Content

| 3.5 |  | Isolating Physical Layer Problems |
| 3.5.5 |  | Interface configuration |

Prior to examining the interface configurations on network devices, it is important to discount the physical causes first:

- Verify cable connectivity.
- Verify that the power supply is on and running.
- Verify the router LED status. If all LEDs are down, it is most likely an issue with the power supply of the device, or with incorrectly seated modules in a modular router or switch.

When examining interfaces on a Layer 2 switching device:

- Start by looking for duplex and speed mismatches.
- Ensure that the correct VLAN encapsulation method has been applied if the port is a trunking port, and that the port has been designated for trunking rather than access.
- Ensure that individual ports have been assigned to the EtherChannel group for trunking ports combined to form an EtherChannel.
- Ensure that the port has not been accidentally assigned an IP address. On 3550 and 6500 series switches, the no switchport command and an IP address converts the switch port to a routing port.
- Look under port configuration to make sure it has not been assigned as a RSPAN reflector port.

When examining a Layer 3 device:

- Ensure that the IP addressing on both sides of the link is within the same subnet.
- Ensure that the interface is not administratively shut down.
- Ensure that serial ports designated as Data Clocking Equipment (DCE) end are assigned the correct clock rate for the link.

Console Not Responding
Console problems occur when the router becomes unresponsive to input at the console port. If the console is not responsive, it means that a high priority process prevents the console driver from responding to input. If traffic is still flowing through the device, try disconnecting network interfaces and see if the router starts responding. Many times the router thinks it is doing something too important to service exec sessions.

Another possibility is that one of the device interfaces may be down. There are multiple events that could cause this, such as a wrong configuration command, or a hardware failure of the interface or the cable. If some interfaces appear down when using the show interfaces command, try to find out what caused it.