Hi,
There are currently three defined RD types: 0, 1, and 2.
If a Type 0 RD is specified, then the Administrator subfield and Assigned Number subfields are 2 bytes and 4 bytes, respectively:
0:AS:#
If a Type 1 RD is specified, the Administrator subfield and Assigned Number subfields are 4 bytes and 2 bytes, respectively:
1:IP:#
If a Type 2 RD is specified, the Administrator subfield and Assigned Number subfields are 4 and 2 bytes, respectively:
2:AS:#
Type 0 and 1 RDs are used when translating IPv4 prefixes into VPN-IPv4 prefixes. Type 2 RDs can be used to signal Multicast VPNs (MVPNs) (to distinguish it from unicast VPNv4 prefixes), the problem is that RD Type 2 is not standard, the new address family based advertisement method, documented in draft-nalawade-idr-mdt-safi overcomes this limitation, please do check this document:
http://www.cisco.com/en/US/products/ps6604/products_white_paper0900aecd80581f3d.shtml
HTH,
Mohammed Mahmoud.