"Route-target export" is used to export routes form VRF to mBGP - i need to know the rules which tells which routes will be exported.
For example : if VRF-RED have 10 Routes which include static routes, connected routes, OSPF routes and routes it imported from VRF-YELLOW. Now if VRF-RED export and VRF-GREEN import, then which of the 10 VRF-RED's route VRF-GREEN will get?
It's depends on RT value. If you are using same RT value for VRF-YELLOW , VRF-RED and VRF-GREEN then you will get all the routes in VRF-GREEN. Otherwise, if you are using different RT value for VRF-YELLOW & VRF-RED and different for VRF-RED & VRF-GREEN, you will get all routes except the routes received from VRF-YELLOW.
Don't forget that VPN's depend on the RT's for the import/export policies. RT's are carried with the prefix along the VPNV4 space to identify the origin of the prefix.
"Route-target export" is used to export routes form VRF to mBGP
Wrong.
Export policy is what tells which routes to export from VRF into MP-BGP.
Export policy could, for example, allow only connected VRF routes to be exported into MP-BGP.
As for RT export. It is carried in each route update, as an extended community of the route, from the VRF (if the route matched the export policy criteria) to the remote PE.
In the remote PE, if there is a VRF with a matching Import RT value to the Export RT value received in the route update, then this route will be imported to this remote PE's VRF.
