Level 3 BGP Advertisements

William Herrin bill at herrin.us
Wed Aug 29 20:35:57 UTC 2012

On Wed, Aug 29, 2012 at 3:28 PM, Nick Olsen <nick at 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.

Bill Herrin

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

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