On 6/6/2012 5:26 AM, Peter Coghlan wrote: No, you have not misunderstood. You are just not getting the full picture. Yes, the Cisco box will transport the DECnet frames inside something. At the bottom there will be IP, but most likely the DECnet frames are actually sitting inside UDP... My understanding of it is that GRE is another IP protocol on the same level as TCP and UDP rather than using UDP. You are correct. GRE is Layer 4 just like TCP and UDP. -brian