Route-Targets are attached to prefixes in MP-BGP when the VRF exports the prefixes to BGP (or you can think of BGP importing the prefixes from the VRF - the import/export under the VRF are from the perspective of the VRF, so it is really better to think of the VRF exporting the prefix to BGP).
Under the VRF, the route-target export <RT> or export-map <route-map> determines which route-target is attached to the prefixes as they are exported from the VRF into BGP.
The route-target inport <RT> or import-map <route-map> determines which RT(s) will be matched to import prefixes from BGP into the VRF.
I don't know where you would need to change a RT value once it has been assigned - you can import multiple RTs from BGP to a VRF to implement route leaking: http://www.cisco.com/en/US/tech/tk436/tk832/technologies_configuration_example09186a0080231a3e.shtml
Reviewing things and browsing the Vol 1 labs, i realised that i don't exactlly know when the RTs are attached to routes.
For example in the Vol1 "MPLS VPNs and VRF Export MAP" the sollution is to make a export map and change at this level the RTs. I tested to change them at the redistribute rip to bgp level and nothing happened.
So the dilema is: where the RT is first attached and where can it be changed?
Darrell A. Escola, B.Sc. IT
MCSE, MCDBA, MCSD, Linux+