Theoretically, BGP does not advertise iBGP-learned routes to eBGP peers. Hence, I did not filter iBGP learned routes to my eBGP peers. I didn't realise my eBGP router did advertise these iBGP routes (not meant for transit) until our partner informed me and I did a "show ip bgp vpnv4 VRF-name neighbors 1.1.1.1 advertised-routes" to verify. So, is this theory wrong?
Well, it comes with a caveat. This theory is true only if BGP synchronisation is enabled. The rule of synchronization states that routes learned via BGP must be validated by the interior routing table before they can be advertised to remote peers. So, what does this mean? Click here for explanation.
In older Cisco IOS versions, Synchronization is enabled by default. However, it is disabled by default in Cisco IOS Software Release 12.2(8)T and later. If you do not intend for the iBGP learnt routes to be on transit among different AS, it is probably safer to enable BGP synchronization.
