Level 3 BGP Advertisements

STARNES, CURTIS Curtis.Starnes at granburyisd.org
Wed Aug 29 20:55:51 UTC 2012

Sorry for the top post...

Not necessarily a Level 3 problem but;

We are announcing our /19 network as one block via BGP through AT&T, not broken up into smaller announcements.
Earlier in the year I started receiving complaints that some of our client systems were having problems connecting to different web sites.
After much troubleshooting I noticed that in every instance the xlate in our Cisco ASA for the client's IP last octet was either a 0 or 255.
Since I am announcing our network as a /19, the subnet mask is, that would make our network address x.x.192.0 and the broadcast x.x.223.255.
So somewhere the /24 boundary addresses were being dropped.

Just curious if anyone else has seen this before.

-----Original Message-----
From: William Herrin [mailto:bill at herrin.us] 
Sent: Wednesday, August 29, 2012 3:36 PM
To: nick at flhsi.com
Cc: nanog at nanog.org
Subject: Re: Level 3 BGP Advertisements

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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