Populating BGP from Connected or IGP routes

Mon Jan 23 21:25:08 UTC 2012

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

> I'm looking for a best practice sort of answer, plus maybe comments on 
> why your network may or may not follow this.
>
  > 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."

That depends on your architecture.  There are several ways to deploy 
sane/scalable IGP and EGP architectures.

> 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?

Keep in mind that "redistribute connected" and a "network" statement in 
your IGP do two different things.

For example, in OSPF, adding a network statement for an interface will 
enable OSPF on that interface, and your router will try to find other 
OSPF speaking devices that are connected to that interface and form an 
adjacency with them, unless you make the interface passive, which would 
negate the network statement.  Routes for connected interfaces that are 
imported/redistributed into your IGP might carry a different origin, LSA 
type and/or metric, depending on how you import them.  "passive-interface 
default" is your friend :)

jms