By default, MAC address learning is enabled on all interfaces and VLANs and MAC address aging is set to 300 seconds on a Cisco switch. You can control MAC address learning on an interface or VLAN to manage the available MAC address table space. Before you disable MAC address learning, be sure that you are familiar with the overall environment which includes network topology and the router system configuration. Disabling MAC address learning on an interface or VLAN could cause flooding in the network. You can disable MAC address learning on a single VLAN ID from 1 to 4094 (for example, no mac address-table learning vlan 229) or a range of VLAN IDs, separated by a hyphen or comma (for example, no mac address-table learning vlan 1-100, 16).
You can display the MAC address table by using one or more of the privileged EXEC commands.
show mac address-table address – Displays MAC address table information for the specified MAC address.
show mac address-table aging-time – Displays the aging time in all VLANs or the specified VLAN
If the configuration shows a port as enabled, but software on the switch detects an error situation on the port, the software shuts down that port. In other words, the port is automatically disabled by the switch operating system software because of an error condition that is encountered on the port. When a port is error disabled, it is effectively shut down and no traffic is sent or received on that port. The port LED is set to the amber and if you issue the show interfaces command, the port status shows err-disabled. Here is an example of what an error-disabled port looks like from the command-line interface (CLI) of the switch:
Switch#show interfaces gigabitethernet 5/1 status
Port Name Status Vlan Duplex Speed Type
Gi4/1 err-disabled 100 full 1000 1000BaseSX
Or, if the interface has been disabled because of an error condition, you can see messages that are similar to these in both the console and the syslog:
Received BPDU on port GigabitEthernet4/1 with BPDU Guard enabled. Disabling port.
bpduguard error detected on Gi4/1, putting Gi4/1 in err-disable state
In order to recover a port from the errdisable state, first identify and correct the underlying cause, and then re-enable the port. If you re-enable the port before you fix the actual problem, the ports could just become error disabled again. After you fix the root problem, the ports are still disabled if you have not configured errdisable recovery on the switch. In this case, you must re-enable the ports manually. Issue the shutdown command and then the no shutdown interface mode command on the associated interface in order to manually re-enable the ports.
Major reasons for errdisable are:
There are 3 types of MTU that can be configured on a switch: