Acee -

From: Acee Lindem (acee) <>
Sent: Wednesday, August 05, 2020 12:31 PM
To: Tony Li <>; Bruno Decraene <>
Cc: Les Ginsberg (ginsberg) <>;
Subject: Re: [Lsr] draft-ietf-lsr-isis-area-proxy-02

Hi Tony, Bruno, Les,

From: Lsr <<>> on behalf of Tony 
Li <<>>
Date: Tuesday, August 4, 2020 at 11:26 AM
To: Bruno Decraene <<>>
Cc: "Les Ginsberg (ginsberg)" <<>>, 
"<>" <<>>
Subject: Re: [Lsr] draft-ietf-lsr-isis-area-proxy-02

Hi Bruno,

[Bruno] Agreed so far.
Do we agree that draft-ietf-lsr-isis-area-proxy uses the SID/Label sub-TLV? We 
both agree that this sub-TLV has no mention of the global flag nor the routing 
algoto be used.

So far, we do NOT have agreement on that.  Your argument yesterday (backed by 
Robert) is pretty compelling: go ahead and assign a prefix and now the Area SID 
may be advertised as a Node SID in the Proxy LSP. If we take that direction, 
this discussion is moot.

Why wouldn’t we take this approach? Since the border routers are abstracting 
the area as a node, why wouldn’t we do the same for the Node-SID?

[Les:] The original idea proposed by the draft authors was to have a SID which 
could be used to forward traffic to the inside area. Conceptually this does not 
require a prefix – and the encodings currently defined in the draft reflect 

Bruno has since commented that he prefers a prefix to be associated with the 
Area SID.
I agree this is a viable approach, but if we go that way then I think what is 
needed is:

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

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?




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