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

Stewart Bryant <> Mon, 29 March 2021 13:15 UTC

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From: Stewart Bryant <>
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Date: Mon, 29 Mar 2021 14:15:34 +0100
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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> On 26 Mar 2021, at 11:06, Loa Andersson <> wrote:
> Stewart,
> I understand evbery single piece yuor argument, but can't get my head around the it ll in the end.
> You say:
>   You only pop the label to see the xSPL if you are the destination.
> So my conclusion is that if you don't pop the top label you can't see the info encoded in the xSPL, unless you scan the stack without popping the top label.  Crossing the Rubicon!
> I guess that this is fine, but that seems to indicate that you have to scan every label stack that passes a node. And when you scan how do you know when to stop, it might be that the first xSPL that you encounter is the important one, it might be deeper down.
> Wouldn't it be better to have something in the top label that tells you where you need to look and what you ar looking for?
> I'd say that this is a FEC.
> /Loa


If you are going to use a FEC, you might as well use a FEC and only a FEC, though the downside of a FEC is the need to advertise the FEC. That is not an issue with something like RSVP, a reduced flooding scope PPR, SDN etc, where scope of knowledge/flooding is limited to the nodes that need to know, but with the general fooding label distribution methods increases the number of labels to flood, and worse allocate.

A lot of routers scan the whole stack anyway so we know you could scan to find an xSPL somewhere in the stack.

The alternative is to have a scan one below implicit action which would allow you to have a scheme where you looked at ToS if you knew no better, but look one more below if you understood the game.

So I was thinking of a top label with the meaning "Go to Foo", and an SPL below that indicating “using Bar” so the pair had the meaning “Go to Foo using the information you get from Bar”. So the label pair becomes a compound label at ToS.

We can create all sorts of syntax for this, and I am proposing to write a draft examining  some of the possibilities and issues.

So bottom line: I am thinking of using a label group rather than searching the stack in the hope of finding something.

- Stewart