Re: [Isis-wg] Conflicting MS entries

"Aissaoui, Mustapha (Mustapha)" <mustapha.aissaoui@alcatel-lucent.com> Wed, 24 June 2015 13:37 UTC

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From: "Aissaoui, Mustapha (Mustapha)" <mustapha.aissaoui@alcatel-lucent.com>
To: "bruno.decraene@orange.com" <bruno.decraene@orange.com>
Thread-Topic: Conflicting MS entries
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Date: Wed, 24 Jun 2015 13:36:55 +0000
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Cc: "spring@ietf.org" <spring@ietf.org>, "isis-wg@ietf.org list" <isis-wg@ietf.org>, "Stefano Previdi \(sprevidi\)" <sprevidi@cisco.com>
Subject: Re: [Isis-wg] Conflicting MS entries
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Hi Bruno,
Indeed what you are describing would be similar to having tunnels to the same destination prefix resolved via two different protocols (LDP and SR). As you said, maybe I am just not seeing a use case for allowing this. I can however see that one can program two SIDs for the same prefix using two different SPF algorithms.

Regards,
Mustapha.

> -----Original Message-----
> From: spring [mailto:spring-bounces@ietf.org] On Behalf Of
> bruno.decraene@orange.com
> Sent: Wednesday, June 24, 2015 3:38 AM
> To: Aissaoui, Mustapha (Mustapha)
> Cc: LITKOWSKI Stephane SCE/IBNF; spring@ietf.org; isis-wg@ietf.org list;
> Stefano Previdi (sprevidi)
> Subject: Re: [spring] Conflicting MS entries
> 
> Hi Mustapha,
> 
> > From: Aissaoui, Mustapha (Mustapha) [mailto:mustapha.aissaoui@alcatel-
> > lucent.com]
> > Sent: Tuesday, June 23, 2015 10:34 PM
> >
> > Hi Bruno,
> > I can see that one can program multiple SIDs if these are for routes
> > of the same prefix which are advertised in multiple ISIS instances or
> > in multiple topologies. But within a single instance and topology, how
> > do you reconcile the multiple MS entries with a single active route to that
> destination prefix?
> 
> I'm probably missing something, but I don't see a need to reconcile. I see 2 LSPs
> and for each a (N:1) indirection between the (label, FEC element) and the IP FIB. A
> bit similar to multiple BGP routes using the same IP prefix to resolve their BGP
> Next-Hop.
> 
> e.g. MS advertises 2 SIDs for prefix1 --> (via SRGBs) 2 incoming & 2 outgoings
> labels. LL1, LL2 for local/incoming labels. LN1, LN2 for neighbor/outgoing label.
> IP FIB:
> Prefix1 --> eth1
> 
> NHLFE:
> LFE1: eth1, swap LN1
> LFE2: eth1, swap LN2
> 
> ILM:
> LL1 --> LFE1
> LL2 --> LFE2
> 
> 
> Looks also similar to the case where a node(LSR) is both SR & LDP and for the
> same FEC element/IP prefix  gets 1 label from LDP and 1 from SR/MS.
> 
> That being said, if we are all in favor of selecting a single MS entry, the discussion is
> purely theoretical.
> 
> /Bruno
> 
> > Mustapha.
> >
> > > -----Original Message-----
> > > From: spring [mailto:spring-bounces@ietf.org] On Behalf Of
> > > bruno.decraene@orange.com
> > > Sent: Tuesday, June 23, 2015 9:48 AM
> > > To: Aissaoui, Mustapha (Mustapha)
> > > Cc: LITKOWSKI Stephane SCE/IBNF; spring@ietf.org; isis-wg@ietf.org
> > > list; Stefano Previdi (sprevidi)
> > > Subject: Re: [spring] Conflicting MS entries
> > >
> > > Hi Mustapha,
> > >
> > > Thanks for the discussion. Please see inline.
> > >
> > > > From: Aissaoui, Mustapha (Mustapha)
> > > > [mailto:mustapha.aissaoui@alcatel-> lucent.com] > Sent: Friday,
> > > > June 19, 2015 8:19 PM
> > > >
> > > > Hi Stephane and Bruno,
> > > > I do not think programming multiple SIDs makes sense. While there
