I found a cisco web page that said the 2613 ws the only 2600 series router that did not support a fast ethernet card. This does not seem logical, as it is a token ring, and the 2612 has both token ring and ethernet but (by implication) does support fast ethernet. Maybe the web page was from a time when the 2613 was the only 261x series still in manufacturing, or it refered to stock routers, and the 2613 was normally shipped with less memory than other models (so if you upgraded memory, the 2613 would support fast ethernet). Or maybe the page was in error.
Does anybody know what the story is?