[j-nsp] bgp soft clear and metric minimum-igp, bug?

[Richard A Steenbergen]

On Tue, Jan 20, 2004 at 06:37:26PM -0500, Jeff Wheeler wrote:
> I've noticed that when changing a neighbor's export policy from any
> configuration, to a configuration that sets metric minimum-igp <offset>
> on an exported route, the advertised MED will not be updated if the
> value would increase as a result, even if the previous configuration did
> not use a minimum-igp based MED value.
> 
> Is this the intended functionality, or an implementation quirk?

This is an intended effect of the "minimum-igp" command. As far as I know 
it doesn't actually state a reason for this in the documentation, but I 
would assume it is to prevent unnecessary route flapping in the event of 
an IGP cost change, by not forcing a route update if the metric value 
increases past the previous lowest known value.

Of course this begs the question, exactly how long will it stay this way
(forever?) and what happens if you make a permanent routing change which
takes away the lowest cost route? Would you have to clear it in order for
the updates to be processed, and if so would a soft clear out do the
trick? Also, in the event of a sustained outage of the normal lower cost
path (or anything more than a brief flap I would think), it might make
more sense to propagate the new MEDs so that anyone listening to them can
shift traffic accordingly. Perhaps it would make even more sense for there
to be a time limit, where if the cost does not return to its previous
value within X amount of time, an update is sent? Or am I completely
reading this feature wrong?

At any rate, if you don't want the minimum igp cost, leave off the 
"minimum-" part. :)

-- 
Richard A Steenbergen <ras at e-gerbil.net>       http://www.e-gerbil.net/ras
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)