Router failures come in many forms, and many people are still unfamiliar with the troubleshooting methods. Below, using a NETGEAR router as an example, we introduce a relatively complex fault that is quite representative.
1. With a NETGEAR router serving as the WAN backbone router, after the network setup was completed, it was discovered that the 2M main egress line protocol was in a down state, causing the connected network to be interrupted. The “show running-config” command was used to check all running parameters, and no errors were found. The “show interfaces serial” command was also used to check the serial ports, revealing some ports were physically up but the line protocol was down. All serial ports exhibiting this issue were located on the same serial port board (Board A), while other modules worked normally.
Upon inspection, all physical lines were also normal, thus ruling out the possibility of a line fault. However, under normal circumstances, the likelihood of multiple serial ports on the same board simultaneously experiencing line failures is quite low.
2. At this point, it could be preliminarily determined that Board A likely had a problem. When entering global configuration mode and then trying to enter the port configuration mode for the serial ports on Board A, it was impossible to do so. Every time “interface serial [serial-number]” was typed, an error was always reported, yet the same command could successfully enter the port configuration mode for other serial ports. This suggested a certain software glitch within the IOS itself. After restarting the NETGEAR router using the reload command, the system entered ROMmonitor mode with the prompt “rommon>”. Typing the command “boot” could start the NETGEAR router, but the fault persisted, and all configuration statements for Board A’s serial ports were reported as errors.
3. After powering off and then powering on the NETGEAR router, the following was observed: The router’s AC (or DC) OK LED was green, indicating the system’s power supply subsystem was working normally; the fan rotated normally and the error output LED was not lit, indicating the cooling subsystem was working normally; the Route Processor (RSP) LED was green, also indicating the system was operating normally. The Enable LEDs for each interface were green, showing the RSP had completed the initialization of the interface processors.
However, all LEDs on Board A flashed once and then went out. After executing the “reset” command and then using “boot” to guide the system, all configuration statements for Board A’s serial ports had completely disappeared.
4. Upon further inspection of Board A, it was found to be model VIP2. Only one four-port serial sub-module was installed out of the two available sub-module slots, so Board A only had four serial ports (expandable to 8), with three in use and one idle. Normally, the LEDs for the three used serial ports should be lit, and the LED for the unused port should be yellow. Currently, however, none of the four serial port LEDs were lit. After the system started, using the command “show version” revealed that the system had found Board A and identified its model as VIP2, but no detailed information was available.
5. In the last line of the display information, it was found that the NETGEAR router’s configuration register value was 0x0. After modifying it to 0x2102 and rebooting the NETGEAR router, and then copying the backup configuration from a tftp server to the running-config, the NETGEAR router returned to normal operation. This resolved the fault, which