what is local AS attribute in BGP?
when it is used and how we can utilise it?
and which will be prefered first: AS-Path or MED?
and what is the proper chatecristic of the IGP in the Service Provider Network?
How should IGP interact with IBGP and vice versa?
to route the traffic from the local as to some other AS... IGP should have knowledge to reach that router or route in the external AS... now you can have multiple way to achieve the same function... one is to use the redistribution of BGP routes in to the IGP routing table (at that time you need the synchronization) and other way is to go for the full mesh of IBGP session (no need for the synchronization)... so in short synchronization you can define as the full reachability information knowledge between IGP and BGP...
try to search for the NEXT_HOP or SYNCHRONIZATION in BGP in google or in cisco and read some documents on it so you will have proper idea of it...