Before you go looking at IP Routes, OSPF or EIGRP configurations or other less likely problems, remember to always suspect the physical layer first:
- Do you have an Ethernet link light? 如果不能，为什么？
- Do you have ANY lights at all on the device? If not, check power!
This may seem elementary or in some cases even silly but it should be the FIRST thing you verify before moving on to other possibilities.Some estimates are that between 75 – 85 percent of all troubles are found at the physical layer.
Key questions BEFORE your investigation:
- Do we have a working one of these to compare?What are the differences between them?
- What is the health of the path from that device into the network?To the WAN/Internet?
- Begin the process of elimination…i.e., Connect the suspect device directly to a switch with a known working (known good) patch cord
- What time and date did this begin?
- Has there been and event such as a power surge or brown out that coincided with this trouble?
- Is the device’s Real Time Clock (RTC) in synch with NTP?(that is, can we believe the log times?
- What other event happened at that time? (Fred changed the water in the fish tank above the router).
- Use the “show log” command starting with the site’s router, then move on to other devices.
- See a before and after snapshot?(compare prior test result reports)
- Ping the device’s own IP address?(test its own stack/NIC, verify network layer connectivity)
- Ping the device’s Default Gateway?
- Swap components (Cable, Switch, WAP)