Subject: FYI: Route Flapping

From: Erblichs <[email-masked]>
Date: Mon, 9 Feb 2004 18:07:20 -0800

Group,

        I have a concern that Route Flapping algorithms
        are being employed by routers which
        violate the values of MinLSInterval/Arrival.

        In a heterogeneous environments where OSPF routers
        are from different vendors, these values IMO must
        be followed. However, it has been argued that
        convergence where routes are incorrect due to
        flapping is less desirable than the logical splitting
        of an area, due to some routers ignoring the
        above values and implementing some Route Flap
        algorithm.

        Feedback please...

        Mitchell Erblich

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From: Dave Katz <[email-masked]>
Date: Mon, 9 Feb 2004 18:58:04 -0800

MinLSInterval should have never been an architectural constant.
MinLSArrival should never have been an architectural constant either,
and arguably should never have existed (as it makes manipulation of the
transmit interval more problematic.)

Note that ignoring either of these values should have *no* effect on
interoperability, as any system that enforces MinLSArrival will drop
excessively rapid LS changes (and in any case, enforcing MinLSArrival
is insufficient to protect a system against excessive LSAs, and
building a system that is well protected against such things should be
outside the scope of the specification.)

It has been demonstrated that convergence of LS networks is far from
simultaneous even when they are homogeneous (there are way too many
factors that stand in the way) and in some cases may take multiple SPF
spins (particularly in the case of a link coming up.)  The only way to
improve the global convergence time, IMHO, is to do everything as
rapidly as possible (while protecting against meltdown, of course.)

Absurdly long SPF delays are the biggest impediment to progress in this
area.

For what it's worth, we dropped support for MinLSArrival some time ago,
and will regenerate a limited number of LSAs at a much more rapid
interval than 5 seconds before slowing down.

Grumpy as ever,

--Dave