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Re: Level 3 BGP Advertisements
From: William Herrin <bill () herrin us>
Date: Wed, 29 Aug 2012 16:35:57 -0400

On Wed, Aug 29, 2012 at 3:28 PM, Nick Olsen <nick () flhsi com> wrote:
In practice, We've always advertised our space all the way down to /24's
but also the aggregate block (the /20 or the /21). Just so there was still
reachability to our network in the event that someone made the foolish
mistake of filtering lets say prefixes smaller /23...

Anyways, I've always thought that was standard practice.

That's very poor practice. Each announcements costs *other people* the
better part of $10k per year. Be polite with other peoples' money. If
the /24 shares the exact same routing policy as the covering route,
announce only the covering route.

For all the good it'll do you, you can break it out to /24's when and
if someone mis-announces one of your address blocks. Competing
announcements of the /24 still won't leave you with correct
connectivity. If anything, putting the /24 announcement in ahead of
time will delay your detection of the problem by causing a partial
failure instead of a total one.


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?

That's insane. Assuming you're authorized to announce that address
space, Level 3 should be propagating your announcements exactly as you
make them. As only one of your peers, they're in no position to
understand the traffic engineering behind your announcement choices.
If they are acting as you say, they are dead wrong to do so.

Regards,
Bill Herrin



-- 
William D. Herrin ................ herrin () dirtside com  bill () herrin us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004


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