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Re: Level 3 BGP Advertisements
From: Jon Lewis <jlewis () lewis org>
Date: Wed, 29 Aug 2012 15:48:00 -0400 (EDT)

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 properly.

 Jon Lewis, MCP :)           |  I route
 Senior Network Engineer     |  therefore you are
 Atlantic Net                |
_________ http://www.lewis.org/~jlewis/pgp for PGP public key_________

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