--There is a rule that iBGP peers do not advertise routes between themselves so if you get a route in the routing table with an AD of 200 does that mean that one of the routers has split-horizon turned off?
You can not disable split horizon, it is always on no matter what.
--Why would you prefer routes learned by eBGP over those learned by iBGP?
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtmlI am assuming you mean step 7. no? I have not really thought about that before, it is good that you are asking questions like this one. You can look at it from this perspective, what is BGP used for? To exchange NLRI between ASes, correct? I believe that is the primary objective,so anything that is internal should be secondary and external ("e"BGP) should be primary, hence the lower AD (20).
Billy