Hi Tony,

Thank you for your reply.
Top posting the description of the use case that I have in mind.

Ø  First off, the Area SID is 100% optional. If you choose not to use it, then 
the Proxy LSP should be 100% compatible with a standard L2 node.
Good. But I think that the idea of the Area SID is a good one, and I choose to 
use it. Then I’d like to get it for free ;-)

Please fine below a network topology:

My understanding is that the L2 topology seen by node S is the following

P been the Proxy LSP.

S wants to protect from the failure of link S-C by using TI-LFA. For the 
destination C, it would push 2 (*) node SIDs: P, C
So S needs a Node SID for P:

a)      If P does not redistribute the Node SIDs from its area members, P does 
not seem to advertise any node SID currently. There is a chance that C’s TI-LFA 
implementation would not like it and hence would not install protection for 
this (link, destination)

b)      If P redistributes the Node SIDs from its area members, P advertises 3 
node SIDs (1,2, 3). S could pick any one at random. If it picks 3, the 
forwarding path would be S, A,B, 1, 2, 3, 2 , 1, C,  which is suboptimal.

Two solutions I could think of:
- when redistributing the node SID, P changes the SIDs values and replace them 
with the value of the Area SID. This works for this use case, but it is 
borderline. (e.g. if some a L2 node learn both the original and ‘NATed’ SID, we 
have some SID conflict. Let’s try to avoid this subject).
- P could advertise its own Node-SID with the Area SID value. This is what I’m 
proposing. Both the IP loopback and the Area SID of this Node SID  are likely 
configured by the network operator so this does not seem like a significant 
effort from the implementation.

As you say, this does not involve any protocol extension. But the goal is to 
improve interop with existing/legacy L2 nodes so this may be valuable in the 
draft. This point could be pushed to a deployment consideration section if you 
don’t want any impact on the protocol extension.


(*) Assuming the right metrics on the links. This is not the subject of this 

From: Tony Li [mailto:tony1ath...@gmail.com] On Behalf Of tony...@tony.li
Sent: Thursday, July 30, 2020 7:39 PM
To: DECRAENE Bruno TGI/OLN <bruno.decra...@orange.com>
Cc: lsr@ietf.org
Subject: Re: [Lsr] New Version Notification for 

Hi Bruno,

Thank you for your comments.

On Jul 30, 2020, at 9:22 AM, 
bruno.decra...@orange.com<mailto:bruno.decra...@orange.com> wrote:

Hi Tony,

Thanks for the updated draft.

“ The Area SID Sub-TLV allows the Area Leader to advertise a SID that
   represents the entirety of the Inside Area to the Outside Area.  This
   sub-TLV is learned by all of the Inside Edge Nodes who should consume
   this SID at forwarding time.”

Excellent, from my perspective.

>  - The Area Segment SID TLV has been replaced by extending the Binding SID 
> TLV.

“When SR is enabled, it may be useful to advertise an Area SID which

   will direct traffic to any of the Inside Edge Routers.  The Binding/

   MT Binding TLVs described in RFC 8667 Section 
2.4<https://tools.ietf.org/html/rfc8667#section-2.4> are used to

   advertise such a 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) »

This works.


However I may have a different deployment environment than the one you have in 
mind. Even if those issues may be mine, allow me to share them with you.
In many WAN networks than I’m used to, there are routers from different 
vendors, platforms, software, generations. Requiring all those routers to 
support the new Binding SID TLV T-Flag will take time. Some platform may even 
be end of engineering (evolutions) so would never support such new features.
In my environment, ideally, I would prefer a solution which do not require any 
new feature on external L2 nodes, while all existing L2 features keep working, 
in particular SR, SR-TE, TI-LFA, SR uloop avoidance… This would require the 
Proxy LSP to be not (significantly) different than the LSP of a vanilla L2 node.

First off, the Area SID is 100% optional. If you choose not to use it, then the 
Proxy LSP should be 100% compatible with a standard L2 node.

I cannot claim that we’ve exhaustively tested our implementation against all of 
the features that you cite, so there may still be corner cases, but our intent 
is to make that doable.  For exaple, the Proxy LSP can still contain a node 
SID, adjacency SID, and prefix SID as before. There’s no change there.

For SR, I think that this would require this Proxy LSP to advertise a 
Prefix/Node SID with the Area SID attached. One drawback is that a Node-SID is 
advertised with an IP address that would need to be provisioned.

That’s certainly doable and requires no new protocol machinery. If the WG 
prefers this mode of operation, I’m not opposed.

Both approaches are not mutually exclusives. I’d be happy enough with an option 
for the Proxy LSP to advertise an Area Node SID with the Area SID attached.

Finally, there is no requirement to make me happy ;-) . The above could also be 
a local implementation knob not mentioned in the draft.

Our goal is to make as many customers as happy as possible.  ;-)



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