Level 3 BGP Advertisements

Randy randy_94108 at yahoo.com
Wed Aug 29 19:49:24 UTC 2012

--- On Wed, 8/29/12, Nick Olsen <nick at flhsi.com> wrote:

> From: Nick Olsen <nick at flhsi.com>
> Subject: Level 3 BGP Advertisements
> To: nanog at nanog.org
> Date: Wednesday, August 29, 2012, 12:28 PM
> Greetings all.
> 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. And
> its never been 
> a problem. Until we brought up peering with level 3..
> 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?
> Is what I'm doing (Advertising the aggregate prefix) a good
> rule of thumb?
> Any other thoughts?
> Nick Olsen
> Network Operations (855) FLSPEED  x106
my 2 cents: I would think L3 would announce the /20 and /21's and no-export the /24

Why announce more-specifics if you can get away with a few shorter-prefixes.

Do you have a setup where you have to announce /24's? If you can do with a /20 and two /21's, that would be the way to go.

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