I’ve received several e-mails responding to the mismatched OSPF subnet challenge. Some of the readers claimed that the configuration would work as-is; if you were one of them, I would advise you do some lab test the next time. A few of the respondents also noted that it was more a review question than a challenge (since I’ve been writing about this topic a few days back) and everyone who decided the configuration has to be fixed has provided the correct solution: you have to configure the Fast Ethernet as a point-to-point OSPF interface and the routers stop complaining about the OSPF subnet mask mismatch.
Unfortunately, someone decided to prevent everyone else from having real fun figuring out the solution and posted the solution as a comment to my post almost immediately after I wrote it (but I’m positive that those readers that sent me e-mails did not read that comment first). Lesson learned: the next time I’ll disable comments in the challenges.