Bruno -

Regarding the A-flag...
It may not matter much whichever way we decide - but the A-flag was invented 
because at the time (prior to RFC 7794) there was no way to determine from 
looking at a prefix reachability advertisement whether it was originated by the 
advertising node or had been leaked from another area.
If RFC 7794 had existed when the SR-MPLS draft was first being developed there 
would have been no need for the A-flag.

As regards Area-SID, there is no prefix to be leaked - so I do not see that the 
A-flag serves any useful purpose.
If you want it to be set just to be consistent with its use in PSP logic - OK - 
but I think this is unnecessary.


From: Lsr <> On Behalf Of
Sent: Thursday, July 30, 2020 9:46 AM
Subject: [Lsr] draft-ietf-lsr-isis-area-proxy-02

Hi authors,

Please find below some feedback for information. Feel free to ignore.

4.4.13.  The Area SID<>

   "The following extensions to the Binding TLV are defined in order to
   support Area SID:

      A new flag is defined:

         T-flag: The SID directs traffic to an area.  (Bit 5)

         When T-flag is set:

            M and A flag MUST be clear"

My understanding is that the Area SID is installed in the FIB of all inside 
edge routers. Based on this, I would argue that the  A flag could and SHOULD be 
"A-Flag: Attached Flag. The originator of the SID/Label Binding TLV MAY set the 
A bit in order to signal that the prefixes and SIDs advertised in the SID/Label 
Binding TLV are directly connected to their originators."
4.4.7.  Reachability TLVs

   If the Inside Area supports Segment Routing and the selected TLV

   includes a Prefix Segment Identifier sub-TLV (3) 
[RFC8667<>], then the

   sub-TLV SHOULD be copied as well. The P-Flag SHOULD be set in the

   copy of the sub-TLV to indicate that penultimate hop popping SHOULD

   NOT be performed for this prefix.  The E-Flag SHOULD be reset in the

   copy of the sub-TLV to indicate that an explicit NULL is not

   required. The R-Flag SHOULD simply be copied.

May be it would be more generic to say that those prefix are handled as 
redistributed prefix. and already defines the 
behaviour for respectively Prefix-SID propagation and P and E flags handling, 
so probably no need to re-specify:
When propagating (from either Level-1 to Level-2 or Level-2 to Level-1) a 
reachability advertisement originated by another IS-IS speaker, the router MUST 
set the P-Flag and MUST clear the E-Flag of the related Prefix-SIDs.
That would also cover for the handling of Prefix Attribute Flags sub-TLV 

I would argue that the R-Flag MUST be set (rather than simply copied). As per

Best regards,


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