Folks, I just want to confirm that BGP synchronization is related to iBGP only, never applied to eBGP, correct?
"If BGP synchronization is enabled, there must be a match for the prefix in the IP routing table in order for an internal BGP (iBGP) path to be considered a valid path".
Seth, although you are talking to Peter, I wanted to say a couple of things. NetPro members do not usually have a problem to answer questions or explain things, since reading the notorious manuals doesn't always help (although some other well-known communities seem to religiously believe that it does). If I remember correctly, you are studying for the BSCI and you are reading the famously very well-written certification guide we all have learnt to love and can't live without. If you want more specialized reading about BGP, I believe you should read "Internet Routing Architectures" by Sam Halabi. Reading the right book can save you days of standing in front of the computer (which has caused serious neck pain to me right now).
Going to your parenthesized question: I have partly answered in my previous post. Synchronization doesn't "apply" to iBGP or eBGP. This kind of loose use of terminology causes confusion. Synchronization occurs within the AS and doesn't allow routes to go to the outside of the AS if routers internal to the AS are not synchronized.
The general synchronization scenario involves an AS that provides transit service. This means that the transit ASx receives routes from an eBGP peer in ASy and passes those to an eBGP peer in ASz, so that ASz can reach ASy destinations via ASx. If synchronization is on in ASx, this AS needs to have the routes of ASy in its IGP. Because this is obvious waste and practically impossible, synchronization is turned off and other techniques are used avoid black-holing.
If a route is received via an iBGP session, that doesn't make it an AS internal route. If ASy sends a route to ASx border router1 and router1 passes it to ASx border router2 via iBGP, this route is still external to the AS. Synchronization says router2 has to have external route in its IGP before passing it to an external peer in ASz, and that is the problem in the first place. If router 1 has another eBGP peer of its own, router 1 doesn't have to synchronize with any other router within its AS, so it gets away with the synchronization rule.