> > > > are multiple MS prefix sub-TLVs, there is only single active route
> > > > for the prefix with potentially ECMP next-hops which was resolved
> > > > from a received IP reachability TLV.
> > >
> > > I don't see what prevents us from programming multiple SIDs/labels
> > > for a single prefix. i.e. setting up multiple LSPs for a given prefix.
> > > e.g. BGP seems to specifically allow for this
> > > https://tools.ietf.org/html/rfc3107#section-4
> > >
> > > That being said:
> > > - I'm not seeing benefit (which, may be, is what you meant by "makes
> > > no sense")
> > > - St├ęphane expressed that from an operational point, this may make
> > > things
> > harder.
> > > - eventually some implementation may find this harder compared to
> > > having a single one.
> > >
> > > So, so far, it looks like everyone expressed a preference to use
> > > only one, based on deterministic criteria.
> > >
> > > > I agree that selecting one of the entries is preferable to dropping traffic.
> > > Ok.
> > >
> > > > We
> > > > could come up with selection criteria but the reality is that
> > > > there no way for the router to check if any of the MS entries is legitimate or
> not.
> > >
> > > What do you mean by "legitimate"?
> > > In case of multiple SID advertisement for a prefix, they seem all
> > > equally legitimate to me. Looks like giving multiple names to
> > > something. e.g. we'll call this LSP/segment "A" or "R" or "Y".
> > > Now we may choose to only use one, based on a criteria TBD. e.g.
> > > smallest SID (which a priori improve the probability of fitting
> > > inside the
> > SRGBs).
> > >
> > > /Bruno
> > >
> > > > As a result, I
> > > > would think that once an entry is selected based on the criteria
> > > > and programmed, we should not be changing it unless the MS entry
> > > > is
> > withdrawn.
> > > >
> > > > Mustapha.
> > > >
> > > > > -----Original Message-----
> > > > > From: spring [mailto:spring-bounces@ietf.org] On Behalf Of
> > > > > stephane.litkowski@orange.com
> > > > > Sent: Friday, June 19, 2015 5:17 AM
> > > > > To: DECRAENE Bruno IMT/OLN; Stefano Previdi (sprevidi)
> > > > > Cc: spring@ietf.org; isis-wg@ietf.org list
> > > > > Subject: Re: [spring] Conflicting MS entries
> > > > >
> > > > > Even if choosing any IP to MPLS entry does not break anything,
> > > > > I'm not sure this is a good idea from an operational point of
> > > > > view to let it
> > > > undeterministic.
> > > > >
> > > > >
> > > > > -----Original Message-----
> > > > > From: spring [mailto:spring-bounces@ietf.org] On Behalf Of
> > > > > bruno.decraene@orange.com
> > > > > Sent: Thursday, June 18, 2015 09:29
> > > > > To: LITKOWSKI Stephane SCE/IBNF; Stefano Previdi (sprevidi)
> > > > > Cc: spring@ietf.org; isis-wg@ietf.org list
> > > > > Subject: Re: [spring] Conflicting MS entries
> > > > >
> > > > > Hi St├ęphane,
> > > > >
> > > > > > From: stephane.litkowski@orange.com> Sent: Thursday, June 18,
> > > > > > 2015
> > > > > > 9:23 AM
> > > > > >
> > > > > > Hi Bruno,
> > > > > >
> > > > > > "	 1) I don't really the issue. From a forwarding standpoint, looks
> > like
> > > > > > we can simply program multiple SIDs in the FIB."
> > > > > >
> > > > > > [SLI] What about the IP to MPLS entry ?
> > > > >
> > > > > [Bruno] If transit LSRs install all SIDs, an ingress may use any
> > > > > SID, no? Local decision.
> > > > >
> > > > > Bruno
> > > > >
> > > > >
> > > > >
> > > > >
> > > >
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