BGP router with synchronization enabled will not advertise iBGP-learned routes to other eBGP peers if it is not able to validate those routes in its IGP. Assuming that IGP has a route to iBGP-learned routes, the router will announce the iBGP routes t…
http://www.ebrahma.com/2014/04/unable-to-announce-ibgp-learned-routes/

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