Tony -

From: Tony Li <>
Sent: Wednesday, August 05, 2020 1:08 PM
To: Les Ginsberg (ginsberg) <>
Cc: Acee Lindem (acee) <>; Bruno Decraene 
Subject: Re: [Lsr] draft-ietf-lsr-isis-area-proxy-02


a)Advertise the “Area Prefix” in the Area Proxy TLV – much as we do a router-id 
today in the Router-ID TLV.

This would make the Area Prefix mandatory for Area Proxy, which is not desired. 
 We would prefer it to remain optional and thus part of the Area SID sub-TLV.

[Les2:] You can advertise the Area Prefix in an optional sub-TLV – just as you 
did with the Area SID. That is what I expected you would do.

b)The remaining info (reachability and SID) can then be provided using existing 
Prefix Reachability advertisements – no need for new sub-TLV for “Area SID”. 
This eliminates any potential issues if the SID advertised by “Area SID 
sub-TLV” were to differ from the SID advertised in Prefix Reachability for the 
same prefix.

As we discussed privately, we view this as a non-issue.  The Area Leader is the 
one advertising both the Area SID sub-TLV and the Proxy LSP. If there’s a 
coding error, there’s a coding error. There is a single source of truth (the 
Area Leader’s config) and we cannot protect against every possible coding 
error.  Reconciling the prefix with a separate advertisement has a non-trivial 
chance of being broken too, and IMHO, much larger.

[Les2:] You can define the advertisements in a way which reduces the 
possibility of ambiguity – which seems like a good thing to me.
And rest assured that you will be asked by someone to define the expected 
behavior when there is an inconsistency. 😊
Since prefix SID and Prefix reachability are directly related in forwarding, it 
makes far more sense to me to have those two together.
If you find correlating information in two different TLVs too challenging, you 
could opt for a new bit in the prefix attributes sub-TLV to identify a prefix 
as an “Area Prefix”. Then you would not need any additional info advertised in 
the Area Proxy TLV at all.

 There then remains the question as to whether the “Area Prefix” is anycast or 
unicast i.e., is it common to all IERs or is it unique to whomever gets elected 
Area Leader?

Does it matter? We have no clear semantics for this prefix. A difference that 
makes no difference is no difference.

[Les:] This question needs to be directed at those who prefer the Area Prefix 
approach. It matters as it impacts configuration and advertisement semantics. 
An anycast prefix is NOT a Node Prefix.
And it impacts how traffic is forwarded into the area.



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