Which reserved AS number or range of numbers is used for backward compatibility between old
BGP peers using 16-bit AS number and new BGP peers using 32-bit AS number?

A.
AS 65001 to 65535

B.
AS 65512 to 65535

C.
AS 12345

D.
AS 23456

E.
AS 64001

Explanation:

http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6554/ps6599/4byte_asnios.pdf
New Reserved AS#
AS_TRANS = AS #23456
2-byte placeholder for a 4-byte AS number

Used for backward compatibility between OLD and NEW BGP speakers