Level 3 BGP Advertisements

Jon Lewis jlewis at lewis.org
Wed Aug 29 19:48:00 UTC 2012

On Wed, 29 Aug 2012, Nick Olsen wrote:

> Anyways, I've always thought that was standard practice. And its never been
> a problem. Until we brought up peering with level 3..

No...I'd call that global table pollution.  In general, there's no reason 
you should announce your CIDRs and all their /24 subnets.

> I noticed that while the /24's made it out to the world. The larger
> counterparts (2 /21's and a /20) did not. So, I start sniffing around. Find
> that I do indeed see the prefixes in Level 3's looking glass but they
> aren't handing it off to peers. So, Naturally, I land on this being some
> kind of prefix filtering issue and open a ticket with Level 3. They tell me
> this is standard practice. And If I want to see the /20 or /21's make it
> out to the rest of the world, I need to stop sending the /24's.
> Does this sound normal?

No.  I announce to Level3 our IP space and 2 subnets of each CIDR (i.e. 
/17 + 2 /18 subnets of that /17, etc.), but I use community tags (and 
other tricks) to mark the more specifics as advertise to [certain] L3
customers only, and let the less specifics out to the world.  The only 
problems I've had with this have been when L3 peers have become customers, 
and one L3 customer doing something odd (never did find out what) that 
caused them to effectively null route our space until I kept them from 
seeing the more specifics (creative abuse of loop detection).

Level3's prefix filter for your session should be built based on IRR data. 
If it's not doing what you want, you probably haven't setup the IRR data 

  Jon Lewis, MCP :)           |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
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