Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-isis-mpls-elc-12: (with DISCUSS and COMMENT)

Alvaro Retana <> Fri, 22 May 2020 14:59 UTC

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From: Alvaro Retana <>
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Subject: Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-isis-mpls-elc-12: (with DISCUSS and COMMENT)
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On May 21, 2020 at 3:39:03 PM, Benjamin Kaduk wrote:



> With respect to Alvaro's clarification, your answer for (1) makes sense;
> thanks! I think Alvaro has offered to help work out what (if any)
> additional text we might want to be sure that the answer to (2) is clear in
> the document.

I think that #1 is where some clarification could be useful. :-)

I'm including both ISIS and OSPF suggestions below to consolidate the

> > My interpretation of Ben's question is two-fold:
> >
> > (1) Would ISIS routers normally propagate the information to a
> > different level? The ELC is a new prefix attribute flag -- are prefix
> > attributes always propagated (unchanged) to other levels? If so, then
> > the requirement (MUST) is not needed. My reading of rfc7794 is that
> > the propagation is optional...
> depends on the attribute or a bit. Some are propagated some are not.
> That's why we are saying this one MUST be preserved.


For ISIS I think the current text is in line with the specification of
the other bits in rfc7794.  No changes are needed.

If anything, you may want to change the order of this sentence to
address Ben's comment:

   When a router propagates a prefix between ISIS levels ([RFC5302], it
   MUST preserve the ELC signaling for this prefix.

   The ELC signaling MUST be preserved when a router propagates a prefix
   between ISIS levels ([RFC5302]).

[Similar for OSPF.]

I think that for OSPF it is not that simple...

For OSPFv2: rfc7684 says that the "scope of the OSPFv2 Extended Prefix
Opaque LSA depends on the scope of the advertised prefixes", which I
assume means that for intra-area prefixes the scope will be the ABR wouldn't simply propagate it; it would have to
originate a new LSA.

Suggestion (Add to 3.1)>
   When an OSPFv2 Area Border Router (ABR) distributes information between
   connected areas it SHOULD originate an OSPFv2 Extended Prefix Opaque LSA
   [RFC7684] including the received ELC setting.  If the received information
   is included in an LSA with an AS-wide scope, then the new LSA is not needed.

For OSPFv3: The PrefixOptions are *in* the LSA, but I couldn't find
anything in rfc5340 saying that the received values should be copied
into the Inter-Area-Prefix-LSA (nor that they should not).

Suggestion (Add to 3.2)>
   When an OSPFv3 Area Border Router (ABR) distributes information between
   connected areas, the setting of the ELC Flag in the Inter-Area-Prefix-LSA
   MUST be the same as the received value.

> > (2) If the propagation is not automatic, and the L1L2 router doesn't
> > support this specification, then what are the drawbacks/failure
> > scenarios? IOW, for multi-level operation is it a requirement that
> > the L1L2 support this specification?
> drawback are identical to what is mentioned in the Security
> Considerations section.

I think that text is ok.