Populating BGP from Connected or IGP routes

Jon Lewis jlewis at lewis.org
Mon Jan 23 21:26:02 UTC 2012

On Mon, 23 Jan 2012, Eric C. Miller wrote:

> First, when running a small ISP with about the equivilent of a /18 or 
> /19 in different blocks, how should you decide what should be in the IGP 
> and what should be in BGP? I assume that it's somewhere between all and 
> none, and one site that I found made some good sense saying something to 
> the following, "Use a link-state protocol to track interconnections and 
> loopbacks only, and place all of the networks including customer 
> networks into BGP."

The simple answer, for an ISP of small size, is use a traditional IGP such 
as OSPF or ISIS for internal routing (if any dynamic routing is even 
needed), and BGP for internet routing, with iBGP between your transit 
routers if you have more than one transit router.

> Secondly, when is it ok, or preferable to utilize "redistribute 
> connected" for gathering networks for BGP over using a network 
> statement? I know that this influences the origin code, but past that, 
> why else? Would it ever be permissible to redistribute from the IGP into 
> BGP?

I haven't seen one.  It's too easy to screw up and let routes out that 
shouldn't if you redistribute into BGP...the only exception being a well 
filtered setup for real time blackhole routing.

For a small ISP, I'd suggest just using network statements and high metric 
static routes to null0 to make those network statements always advertise.

If you're a little bigger and have BGP customers, then I highly recommend 
use of BGP communities to control your outbound route filtering.  By 
defining and setting communties on received customer routes, you can turn 
up new BGP customers without having to modify anything beyond the router 
they're connected to.  It amazes me that there are large networks still 
not setup this way.  "You need an after hours maintenance window to turn 
up a BGP customer?"  "Yeah, we have to modify the prefix list filters on 
all our backbone routers."  WTF?

  Jon Lewis, MCP :)           |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
_________ http://www.lewis.org/~jlewis/pgp for PGP public key_________

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