BGP - Synchronization Rule
The BGP synchronization rule is an old rule from the days when the configuration of a full mesh peering wasn't mandatory for iBGP. That meant that some routers within an autonomous system may not be running iBGP. The rule states that:
A BGP router should not advertise a route to an external destination (an eBGP peer) unless that route is also known by the IGP of the router's own AS. This means the route must be present in the router's IGP routing table.
The synchronization rule was designed to prevent routing loops. The idea was to ensure that all routers in the AS are aware of the route. This prevents scenarios where a router might receive traffic for a destination that it cannot reach because it's not aware of the route, potentially creating a routing loop.
In today's networks, it is best practice to ensure that you have a full mesh of iBGP peerings. This design approach, along with the use of the iBGP split horizon rule resolves the problem of the creation of potential loops, making BGP synchronization more or less obsolete.
In most modern network devices, this feature is disabled by default, but it can be enabled if needed for specific network designs.
Links:
https://forum.networklessons.com/t/bgp-synchronization/1110/47?u=lagapidis