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From: Dmytro Shypovalov <dmytro@vegvisir.ie>
Date: Fri, 31 Oct 2025 11:01:55 +0000
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 <CAEBHQ-NXncAprcZJc2=uBCKAwnb4LmQkxs6XextfzyAPTx9UWg@mail.gmail.com>
To: mpls@ietf.org
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Subject: =?utf-8?q?=5Bmpls=5D_RFC_8277_clarifications?=
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 <https://mailarchive.ietf.org/arch/msg/mpls/akPor7VCHUttvZnZi5wpJs17KgE>

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Dear MPLS WG,

I've been working on an SR-TE project using BGP-LU to influence traffic
engineering paths. This requires the advertisement of multiple labels.

RFC8277 seems to have 2 conflicting statements

Section 2.1:

   the Count is the maximum
   number of labels that the BGP speaker sending the Capability can
   process in a received UPDATE of the specified AFI/SAFI.  If the Count
   is 255, then no limit has been placed on the number of labels that
   can be processed in a received UPDATE of the specified AFI/SAFI.


This assumes the BGP update can have up to 255 labels (in theory).

Section 2.3:

   - Length:

      The Length field consists of a single octet.  It specifies the
      length in bits of the remainder of the NLRI field.

      Note that for each label, the length is increased by 24 bits (20
      bits in the Label field, plus 3 bits in the Rsrv field, plus 1 S
      bit).


If we use BGP-LU with multiple labels for SR-TE, it will always advertise
host routes, which given max 255 bits of NLRI, leaves us with theoretical
maximum of 9 labels for IPv4 and 5 labels for IPv6. The RFC message format
will not be able to support more labels, regardless of platform
capabilities.

Am I missing something? Is there a way to advertise more labels in BGP-LU?


Regards,
Dmytro

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<div dir=3D"ltr">Dear MPLS WG,<div><br></div><div>I&#39;ve been working on =
an SR-TE project using BGP-LU to influence traffic engineering paths. This =
requires the advertisement of multiple labels.</div><div><br></div><div>RFC=
8277 seems to have 2 conflicting statements</div><div><br></div><div>Sectio=
n 2.1:</div><div><br></div><div><pre class=3D"gmail-newpage">   the Count i=
s the maximum
   number of labels that the BGP speaker sending the Capability can
   process in a received UPDATE of the specified AFI/SAFI.  If the Count
   is 255, then no limit has been placed on the number of labels that
   can be processed in a received UPDATE of the specified AFI/SAFI.</pre></=
div><div><br></div><div>This assumes the BGP update can have up to 255 labe=
ls (in theory).</div><div><br></div><div>Section 2.3:</div><div><br></div><=
div><pre class=3D"gmail-newpage">   - Length:

      The Length field consists of a single octet.  It specifies the
      length in bits of the remainder of the NLRI field.

      Note that for each label, the length is increased by 24 bits (20
      bits in the Label field, plus 3 bits in the Rsrv field, plus 1 S
      bit).
</pre></div><div><br></div><div>If we use BGP-LU with multiple labels for S=
R-TE, it will always advertise host routes, which given max 255 bits of NLR=
I, leaves us with theoretical maximum of 9 labels for IPv4 and 5 labels for=
 IPv6. The RFC message format will not be able to support more labels, rega=
rdless of platform capabilities.</div><div><br></div><div>Am I missing some=
thing? Is there a way to advertise more labels in BGP-LU?</div><div><br></d=
iv><div><br></div><div>Regards,</div><div>Dmytro</div></div>

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