Lots of times we are troubleshooting cisco switches connectivity and performance. Under the stress of the moment we forget some essential IOS commands that can save us time and effort.
The most common IOS command to give us a clear picture of the switch interface is show interface or show interface description command. It give us a lot of information about the interface status, the working configuration, protocols, errors, collisions etc.
The switch show interfaces command lists the two-code status just like routers. The two codes are named line status and protocol status. They generally refer to whether Layer 1 is working (line status) and whether Layer 2 is working (protocol status). LAN switch interfaces typically show an interface with both codes with the same value, either up or down.
But what will happen if you want to check the status of more than one interface, let’s say 48 FastEthernet interfaces of a switch? It would be difficult to follow the output omitted by the show interface command for more that 3-5 interfaces because the output will be in tens of lines. A very useful IOS command you can use in this case is the show interfaces status command. This command will generate a list with brief information of all interfaces of the switch, including the status, the vlan id, the speed and the duplex status and how this values were achieved. An a- before any state means that this value was found through autonegotiation process in the interface.
The show interfaces status command lists a different single interface status code than the show interfaces command. This single interface status code corresponds to different combinations of the traditional two code interface status codes and can be easily correlated to those codes.
In the following table there are the code compilations and some typical causes that could cause the interface status:
| # | Line Status | Protocol Status | Interface Status | Typical Cause |
| 1. | Administratively Down | Down | Disabled | The interface is configured with the shutdown command. |
| 2. | Down | Down | Notconnect | No cable; bad cable; wrong cable pinouts; the speeds are mismatched on the two connected devices; the device on the other end of the cable is powered off or the other interface is shutdown. |
| 3. | Up | Down | Notconnect | An interface up/down state is not expected on LAN switch interfaces. |
| 4. | Down | Down (Err-disabled) | Err-disabled | Port security has disabled the interface. |
| 5. | Up | Up | Connect | The interface is working. |
Any switch interface status other than connect or up/up means that the switch will not forward or receive any frame on the specific interface.
With the use of both previous commands and decode the status codes we can point at specific points in our network infrastructure and find the causes of the not connectivity or low performance.
Enjoy!