Re: BGP-4 changes

[Yakov Rekhter <[e-mail]>]

Rich,

> 
> >> I agree with your intent, but this is too easy to misinterpret.  For
> >> example, suppose a router has three routes for a prefix, with the same
> >> local preference: one from AS X w/MED 5, one from AS X w/MED 10, and one
> >> from AS Y w/MED 15.  I think the consensus is that the router should not
> >> compare the MED values 5 and 10 from AS X with the MED value 15 from
> >> AS Y.  On the other hand, we should also make it clear that as part of
> >> MED comparison, the route from AS Y w/MED 15 should be preferred over
> >> the route from AS X w/MED 10 -- otherwise, you can get routing loops.
> 
> > Comparing MEDs on routes coming from different ASs is not very
> > meaningful, as MED is intended to reflect IGP distance within a
> > particular neighboring AS, and one shouldn't expect that IGP distances
> > would be comparable across all neighboring ASs.
> 
> If I *did* compare MED values on routes from different ASs, I wouldn't
> have chosen the route from AS Y w/MED 15 over the route from AS X
> w/MED 10 -- instead, I would have chosen the route from X since 10 < 15.
> 
> When I say to choose the route from AS Y w/MED 15 over the route from
> AS X w/MED 10 at MED comparison time, you may re-interpret this using
> John Scudder's text as: at MED comparison time, keep the route to AS Y
> w/MED 15, and eliminate the route from AS X w/MED 10 (it's inferior
> MED-wise to the route from AS X w/MED 5).

Got it.
  
> > Thus while I certainly agree with your statement that "the router
> > should not compare the MED values 5 and 10 from AS X with the MED value
> > 15 from AS Y", I don't think that "the route from AS Y w/MED 15 should
> > be preferred over the route from AS X w/MED 10" (as it compares
> > uncomparable things).
> 
> Hmmm. I thought John and I were saying essentially the same things
> (in different ways)... John wins the clarity contest -- and I'm quite
> satisfied.

Yes, it was just a matter of clarification.

Yakov.