Communities BCP [was: RE: BGP Path Filtering]

Danny McPherson danny at
Fri May 16 22:16:04 UTC 2003

On 5/16/03 4:01 PM, "Stephen J. Wilcox" <steve at> wrote:

> Hmm.. assuming you're interconnecting at multiple points with the same
> ASN they  will probably want to indicate to you where to send traffic
> to them using MEDs, if you go stripping them out you lose that info,
> check the peering policy..  this may put you in breach.
> Even without a breach of policy the MED will help find the best path to
> a route thats identical at two points and if you take it out you lose
> that.

Perhaps folks are foolish enough to put "MUST accept MEDs" in a bilateral
peering agreement (note that if money is attached the circumstance change,
but routing breaks all the same).  They should be cautious, however, not
only are MEDs the most obvious trigger for persistent route oscillation,
they _very often break with aggregation (unless you're foolish enough to
accept more-specifics as well!) and if compared between multiple ASes (or
even in the same AS) often turn your "cold-potato routing" into
"mashed-potato-routing" because of disjoint policies between different
domains, or downright stupid MED-derivation mechanisms.

Not intending to turn this into an MED rant, but more than a few of us have
experienced brokenness on account of MEDs in the past .. NANOG and IDR WG
archives have addressed these concerns a number of times.


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