MED and community fluctuation

Zhen Wu zwu at
Fri Oct 8 18:40:54 UTC 2004

I understand the usage and application of MED and community attributes. 
But we watch some interesting policy/attribute fluctuation of MED and 

We analyze BGP updates sent out by RouteViews peers. We observed a lot 
of AADupType2 instabilities. We define AADupType2 event as: A route is 
implicitly withdrawn and replaced
with a duplicate of the original route. The two messages have the same 
NEXTHOP and ASPATH attribute, but differ between at least one other 
attribute (like MED). Note that we only pair two updates from the same 
RouteViews' peer / neighboring AS.

A fine-grained analysis revealed that most AADupType2 instabilities 
just change the MED or community attribute (or both), which means that 
MED and community values associated with prefixes are changing 
dynamically and frequently. We are not claiming it is a common practice 
of the Internet, this is just the cases observed from RouteViews peers.

We are thinking of the motivation of doing this? Why the ISPs 
configured their network so that the MED values oscillate? Did ISPs 
configure MED values dynamically calculated from IGP metrics?

The most strange thing to us is why the community attribute associated 
with prefixes also oscillate frequently. We know that community are 
used to simply operational complexity: group a set of prefixes to apply 
the same policy. It didn't make sense to me to change it dynamically. 

Any comments or discussions? Thanks!

Zhen Wu

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