Each instance of a VRF must have a unique Route Distinguisher (RD) assigned to it. The RD is pre-pended on any address being routed or advertised. The RD can be defined as either ASN-relative or IP address-relative. Because the RD is unique to an instance of a VRF, it allows the same IP address to be used in different VPNs without creating any conflict.
To assign a Route Distinguisher (RD) for a VRF based on the AS number 3 and the arbitrary identification number 6, enter the following command.
(config-vrf)# rd 3:6