Utilising upstream MED values

Sat Mar 19 23:37:09 UTC 2005

> -----Original Message-----
> From: owner-nanog at merit.edu [mailto:owner-nanog at merit.edu] On 
> Behalf Of Sam Stickland
> Sent: Friday, March 18, 2005 9:05 AM
> To: nanog at nanog.org
> Subject: Utilising upstream MED values
> 
> 
> 
> Hi,
> 
> We're looking at doing outbound traffic values based on 
> upstream ("tier1") 
> MED values. But, of course, there's no standard for MED 
> values. Assuming I 
> can get definations from the upstreams as to what their MED 
> values mean, I 
> have to rebase them into a common range.

You've answered the question right there.  Since one provides uses 
apples for its MEDs, and another uses oranges, comparing them to each
other. . .

> However, a route-map (cisco, foundry) only allows you to set, add or 
> subtract to an MED value, so this doesn't seem possible.

Theoretically, if you can establish a conversion constant for however
you define "far" to however ISP Apple defines it and however ISP Orange
defines it, you just have to apply that value.  Match all routes 
learned from AS Orange and add or subtract the conversion value.  Only
works if the range of values is pretty close, though.

You *could*, if the range is small enough, write route maps to match
metric and set metric.  

> Is anyone doing this, and if so how?

Or you can hack a script to do traceroutes to various arbitrary 
destinations, compare to the routes learned, and evaluate the Round Trip
Time (or number of hops, or whatever) to MED, establishing your own 
locally-significant table.  Sounds like a lot of trouble to me, but 
YMMV.  

> 
> Sam
> 

Lee