Hello,
You have a good point.
Regarding both route distinguisher and route target, the ASN format of their value is used just for convenience and becase the RFC 4364 requires it but purely technically, it absolutely does not have to express the real ASN where the VPN route is originated or imported.
The route distinguisher is actually used only for BGP purposes and only in the immediate peering (i.e. the route distinguisher set by router A will be rewritten by the route distinguisher of router B when it further advertises the route to router C). The route-target, again, is just a BGP extended community attribute used to express where a VPN route can be imported and where it will be imported (export and import lists). Thus, the value of the ASN in the ASN:nn format is a convenient form but not actually tied to any real ASN.
There is a new RFC 5668 that describes the new format of extended community for route target for 4B ASNs. Also, the RFC 4364 already describes the format of the route distinguisher for 4B ASNs. So actually, there is already infrastructure in place that should allow you to use 4B ASNs in route distinguishers/targets. However, if your current IOS does not support it, it is not actually a problem, as you can use any values of the ASN:nn format as long as it is unique.
Best regards,
Peter
