3.7.h [ii] BGP synchronization
this is arguably the best definition i have yet read concerning sync in bgp, until the very end..
ccie quick ref guide pg 83
For an iBGP route to be added to the BGP table, the exact prefix must be in the routing table from an IGP.
The synchronization rule is a method that guarantees that a route is known to all routers within the AS even if they are not running BGP. If a route is advertised via iBGP and a non-BGP router sits logically between the BGP peers, the non-BGP router will black hole the traffic because the destination is not known via IGP first. The synchronization check can be turned off (and is by default as of IOS version 12.2(8)T) with the router configuration command:
Router(config-router)# no synchronization
If disabled, it must be guaranteed that a routing black hole exists within the AS by creating a full-mesh iBGP network or using a BGP tool such as route reflectors or confederations.
read that last sentence again… the opposite, i believe is true… if synchronization is to be turned off, it must be guaranteed that a routing black hole DOESN’T exist…