Subject: RE: Multiple-topology IS-IS [RE: [Isis-wg] Feedback on IS-IS AutomaticEncapsulation]

From: "Tony Li" <[email-masked]>
Date: Wed, 23 Jul 2003 00:22:19 -0700

Another alternative that I would expect to see is a 'pilot'
deployment of a protocol in a very limited fashion.  During
this pilot, the reliability of v6 would not be part of an
SLA, so the packet losses resulting from concavity would not be 
catastrophic.

Tony

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From: Albert Tian <[email-masked]>
Date: Wed, 23 Jul 2003 01:41:13 -0700

Pekka Savola wrote:
> 
> On Wed, 23 Jul 2003, Tony Li wrote:
> Very true.
> Note that you should also take care not to include v6-only (or boxes which
> should be mainly for v6, as seems to be common in ISPs' IPv6 deployments)
> boxes in the network -- to avoid the possible issue of routing _v4_
> through those _v6_ boxes.
> 
> In any sane network, this should not be an issue due to the very high link
> costs assigned to links to such routers, though.

So we all agree that maintaining convexity while considering the failure cases
is very difficult, correct? I actually do not think the failproof solution can
be easily calculated without special tools. Also because this requires tweaking
the link cost, it would disturb the carefully engineered v4 traffic pattern,
which is undesirable(this is another example showing the limitation of sharing
the cost between v4 and v6). The situation will be even worse if the pilot
involves more than one phase and it requires you to expand the convex once or
more.

Given all the above, relaxing the SLA seems to be the only solution. Still there
may be troubleshooting issues. If the pilot lasts for extended period of time,
various failure cases are bound to happen. When it happens, the situation will
be quite undebugable IMHO. Changing a cost or shutting down an interface may
break your convexity, and it may be hard to find the causality. (Guess vendors
need to add the "concave detector" in the spf then).

Also in somecases the pilot may be driven by where your first v6 costumers are
so the convex may not come in handy(say you want to enable 3 sites but the
trangle pretty much covers all your network).

In short, my point is that the convex has its limitations when used as a generic
transitional strategy.

my 2c,

Cheers,

Albert

> 
> --
> Pekka Savola                 "You each name yourselves king, yet the
> Netcore Oy                    kingdom bleeds."
> Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings