Re: BGP-4 changes

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*** Resending note of 08/30/96 10:11
Subject: Re: BGP-4 changes
Yakov,

>> 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).

> 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.

> Yakov.

-- Rich