Re: BGP-4 changes

[Curtis Villamizar <[e-mail]>]

In message <[e-mail]>et>, [e-mail] write
s:
> *** Resending note of 08/31/96 15:15
> Subject: Re: BGP-4 changes
> John,
> 
> I now have second thoughts about changing the MED value; stripping the
> attribute is of course still OK.  The problem is not route loops,
> but non-deterministic routing (i.e. not predictable).
> 
> >As far as I'm concerned, being able to strip the attribute at the border
> >achieves this goal fine.  Either or both is OK with me.
> 
> >In private email Curtis mentioned a case (which in retrospect I should have
> >seen) where we should be able to strip MED after route selection but before
> >export into IBGP.  The case applies when I have a router facing two
> >neighbors across a DMZ.  I want to consider the MED to get the next hop
> >"right" across the DMZ, but I want to use shortest-path inside my AS.  I
> >believe that this stays loop-free since we only consider MED once we
> >have already decided to choose an external route.
> 
> Let's assume that a router considers the original EBGP MED in the
> route selection process, but sends the modified MED in the
> IBGP export.  Further assume a provider modifies the MED to value
> 0 at two border gateways (A & B), in order to try to draw traffic to
> the nearest of these two exits.

Removed MED attribute completely, not modified.

> Suppose border A gets an external BGP route with MED X first.  With no
> competition from B, border A installs the route and exports into IBGP
> with MED 0.  Border B will see this same route only with MED 0.
> 
> Later, border B receives its route with MED Y (same prefix, same
> neighbor AS, same preference, etc.).  So long as Y > 0 (it usually
> is), then border B will reject the external in favor of the internal
> route from A.
> 
> Note that acceptance of the internal route from A over the external
> route of B is based totally on timing issues -- first come, first
> served.  It only occurs when the MED value is decreased -- that's why
> stripping the MED is OK, since missing MEDs have value -1.
> 
> To fix this, one might use the modified MED value for both route
> selection and IBGP export, but one throws away useful information
> as Curtis and John mention above (for optimizing local external
> route selection).
> 
> Do we need the ability to change the MED attribute value, or is
> it sufficient to be able to remove the MED attribute?  Can we get
> away with modifying local preference instead of changing the MED
> value?
> 
> -- Richard Woundy, IBM

If you removed MED completely, with the current rule changes (sounds
like professional sports commission) you don't get a routing loop.

John didn't write anything about changing MED and I didn't write
anything about changing MED to John.

Curtis