[Isis-wg] draft-ietf-isis-traffic-01.txt

[Don Fedyk]

This is interesting. It seems to me the current pratice
of limiting the metric to a maximum that is less than 
infinity ie 63 is incompatible with a new metric beyond 
63. If metrics beyond 63 were infinity for old systems, 
this would not happen.

Don 

> -----Original Message-----
> From: Tony Przygienda [mailto:prz@siara.com]
> Sent: Tuesday, October 26, 1999 8:48 PM
> To: Jeff Learman
> Cc: Tony Li; Fedyk, Don [BL3:2001-I:EXCH]; 
> isis-wg@external.juniper.net;
> te-wg@UU.NET
> Subject: Re: [Isis-wg] draft-ietf-isis-traffic-01.txt
> 
> 
> Jeff Learman wrote:
> > 
> > Tony,
> > 
> > Right: black holes, not loops.
> 
> It's not exactly because of the MAX_PATH but 
> loops are possible in a mixed old-TLV, new-TLV
> network if e.g. the same prefix is 
> being advertised twice and if 
> certain new link metrics are longer >63 and 
> certain are not (assuming that we'd generate
> old-style TLVs for anything <64). In the figure  O denotes
> old-routers, N new routers and the dashed
> lines are links with metrics
> 
> 
> 	X/Y
> 	|
> 	65
> 	|
>        [N]
> 	|
> 	10
> 	|	
>        [O1]
>    	|
>         10
>         |
>        [N1]
> 	|
> 	63
> 	|
>        [O]
> 	|
> 	63
> 	|
>        [O]
> 	|
> 	63
> 	|
> 	X/Y
> 
> So new router N1 will see the shortest path to X/Y
> as 65+10+10=85 whereas O1 will believe that the 
> shortest path is 10+63+63+63>85 since it cannot see
> any links with metric longer than 64 (new TLVs can
> carry it, old ones cannot). Observe that I introduced
> X/Y into the network twice to make a simple example,
> in reality it's enough O1 cannot compute the route 
> to X/Y (because it cannot use the links with >64) and 
> uses aggregated/default forming a loop.
> 
> And yes, trying to make everything >64 to be just 64 
> in old TLVs doens't help either. Examples are easily
> constructed. 
> 
> For a much more thorough (but rather formal so nothing 
> for the faint-hearted) dealilng 
> with this (IMHO) interesting topic look @ this year's infocom
> 'Hop-by-Hop Routing with Node-Dependent Topology Information'
> 
> 
> 	thanks 
> 
> 	-- tony
> 
> 
> > 
> > Never mind,
> > Roseanna Roseannadanna
> > 
> > At 03:51 PM 10/26/99 -0700, Tony Li wrote:
> > >
> > >|  Earlier, I pointed out what I though was a problem with 
> the 24-bit
> > metrics,
> > >|  namely, that MAX_PATH_METRIC increased from 1024 to a 
> much larger value.
> > >|
> > >|  If you are running traffic engineering metrics in 
> parallel with existing
> > >|  TLVs, which value for MAX_PATH_METRIC should be used?  
> Must all routers
> > >|  in the subdomain support the new TLVs before they are 
> used at all,
> > >|  to avoid routing loops that would occur if some paths 
> exceed the smaller
> > >|  MAX_PATH_METRIC?
> > >
> > >
> > >Would inconsistent MAX_PATH_METRICs lead to forwarding 
> loops?  Or to
> > >blackholes?
> > >
> > >In any case, if you're going to run with both metrics, 
> then in normal
> > >operations, the metric would have to be smaller than 1024. 
>  If it were to
> > >exceed this, presumably due to an outage, then newer 
> systems might find a
> > >path while older systems didn't.  This would seem like a blackhole.
> > >
> > >And of course, if everything is a new system, you have a 
> valid path.
> > >
> > >I'm not sure I see the problem.
> > >
> > >Tony
> > >
> > >
> > ____________________________________________________________________
> > 
> >   Jeff Learman                           Internet:     jjl@one.com
> >   Engineering Manager                    Voice:     (734)-975-7323
> >   ONE (Open Networks Engineering, Inc)   Fax:       (734)-975-6940
> >   2725 South Industrial Pkwy, Suite 100
> >   Ann Arbor, MI  48104  USA
> > ____________________________________________________________________
> > 
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