Errdisable is a feature that automatically disables a port on a Cisco Catalyst switch. When a port is error disabled, it is effectively shut down and no traffic is sent or received on that port.
The error disabled feature is supported on most Catalyst switches running the Cisco IOS software. Including all the following models:
The Errdisable error disable feature was designed to inform the administrator when there is a port problem or error. The reasons a catalyst switch can go into Errdisable mode and shutdown a port are many and include:
When a port is in error-disabled state, it is effectively shut down and no traffic is sent or received on that port. The port LED is set to the orange color and, when you issue the show interfaces command, the port status shows as Errdisabled.
Following is an example of what an error-disabled port looks like:
To recover a port that is in an Errdisable state, manual intervention is required, and the administrator must access the switch and configure the specific port with 'shutdown' followed by the 'no shutdown' command. This command sequence will enable the port again, however, if the problem persists expect to find the port in Errdisable state again soon.
As outlined above, there are a number of reasons a port can enter the Errdisable state. One common reason is the Port Security error, also used in our example below.
Of all the errors, Port Security is more a feature rather than an error. Port Security allows the restriction of MAC Addresses on an interface configured as a layer 2 port. This effectively prevents others connecting unwanted hubs or switches on the network. Port Security allows us to specify a single MAC Address to be connected to a specific port, thus restricting access to a specific computer.
In the case of a violation, Port Security will automatically disable the port. This is the behaviour of the default port security policy when enabling Port Security. Following is a configuration example of port security:
Once a host is connected to the port, we can get more information on its port-security status and actions that will be taken when a violation occurs:
Note that the Violation Mode is set to Shutdown. This means that when a violation is detected, the switch will place gigabitethernet 0/48 in the err-disable shutdown state as shown below:
While it's almost always necessary to know when a port security violation occurs there are some circumstances where autorecovery is a desirable feature, especially durng accidental violations.
The following commands enable the autorecovery feature 30 seconds after a port security violation:
To view the Errdisabled reasons, and see for which reason the autorecovery feature has been enabled, use the show Errdisable recovery command:
We have now confirmed that autorecovery is enabled for port-security violations. If it is required to enable the Errdisable autorecovery feature for all supported reasons, use the following command:
To test our configuration we forced a port security violation, causing the switch to place the offending port in the shutdown state. Notice we've enabled autorecovery for all Errdisable reasons and the time left to enable the interfaces placed in shutdown state by the port security violation:
Seventeen seconds later, the switch automatically recovered from the port security violation and re-enabled the interface:
There are cases where it might be necessary to disable the Errdisable mechanism for specific supported features in order to overcome constant interface shutdowns and auto recoveries. While the Catalyst IOS does not allow disabling all features we can still fine-tune the mechanism and selectively disable a few.
To view the Errdisable reasons monitored by the switch, use the show Errdisable detect command:
2960G# show errdisable detect
ErrDisable Reason Detection Mode
----------------- --------- ----
bpduguard Enabled port
channel-misconfig Enabled port
community-limit Enabled port
dhcp-rate-limit Enabled port
dtp-flap Enabled port
gbic-invalid Enabled port
inline-power Enabled port
invalid-policy Enabled port
link-flap Enabled port
loopback Enabled port
lsgroup Enabled port
mac-limit Enabled port
pagp-flap Enabled port
port-mode-failure Enabled port
secure-violation Enabled port/vlan
security-violation Enabled port
sfp-config-mismatch Enabled port
small-frame Enabled port
storm-control Enabled port
udld Enabled port
vmps Enabled port
As shown, the command lists all supported Errdisable reasons. For our example, let's assume we want to disable the inline-power Errdisable feature.
To achieve this, we simply use the following command:
And verify that Errdisable has been disabled for the feature:
Overall, the Errdisable feature is an extremely useful tool if configured and monitored correctly. Take the necessary time to play around with the supported options of your Cisco Catalyst switch and fine-tune it to suit your network needs.
Decrypt Cisco Type-7 Passwords on the fly!
