4 Byte Autonomous System Numbers

Starting Jan 1 2009 Arin will allocate 4 Byte ASNs by default, 2 Byte ASNs by request only.

http://www.arin.net/policy/nrpm.html#five1

Cisco support documented here.

http://www.cisco.com/en/US/docs/ios/iproute/configuration/guide/irp_bgp_overview.html#wp1055927