Samir,
Check whether AS64600 peering with AS2856.
Check whether AS65600 advertised out the routes to AS2856.
Check whether AS2856 peering with with AS 64620.
Check whether AS2856 advertised out the routes of AS64600 to AS64620.
Check whether AS64620 receives routes from AS2856 and 64600.
Regards,
David Sudjiman