Core Issue
A Border Gateway Protocol (BGP) router with synchronization enabled does not advertise internal BGP (iBGP)-learned routes to other external BGP (eBGP) peers if it is not able to validate those routes in its Interior Gateway Protocol (IGP).
Resolution
To resolve this issue, check the IGP configuration or disable synchronization by issuing the no synchronization command in the router BGP configuration.
For additional information, refer to the Unable to Announce iBGP-Learned Routes section of Troubleshooting When BGP Routes Are Not Advertised.
Type of Route Missing from Routing Table
All routes / All routes from specific neighbor
Valid Route to BGP Next Hop
Valid route to next hop exists
show ip bgp neighbors {neighbor address}
State is established
