How many interfaces Tunel (GRE) are supported in 2851?
The number of interfaces supported (which includes GRE as well as physical interfaces) is dependent on the IDB (Interface Descriptor Block) limit. The IDB limit varies by plarform (and sometimes by IOS version). I do not have a 2851 to check. But as a point of comparison on an 1841 the IDB limit is 1200. I assume that the 2851 would be comparable.