http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd2002.htm#xtocid2292717
grep GRE
"Cisco's Generic Routing Encapsulation (GRE) multiprotocol carrier protocol
encapsulates IP, CLNP, IPX, AppleTalk, DECnet Phase IV, XNS, VINES, and
Apollo packets inside IP tunnels."... assuming you're running Cisco gear,
anyhow.
-carl hirsch
network analyst
sargent & lundy llc