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

[Alvaro Retana <[e-mail]>]

On May 21, 2020 at 6:05:41 AM, Peter Psenak wrote:


Peter:

Hi!


> > ----------------------------------------------------------------------
> > DISCUSS:
> > ----------------------------------------------------------------------
> >
> > As for other reviewers, many of my comments duplicate those for the OSPF
> > document; I expect that the analogous responses apply and am fine if
> > they only appear for one document's review.
> >
> > Here, the question I have about normative language applies to the text
> > in Section 3:
> >
> > When a router propagates a prefix between ISIS levels ([RFC5302], it
> > MUST preserve the ELC signaling for this prefix.
> >
> > The scenario in question is analogous to the OSPF cross-area case: is
> > the router propagating the prefix between ISIS levels required to
> > implement this document; is preservation of the flag value a new
> > requirement from this document vs. a preexisting property; and is this
> > document trying to make normative requirements of devices that don't
> > implement this document?
>
> ##PP
> this is a new requirement and only applies to the routers that support
> this document. We are not making normative requirements of devices that
> don't implement this document, we cannot.
>
> Maybe we can add that it only applies to the routers that supports this
> extension:
>
> "When a router supporting this extension propagates a prefix between
> ISIS levels ([RFC5302], it MUST preserve the ELC signaling for this prefix."
>
> Would it work?


You're right, we can only apply requirements to routers that support
this specification.  IOW, adding the clarification is not necessary.


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...

(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?


Thanks!

Alvaro.