[c-nsp] iBGP

[Justin M. Streiner]

On Fri, 29 Jul 2005, Mark Tohill wrote:

> Another BGP Question!
>
> Consider an ISP with 2 PoP's and one upstream provider peered at each
> PoP.
> Each PoP advertising out own parts of /16. Running iBGP with upstream
> only.

Are you receiving full BGP routes from your upstreams?

> A third PoP (CoLo) is required for DSL aggragation (with different
> provider).
>
> Intention is to change to eBGP and advertise own distinct blocks of that
> /16 from own AS no.
>
> Redundancy/resiliency  cant be fully implemented as yet since
> infrastructure being upgraded.
>
> Questions:
> 1. eBGP IS my only option, right??

If you plan to speak BGP with another AS outside of yours, you're doing 
EBGP.

> 2. Is there a need to configure iBGP neighbors on our side across PoP's
> since these can't be used yet. Any other implications to not using iBGP?

You should speak IBGP with your other POPs for a number of reasons, not 
the least of which is so that your routers can make the best routing 
decision based on the routes received from all of your upstreams.  If the 
physical connections to the other POPs aren't in place yet, you can leave
them administratively down until you're ready to go.

You can run your upstream-facing routers as route reflectors, and if you 
have other routers that need to speak IBGP, you can point them at the 
reflectors to get the BGP information they need.

If you don't do IBGP, you'd have to redistribute the BGP routes you learn 
from each upstream into your IGP (OSPF, EIGRP, IS-IS, etc...), which is a 
bad idea.  Router B needs to know how to reach sites via transit pipe A in 
case transit pipe B goes down and vice versa.

There are several good books on setting up networks like this.  Two of the 
better ones are "Internet Routing Architrctures" by Bassam Halabi et al 
and "High-Avability Networking with Cisco" by Vincent Jones et al.

> 3. What is the best way of favouring inbound traffic on one of two
> routers configured identically in same PoP in a HSRP (Active-standy
> router scenario.)? We had implemented this via local-pref, but this is
> only intra-AS, which we are about to loose?

In the small ISP EBGP world, inbound traffic preference is normally 
controlled by prepending your AS on outbound announcements (shorter AS 
path is preferred over a longer one if everything else is equal) or if 
your upstream offers them, tagging outbound routes with a community that 
your upstream will use to control to some extent what they do with the 
route in their network, such as what they set their local-preference to, 
etc.

jms