Subject: Re: question on microloops

From: "Alok" <[email-masked]>
Date: Wed, 3 Aug 2005 19:02:37 +0530

Thanks for the clarifications Curtis,
posting back the reply to the list,

----- Original Message ----- 
From: "Curtis Villamizar" <[email-masked]>
To: "Alok" <[email-masked]>
Cc: <[email-masked]>
Sent: Wednesday, August 03, 2005 5:52 PM
Subject: Re: question on microloops

>
> In message <[email-masked]>
> "Alok" writes:
> >
> > 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 -ve arcs?
>
>
> A persistant route oscillation and a persistant routing loop are two
> very different things.  Although a persistant route oscillation along
> with a transient routing loop can cause a great deal of trouble.
>
> There is never a negative arc in the RFC 3345 cases.
>
> Where there are multiple metrics, such as Local pref, AS_PATH_LEN,
> Origin, MED you can consider this to be one "metric space".  Comparing
> two values in this metric space is like comparing decimal numbers with
> multiple digits.  You compare the most significant first, and only if
> they are equal move to the next.
>
> The RFC 3345 churn is caused by non-deterministic comparisons based on
> MED.  The first received rule is at fault here and violates the
> current standard.  The best solution would be to always compare MEDs
> or compare border AS number first (rather arbitrary), then MEDs to
> make the comparison deterministic again without comparing MEDs from
> different peers.  Essentially the transitive property does not hold
> true in the metric space when MEDs are compared only if the border AS
> is the same but border AS does not appear earlier in the comparison.
>
> Curtis
>

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