Re: BGP-4 changes

[Yakov Rekhter <[e-mail]>]

Rich,

>  >1. Add to 5.1.4 the following:
> 
>  >  MULTI_EXIT_DISC shall not be considered when comparing routes received fr
om
>  >  different neighboring ASs.
> 
>  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.

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).

Yakov.