How Not to Multihome

Mon Oct 8 21:55:55 UTC 2007

On Mon, 8 Oct 2007, Keegan.Holley at sungard.com wrote:

> I have a client that wants us to advertise an IP block assigned by another
> ISP.  I know that the best practice is to have them request an AS number
> from ARIN and peer with us, etc.  However, I cannot find any information
> that states as law.  Does anyone know of a document or RFC that states
> this?

It's not 'law' per se, but having the customer originate their own 
announcements is definitely the Right Way to go.

Some providers take a pretty dim view of seeing chunks of their address 
space show up in advertisements originating from someone who isn't one of 
their customers.  It can make troubleshooting connectivity problems for 
that customer (from the provider's point of view) very painful, i.e. "Hey, 
this AS, who isn't one of our customers, is hijacking IP space assigned to 
one of our customers!"  The provider could then contact your host's 
upstream(s) and ask them to drop said announcement under the impression 
they're stopping someone from doing something bad.

Also, if some network out there aggregates prefixes in an aggessive/odd 
manner, the disjoint announcement, and the reachability info it contains 
could be washed out of their routing tables, causing connectivity 
problems.

Standard caveats about the block being a /24 or larger also apply.

jms