31 Days Before Your CCNA Exam

350 31 Days Before Your CCNA Exam■■■Serial x is up, line protocol is up (looped)Serial x is up, line protocol is down (disabled)Serial x is administratively down, line protocol is downTroubleshooting Layer 1 ProblemsA down line status on the serial link typically points to a Layer 1 problem. The following listdescribes the most likely reasons:■■■■The leased line is down (a telco problem).The line from the telco is not plugged in to either or both CSU/DSUs.A CSU/DSU has failed or is misconfigured.A serial cable from a router to its CSU/DSU is disconnected or faulty.The details of how to further isolate these four problems is beyond the scope of the CCNA exam.One other common physical layer problem can occur that results in both routers’ interfaces beingin an up/down state. On a back-to-back serial link, if the required clock rate command is missingon the router with a DCE cable installed, both routers’ serial interfaces will fail and end up with aline status of up but a line protocol status of down.To check for this error, use the show controllers command on the side that should be DCE. Youmight be surprised to find that a DTE cable is attached. Or you might discover that no clock is set,as shown in Example 2-11.Example 2-11Problem: No clock rate Command on the DCE EndR1#show controllers serial 0/0/0Interface Serial0/0/0Hardware is PowerQUICC MPC860DCE V.35, no clockidb at 0x81081AC4, driver data structure at 0x81084AC0SCC Registers:Troubleshooting Layer 2 ProblemsTable 2-1 lists three common data-link layer problems.Table 2-1Likely Reasons for Data-Link Problems on Serial LinksLine Status Protocol Status Likely ReasonUp Down (stable) on both ends Mismatched encapsulation commands.orDown (stable) on one end, flappingbetween up and down on the otherUp Down on one end, up on the other Keepalive is disabled on the end in an up state.Up Down (stable) on both ends PAP/CHAP authentication failure.