Subject: Re: question on microloops

From: "Alok" <[email-masked]>
Date: Wed, 3 Aug 2005 15:26:13 +0530

Thanks Curtis
>
>
> FYI- Metrics in ISIS and costs in OSPF are unsigned integers.  There
> is no way to signal a "negative arc" in either link state protocol.
>
> If a local node changes the metric being used from that which was
> signaled and ends up with a negative number that's a local matter (and
> should be considered a bug in the local implementation).
>

What if a local node reduces the metric?
A metric need not be a "value", it can be any order of decision making,
numbers/values are an ordered set y={1,2,3,...}
Y[n]>y[n-1] always holds true

similary any attribute can form an ordered set
X={....origin.as_path,local_pref..}
and define relations X[2]>X[1] etc

a negaitve cycle can be formed anytime one prefers X[k] over X[m] where k <m

for example if i take BGP,

Local pref > AS_PATH_LEN>Origin>MED and so on.....

I understand certain paramters have AS wide significance, but isnt RFC 3345
also a case of negative arcs? Same for weights in eBGP?

> Curtis

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From: mike shand <[email-masked]>
Date: Wed, 03 Aug 2005 11:28:23 +0100

> > is no way to signal a "negative arc" in either link state protocol.

and not only that. Standard Dijkstra breaks with negative arcs (although it 
can be modified to handle them).

>also a case of negative arcs? Same for weights in eBGP?

Currently we are only considering Link State protocols. so the above 
constraints apply.

         Mike

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From: "Alok" <[email-masked]>
Date: Wed, 3 Aug 2005 16:06:08 +0530

ahh thanks,

then you can detect the loop anyways by the node info list
unless u plan to have ring of areas and give "area ids" a meaning.

So on an OSPF speaking node,
if for example i say "no i want to *reduce the actual cost* for this end
point X in all associated LSAs on this node,install it in my table (if it
works out to be the best) and then forward the LSA" it may just be possible
to actually be able ..right? though I dont know why bother when u can just
signal :-)

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From: Curtis Villamizar <[email-masked]>
Date: Wed, 03 Aug 2005 08:45:44 -0400

> to actually be able ..right? though I dont know why bother when u can just

If no one violates the spec by changing the value that should be
advertised with a summary LSA, then everything is fine.

The cost of a summary route advertised into an area is always less
than the cost of the same summary route advertised out since at least
one link is crossed and the link must have a positive cost.

A zero cost can be advertised on a stub such as a loopback address but
should not be advertised for any other stub and never on a link.

Most routers give you plenty of rope to hang yourself.  There are all
sorts of stupid ways to misconfigure a router that both violate
protocol specs and can make a mess of your network.  You can add
arbitrary static routes and advertise them out.  You can lower the
cost on summary routes.  You can truncate the BPG Path attribute.  All
of these can potentially cause a persistant routing loop.  (None of
these is a negative cost just injection of just plain wrong
information).  Since we already know this and have acknowledged it for
well over a decade and you don't seem to have anything new to add,
further discussion along these lines seems pointless.

Curtis