Re: [mpls] how does it work if ....

Stewart Bryant <> Wed, 17 March 2021 16:34 UTC

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From: Stewart Bryant <>
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Date: Wed, 17 Mar 2021 16:34:38 +0000
Cc: Stewart Bryant <>, "" <>, Kireeti Kompella <>
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To: loa Andersson <>
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Subject: Re: [mpls] how does it work if ....
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You only pop the label to see the xSPL if you are the destination.

If you do not know about the xSPL you should not have been sent the packet in the first place. That would be a routing/control plane failure.

If you receive it by mistake, and consider that it is for you (unlikely but possible) so you pop the label, you will find an unknown xSPL and presumably drop the packet.

So I that is safe.

What is going to be difficult is with one of Kireeti’s reused Els. That will work fine at a P router, but a PE is going to not process the packet properly. This cannot be a PW or Detnet since that has another label as a gatekeeper, so we nee think what happens if it is an IP packet, in which case the ECMP defeat nibble will be found. This should be unexpected, so the packet will be dropped.

If there is an old P router that does not understand the xSPL it will just swap and forward as before.

You can always program a P router not to PHP (we did that with MPLS-TP for example). It is more work for the receiving PE, but it needs to be capable of dealing with that or it should not be used in that role with this label stack.

So assuming that Kireeti and I have the same vision of how this works, it looks safe to me. However we have to look at every corner and strong exception to validate the approach.


> On 17 Mar 2021, at 14:18, Loa Andersson <> wrote:
> Kireeti,
> I'm looking at your slides from the joint meeting, in particular slide #3, where you say at thye bottom:
>  "Corollary: such labels MUST NOT reach the top of stack (by popping
>   labels above them)"
> How is that backwards compatible.
> If you  have an old LSR, that don't know to look at the bSPL or eSPL underneath the top label. I assume that the top label will be swapped and the packet forwarded.
> What is the pen-ultimate LSR is "old" and just pop the top label, how can the PE sort this out?
> /Loa
> -- 
> Loa Andersson                        email:
> Senior MPLS Expert                
> Bronze Dragon Consulting             phone: +46 739 81 21 64
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