>     > 
>>     >     On 30/07/2020 16:14, Robert Raszuk wrote:
>>     >      >      > 2：For bgp example，when the pe node down，the bgp peer
>>     >     must down
>>     >      >     within
>>     >      >      > 30 mintus，It will not get it up via cancle advertise pua.
>>     >      >
>>     >      >     for the above it is sufficient to advertise the
>>     >     unreachability for few
>>     >      >     seconds from each ABR independently. That would be a much
>>     >     more solid
>>     >      >     proposal.
>>     >      >
>>     >      >
>>     >      > Not sure about "few seconds" ... IBGP def hold time in number of
>>     >      > implementations is 180 sec :) .. but few minutes will work for sure.
>>     > 
>>     >     depends how you use it.
>>     > 
>>     >     If you can use the unreachable info in a smart way, it's sufficient if
>>     >     it is present for a very short time interval.
>>     > 
>>     >     thanks,
>>     >     Peter
>>     > 
>>     >      >
>>     >      > Thx,
>>     >      > R.
>>     >      >
>>     > 
>> 
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