[mpls] RFC 8277 clarifications
Dmytro Shypovalov <dmytro@vegvisir.ie> Fri, 31 October 2025 11:02 UTC
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From: Dmytro Shypovalov <dmytro@vegvisir.ie>
Date: Fri, 31 Oct 2025 11:01:55 +0000
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Subject: [mpls] RFC 8277 clarifications
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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
- [mpls] RFC 8277 clarifications Dmytro Shypovalov