More specific routes are preferred, and some providers do indeed filter
routes based on prefix-length.  Verio is probably the most well-known
example -- they filter based on prefix-length and class designation:
http://info.us.bb.verio.net/routing.html#PeerFilter

Exactly how common this is, I don't know.  It probably started off as a
hardware resource issue but now seems to be more of a "route aggregation
encouragement" type of thing.  But reasoning aside, the end result is
that you could quite likely have some issues if you continue to only
announce a /20 while the ex-customer announces their more specifics.  

I don't know how a soon-to-be ex-customer convinced you to let them keep
their assigned blocks, but I'd consider it wise to either de-aggregate
(generally frowned upon) the parts of your block that you'll have left,
or tell the customer that you've changed your mind.

-Terry

> -----Original Message-----
> From: Anand Buddhdev [mailto:arb@...> Sent: Saturday, July 12, 2003 11:33 PM
> To: isp-bgp@isp-bgp.com
> Subject: [isp-bgp] Introduction, and questions
> 
> 
> Hi everyone,
> 
> I'm a network engineer for a company based in The 
> Netherlands. I've recently started to use BGP with my 
> upstream, and we announce a /20. At the moment, our 
> configuration is very simple, since we don't do any transit 
> or multi-homing, although this is going to change in just a 
> few months, because we are getting a second upstream.
> 
> I am a newbie with BGP, so there are some things that I am 
> not too clear about. Perhaps someone with more experience can 
> guide me.
> 
> At the moment, we have a customer using a /22 and a /23 from 
> our /20 assignment from RIPE. For various complex reasons 
> this customer is moving to another provider, but needs to 
> keep their IP addresses.
> 
> If the other provider announces those /22 and /23 blocks via 
> their AS, is that ok? Do I need to break up my /20 
> announcement into a /21 and /23?
> 
> I have read that BGP will choose the most specific prefix, so 
> if I understand it right, then I should be able to leave my 
> /20 in place, while the customer can announce their more 
> specific routes via the other provider. Can someone confirm 
> this for me?
> 
> Secondly, I have read that many providers often filter 
> received routes, and typically reject anything more specific 
> than a /22. We don't do that, so in our BGP table, I see many 
> /24 networks too. May I hear opinion on what is common 
> practice on the Internet these days? Since routers nowadays 
> have more memory and power, is it necessary to filter more 
> specific routes? I ask this question to understand the 
> implications of what will happen to our customer when they 
> announce their routes via the other provider, particularly 
> the /23 they have.
> 
> Thanks in advance to everyone for your valuable opinions.
> 
> -- 
> Anand Buddhdev
> http://anand.org
> 
> 




> 

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[isp-bgp] Introduction, and questions, Anand Buddhdev