Gunter – I was not involved in writing RFC 6119 – but I don’t believe what you reference is a typo. The section from which you quote starts with:
“To prevent an SRLG TLV and an IPv6 SRLG TLV in the same logical LSP from causing confusion of interpretation, the following rules are applied.” So it seems clear to me that this was done intentionally. The authors are in the best position to explain why (if they remember after all these years 😊 ). In regards to RFC8919<https://datatracker.ietf.org/doc/html/rfc8919#section-4.3>, as we were defining a new TLV that supported both IPv4 and IPv6, we saw no reason to limit the per link advertisement to a single TLV – especially given the potential for exceeding the 255 byte TLV limit. So we intentionally did not impose this restriction. Note that RFC5307<https://datatracker.ietf.org/doc/html/rfc5307#section-1.4> does not make an explicit statement regarding TLV 138 – so you are correct in thinking that it is possible to have multiple TLV 138 for the same link. This has been made explicit in https://www.ietf.org/archive/id/draft-ietf-lsr-multi-tlv-19.html#name-mp-tlv-for-is-is-top-level- . Given the explicit statement in RFC 6119 forbidding multiple TLVs/link for TLV 139 we could not alter that in the MP-TLV draft w/o introducing backwards compatibility issues. HTH Les From: Gunter van de Velde (Nokia) <[email protected]> Sent: Tuesday, October 28, 2025 7:49 AM To: [email protected]; [email protected]; [email protected]; lsr <[email protected]> Subject: [Lsr] clarification question about IPv6 SRLG TLV procedure documented by RFC6119 [writing as individual contributor without AD hat] Hi folks, LSR WG, I am trying to figure out if RFC6119 has a typo or if it was the intent. In RFC6119 we see the following procedure: “There MUST NOT be more than one IPv6 SRLG TLV for a given link.” https://datatracker.ietf.org/doc/html/rfc6119#section-4.4 So, this forbids more as a single IPV6 SRLG TLV for a single link. The RFC is very clear and formal in this procedure. No issue understanding this. But this seems to deviate from prior SRLG TLV procedures. According RFC8919<https://datatracker.ietf.org/doc/html/rfc8919#section-4.3>, its possible to have multiple Type 238 TLVs for a particular link. “Multiple TLVs for the same link MAY be advertised.” Also according to RFC5307<https://datatracker.ietf.org/doc/html/rfc5307#section-1.4>, for TLV138 the following statement is made. “The SRLG TLV MAY occur more than once within the IS-IS Link State Protocol Data Units.” – So for me its possible to have multiple type 138 TLVs as well. So not sure why this restriction is there only for Type 139 TLVs? Was this intentional or a accidental typo that slipped into RFC6119? Thoughts? Brgds, G/ (as individual contributor)
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