BGP is external gateway protocol as it carries Internet routes; so you want to get outside as soon as possible (shortest way);
If you have internal BGP peers, you do not want to go that way, if there is shorter external path;
It also AD of 20 is better than any other AD's of routing protocols;
Martin beat me to it! I agree.
With that though, be careful of EIGRP summary routes as those will default to AD of 5 if not defined when summarizing. **See Martin comment below**
Message was edited by: Beau Yancy
Even they made BGP special when you redistribute routes into OSPF.
When you redistribute routes into OSPF, Default metric is 20 except BGP which is 1.
BGP is the King;
If you look at the purpose of BGP, it is to enable vector-based routing based on Path Attritubes tied to prefixes, which make up the NRLI that is advertised between BGP neighbors. These neighbors can be eBGP neighbors - those with an AS number that is dissimilar from our own, or those peers can be internal - with an AS that matches our own. Our own AS number is defined as the number in the BGP process configuration line "router bgp <AS>".
With that said, if you have multiple peers, with at least one internal and one external, and you receive the same route from both, return back to the purpose of BGP - to route between discrete AS's. This would dictate that you would want the shortest path between one AS and another - so if you have a direct link to the other AS, you want to prefer that path as opposed to a path that has you traveling further within your own AS before you reach the next AS.
This can get pretty complicated when you have multiple exit paths, even more so when you redistribute at multiple points within a network and learn routes from several different neighbors.
No, you are right in your initial comment. None of the routing protocols advertise the AD, AD is locally significant. So, you could have a route to a destination learned through eBGP with an AD of 20 and if you then create a summary route for this same destination back through your EIGRP domain to another router that has reachability to the destination as well then you basically create a suboptimal routing path. Not good. So, as you said, watchout when creating EIGRP summary routes.
Just my 2-cents.
You would be surprised how easy it is to wind up in that situation.
Engineer 1: Hey - looks like we're good.
Engineer 2: Nah, lets do a summary route, clean up the advertisements.
Engineer 1: I'm not so sure it works that way....
Engineer 2: I'm sure it does, I just entered it in. My routing table looks great!