Re: [Isis-wg] Requesting WG adoption of draft-baker-ipv6-isis-dst-src-routing-06

"Les Ginsberg (ginsberg)" <> Thu, 27 October 2016 14:58 UTC

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From: "Les Ginsberg (ginsberg)" <>
To: David Lamparter <>, Chris Bowers <>
Thread-Topic: [Isis-wg] Requesting WG adoption of draft-baker-ipv6-isis-dst-src-routing-06
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Date: Thu, 27 Oct 2016 14:57:56 +0000
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Subject: Re: [Isis-wg] Requesting WG adoption of draft-baker-ipv6-isis-dst-src-routing-06
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David -

> -----Original Message-----
> From: Isis-wg [] On Behalf Of David
> Lamparter
> Sent: Thursday, October 27, 2016 7:12 AM
> To: Chris Bowers
> Cc:;;
> list (
> Subject: Re: [Isis-wg] Requesting WG adoption of draft-baker-ipv6-isis-dst-
> src-routing-06
> Hi Chris,
> On Thu, Oct 27, 2016 at 12:51:19PM +0000, Chris Bowers wrote:
> > I have two main comments/questions on this.
> Useful feedback, Thanks!
> > 1)  If I understand correctly, one reason to use existing
> > multi-topology routing mechanisms for this is the requirement to
> > support deployments where only a partial subset of routers support
> > source address dependent routing.  Ideally, this would allow an enterprise
> site to start by upgrading a subset of routers to support SADR, starting at the
> site egress routers.
> Yes, that's the intent.
> > However, this statement from section 2.3 seems to imply that all of
> > the routers in the enterprise network would need to support multi-
> topology even if they don't support SADR.
> No, non-MT routers would be ignorant of the MT information and would not
> see any of the SADR routes -- which is exactly the intended goal.  I'll update
> the draft to say this very clearly.
[Les:] For my taste the best way to do this is simply to remove the statement:

" Even installations that previously used only MTID 0
   (i.e.  no M-ISIS) would need to start using MTID TBD-MT0."

You don't need to explain the way MT works because you are making no changes to RFC 5120. Trying to "explain" is only going put you at risk for misinterpretation - and this thread is an existence proof for that. :-)

It is the first sentence of the same paragraph which is important:

" As this compatibility mechanism is not considered optional, M-ISIS
   MUST therefore be implemented for supporting the protocol outlined in
   this document."

(though I would prefer replacing "protocol" with "extensions")


> >    As this compatibility mechanism is not considered optional, M-ISIS
> >    MUST therefore be implemented for supporting the protocol outlined in
> >    this document.  Even installations that previously used only MTID 0
> >    (i.e.  no M-ISIS) would need to start using MTID TBD-MT0.
> Ah, I see where I misworded that.  It needs to say "need to start using MTID
> TBD-MT0 *for transporting SADR routes*".
> Sidenote:  at some point, the draft allowed non-usage of MT for greenfield
> setups (all routers with SADR support and everything in zero MTID / no M-
> ISIS used at all), with homenet in mind for that.  I'm now thinking that was a
> stupid idea.  However, there might be leftover weird wording in the draft
> from that.
> > It would be good to clarify this statement.  I interpret it to mean
> > that all prefix advertisements to use TLV#237, so that even routers
> > that are currently using TLV#236 to advertise IPv6 prefixes would need
> > to start using TLV#237 instead.  But it is not clear what MT-ID a router not
> supporting SADR should use when advertising a prefix in TLV#237, since it
> can't use MT-ID=0.
> No -- all non-SADR routing information stays in MTID 0/TLV 236 (or 2/237, if
> you're using separate IPv4/IPv6 MT topologies).  There is no change to non-
> SADR routes, and by using a separate MTID for SADR, they will be routed
> "around" as non-participants in the SADR topology to ensure proper SADR
> operation.  (That's the core idea why MT is used
> here.)
> As a nice side-effect, the operator also has some good control here in
> feeding information to non-SADR systems.  For example, they can advertise
> non-SADR default routes at one or more selected SADR routers, so traffic
> towards the internet flows towards the "SADR subdomain", where it can
> then be SADR-routed to the proper exit.
> (MT makes sure SADR-routed traffic, once in the "SADR subdomain", will
> never be routed back towards non-SADR routers.  They're not in the MT
> topology, thus not in SPF, thus not in the shortest path used for SADR
> routes.)
> > In any case, it would be good to clarify overall if and how this
> > solution achieves the goal of requiring only a subset of routers to be
> upgraded from a basic non-MT deployment of ISIS.
> > As part of this, it would be good to explain how TLV#222 and TLV#229
> > (the other multi-topology
> > TLVs) are used in the partial deployment case.
> >
> > 2)  For addressing PA multi-homing for IPv6 w/o NAT in existing
> > enterprises,  I would guess that >90% of enterprises running a
> > link-state routing protocol are running OSPF as opposed to ISIS.  Is the
> expectation that enterprises will switch to ISIS
> > in order to address this problem?   I would like to better understand what
> use cases this work
> > is targeted at from a practical point of view.
> There is/was a companion OSPF draft:
> which we need to revive I guess.  The OSPF version has the additional
> constraint that it is built on another draft in progress:
> which led to (my personal) decision to focus on the IS-IS draft and "fix OSPF
> later."  Sorry, OSPF ;)
> Updating the doc -- also planning to explain some protocol-agnostic MT
> considerations in the rtgwg document -- expect another mail from me soon
> (with a diff attached),
> -David
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