Data link layer problems on serial links usually result in at least one of the routers having a serial interface status of up/down. In other words, the line status (the first status code) is up, while the second status (the line protocol status) is down. Table 12-6 summarizes these types of problems.
As with the other troubleshooting topics in this book, Table 12-6 lists some of the more common types of failures but not all.
The first of ...