Hi, Gyan:


Sorry for replying you so late.

You are right about the summary address behavior, but the summary address is 
configured by manually, and if one of the specific prefix within this summary 
range is down, the black hole(route to this specific prefix) will be formed.  
PUA mechanism just want to amend this.

Glad to know Rift has also noticed such issues.  In OSPF/ISIS, such problem 
needs also be solved.


If you are interested this topic, welcome to join us to the solution.



Best Regards


Aijun Wang

China Telecom


From: lsr-boun...@ietf.org [mailto:lsr-boun...@ietf.org] On Behalf Of Gyan 
Sent: Thursday, August 6, 2020 4:44 PM
To: Aijun Wang <wangai...@tsinghua.org.cn>
Cc: Peter Psenak <ppsenak=40cisco....@dmarc.ietf.org>; Robert Raszuk 
<rob...@raszuk.net>; Huzhibo <huzh...@huawei.com>; Aijun Wang 
<wang...@chinatelecom.cn>; lsr <lsr@ietf.org>; Acee Lindem (acee) 
<a...@cisco.com>; Xiaoyaqun <xiaoya...@huawei.com>
Subject: Re: [Lsr] New Version Notification for 


Hi Aijun and authors


I am catching up with this thread after reading through this draft.


I understand the concept that we are trying to send a PUA advertisement which 
sounds similar to Rift Negative Disaggregation prefix now with a  null setting 
when a longer match component prefix that is part of a summary range is down to 
detect prefix down conditions with longer match component prefixes that are 
part of a summary.  


Basically how summarization works with both OSPF and ISIS is that at minimum a 
single longer match within the defined IA summary range must exist for the IA 
summary to be sent.  So the summary is sent conditionally similar to a BGP 
conditional advertisement or even a ospf default originate which requires a 
default in the RIB to exist before the advertisement is sent.  A good example 
of non conditional analogy with BGP if you have a null0 static for a summary 
for an exact match BGP advertisement the prefix is always advertised no matter 
what even if no component prefixes exist.  This can result in black hole 
routing. BGP has conditional feature to conditionally advertisement based on 
existence of a route advertisement of downstream neighbor in the BGP RIB.  So 
with ospf and Isis the summary is in fact conditional similar to a BGP 
conditional advertisement and in the example given in the draft in section 3.1 
when node T2 is down and pt2 becomes unreachable and let’s say that prefix is <>  and the summary is 
<>  and no other component prefix exists within the summary 
range the prefix will not get adversed.  So there will not be any black hole.  


The summary represents all prefixes within the range that would be suppressed 
with the summary when advertised into the backbone area.  However only at a 
minimum one prefix must exist in the range for the summary to be generated.  
That is done by design as the summary represents all prefixes within the range. 
 So let’s say there are a 100 prefixes and let’s say a few devices are down and 
so now only 5 prefixes exist within the range.  By design it is OK for router 
to generate the summary for the 5 prefixes it is representing and that will not 
cause any routing loop or black hole.



I am trying to understand wage gap exists and what we are trying to solve 
related to summarization in the context of IPv6.  Both IPv4 and IPV6 
summarization operates the similarly as far as the requirement of at minimum a 
single component route within the summary range must exist  as a condition to 
be present in the RIB before the summary can be advertised.


Kind Regards 




On Tue, Aug 4, 2020 at 11:25 PM Aijun Wang <wangai...@tsinghua.org.cn 
<mailto:wangai...@tsinghua.org.cn> > wrote:

Hi, Robert:


From: lsr-boun...@ietf.org <mailto:lsr-boun...@ietf.org>  
[mailto:lsr-boun...@ietf.org <mailto:lsr-boun...@ietf.org> ] On Behalf Of 
Robert Raszuk
Sent: Friday, July 31, 2020 6:21 PM
To: Aijun Wang <wang...@chinatelecom.cn <mailto:wang...@chinatelecom.cn> >
Cc: Peter Psenak <ppsenak=40cisco....@dmarc.ietf.org 
<mailto:40cisco....@dmarc.ietf.org> >; Huzhibo <huzh...@huawei.com 
<mailto:huzh...@huawei.com> >; Aijun Wang <wangai...@tsinghua.org.cn 
<mailto:wangai...@tsinghua.org.cn> >; lsr <lsr@ietf.org <mailto:lsr@ietf.org> 
>; Acee Lindem (acee) <a...@cisco.com <mailto:a...@cisco.com> >; Xiaoyaqun 
<xiaoya...@huawei.com <mailto:xiaoya...@huawei.com> >
Subject: Re: [Lsr] New Version Notification for 


[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 


Well if we look at this problem from a distance while on surface it seems like 
an IGP issue (not to mention some which use BGP as IGP :) IMO it is only 
hurting when you have some service overlay on top utilizing the IGP. 

[WAJ] There are situations that the PUA mechanism apply when no service overlay 
running over IGP.  Scenarios described in  
 draft-wang-lsr-prefix-unreachable-annoucement-03#section-3 are not tightly 
coupled with the overlay service.


So typically today if I am running any service with BGP I do count on BGP to 
remove routes which are no longer reachable. IGP just tells me how to get to 
the next hop, which direction to go and not if the endpoint (service CPE or PE 
connected to given CE) is up or down. 


So today smart BGP implementations in good network design can use RD based 
withdraws to very fast (milliseconds) remove the affected service routes. When 
I said should we do it in BGP I meant to ask WG if this is good enough to 
quickly remove service routes. If not maybe we should send such affected next 
hop in BGP to even faster invalidate all routes with such next hop as failing 


Bottom line if you think the problem is IGP then I think Acee's comments apply. 

[WAJ] Which comment is not addressed yet?


Last - See today you are bringing the case to signal transition to DOWN .... 
but for some people and applications it may be not enough. In fact UP/DOWN they 
can get via BGP. But if you have two ABRs and one will due to topology changes 
in its area suddenly will be forced to reach atomic destination covered by 
summary over much higher metric path that for applications running above may be 
much more severe case and not acceptable one too. 

[WAJ] Or else, the application traffic will be broken.


And BGP will not remove service routes nor modify best path in any way as 
summary is masking the real metric to some next hops. So while in the network 
you may have alternate better native transit paths with a lot of free capacity 
if you only switch to a different bgp next hop (not talking about any TE at 
all) you are stuck offering much worse service to your customers. 

[WAJ] if there are other links to reach the affected prefix via the ABR, then 
this ABR will not send the PUA information.


Those cases are starting to be solved by performance routing both at the 
service itself or at BGP nh levels. Should IGP assist here ... I am not sure.

[WAJ] when node become down, it can only depend on other nodes within the same 
IGP to send such unreachability information. IGP can certainly help here J



Many thx,


Lsr mailing list
Lsr@ietf.org <mailto:Lsr@ietf.org> 



Gyan Mishra

Network Solutions Architect 

M 301 502-1347
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Silver Spring, MD


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