Topology:2612-----(X.25)-----Z company switch
The router 2612 is connected to Z company switch through two serial interfaces which are encapsulated with X.25. At the beginning, both physical state and protocol state of two serial interfaces are up; after the router runs 30 minutes, the protocol state of one of interfaces is down; draw and plug the V35 cable of the interface, and the protocol is UP; after 30 minutes, the protocol is down again.
Use dis interface command to check interface state, and the interface protocol turns to down from up state.
Replace with a new serial interface module for test, and the problem persists; when the interface protocol is down, repeat drawing and plugging the V35 cable in the interface, and the protocol can go up, but it can keep up for 10-20 minutes. Because the problem occurs to both old and new modules, it is confirmed that the problem is not caused by modules.
After investigation on symptoms of failure, the customer reports that other devices in the same cabinet were attacked by thunder, and the router cannot work well, but it was restored after while. Moreover, the router keeps using the original software version, and there is no upgrade and other operations. So it is not resulted from software.
The switch at opposite is suspicious of problematic. The customer reports that the switch has also replaced with one module, but the problem persists, so it is not caused by the opposite switch. However, we find a problem during location: one of two serial interfaces works well, and the other does not work well often. Exchange the cables, and the one that works well before cannot be up, and the problematic one can be up then. It is located that the cable is the cause. Replace with a new cable for test, and the problem is solved.
The reasons include:
1. The router module is problematic
2. The router software is problematic
3. The connected cables are problematic.
When troubleshooting, we often pay more attention to the devices, and ignore such details as interface cables. For example, the device is attacked by thunder, we often confirm that the device cannot work; however, the device is likely to work well, but the cables are damaged. During sovling the problems, we should think over all round, not only pay attention to the devices, but also to some details until find the root.