Hello @Rinam Shah ,
BGP synchronization had been introduced for iBGP scenarios intra an AS in the first years of BGP introduction and iBGP use.
BGP started as a protocol spoken only by border routers between different ASes in eBGP sessions.
Now , as a safety measure to avoid internal black holing inside an AS a Router RB could advertise a prefix learned in an iBGP session to an eBGP peer only if the same prefix was present in the IP routing table as an IGP route.
If these conditions were met RB could advertise the prefix to the eBGP peer without risk of having a router in the path between RA and RB not able to route to the prefix ( a non BGP speaker router )
With the introduction of BGP route reflector servers, the introduction of BGP confederations and the key role played by MP BGP for MPLS services, now most SP networks have BGP running on all routers or use MPLS label swapping in inner core.
For this reason BGP sync is now a disabled feature for address family ipv4 unicast.
It never applied to other non ipv4 related address families like vpnv4. It could be disabled in af vrf <name> ipv4 unicast.
Hope to help
Giuseppe
