This is required blogging…and reading for that matter. A good chunk of this is taken from my CCNP posts from last year. Corrections, please.
How does a BGP router decide which BGP route is the best?
Next-hop : Does the router have a route to the next-hop?
Weight : This is a numeric value where bigger is better. Weight is not passed onto other peers and is a Cisco proprietary feature.
LOCAL_PREF : This is a numeric value where bigger is better. All iBGP peers pass this value around amongst themselves.
Local : Is the next hop me (0.0.0.0)?
AS_PATH length : This is the number of AS hops to the destination. If you don’t know this one by now, then you missed something big.
: This is the number of AS hops to the destination. If you don’t know this one by now, then you missed something big.
ORIGIN : Did this route come from a netowork statement in an IGP (I), from EGP (E, which shouldn’t exist any more), or somewhere else (?) like a redistributed route? I is better than E is better than ?.
MED : The Multi Exit Discriminator can be used by one AS to influence routes to that AS. The smaller the better.
Neighbor type : eBGP are better than iBGP routes.
IGP metric : Prefer the next-hop address that’s closest via an IGP like OSPF or EIGRP (or RIP, Ivan).
Route age : Prefer the oldest (and, thusly, the most stable) route.
Lowest BGP neighbor router ID : Do I have to explain that one?
Lowest BGP neighbor IP : You know what this is, right?