Have you ever had a nightmare where your Cisco switch port suddenly crashed, caught in the inevitable err-disable grip? This article will walk you through the intricacies of err-disable, uncover its causes, and offer powerful recovery solutions.
The port errdisabled is the port that has been disabled by the Cisco switch because an error condition is detected. This error condition can be caused by various factors, such as misconfiguration, loopback, link flapping, security, or protocol violations. Errdisabled ports cannot send or receive data traffic until those ports are manually or automatically reactivated.
Errdisabled ports can cause serious network interference, especially if they are connected to critical devices such as servers, routers, or end devices. Therefore, it is important to know how to address errdisabled ports and prevent them from happening in the future.
In this article, we will discuss the following:
- What are the common causes of errdisabled ports and how to diagnose the problem?
- How to re-enable errdisabled ports manually or automatically by using the errdisable recovery feature.
- How to prevent errdisabled ports by using the errdisable detect and errdisable flap features.
- How to monitor the status of errdisabled ports by using the show interfaces status err-disabled and show errdisable recovery commands.
Common Causes of Errdisabled Ports and How to Diagnose the Problem
Many error conditions can cause a port to become errdisabled on Cisco switches. Some of the most common error conditions are:
- Misconfigurasi: Misconfiguration is a misconfiguration that causes conflicts or incompatibilities between connected ports. For example, if the connected ports have different speeds or duplexes, or if the connected ports have different trunking modes, then those ports can become errdisabled.
- Loopback: Loopback is a condition in which a port receives back packets sent by the port itself. This can happen if there is a cable connected between two ports on the same switch, or if a device sends back packets it receives. Loopbacks can cause network traffic to become excessive and interfere with switch performance.
- Link flapping: Link flapping is a condition in which ports repeatedly rise and fall due to physical or logical interference with the connection. This can be caused by damaged cables, loose connectors, electromagnetic interference, or a faulty device. Link flapping can cause network instability and reduce switch reliability.
- Security: Security is a condition where a port violates the security rules set by the switch. This can happen if an unauthorized device tries to access the network, or if a device tries to perform a network attack. Some security features that can cause a port to become errdisabled are Port Security, 802.1x, DHCP Snooping, Dynamic ARP Inspection, and IP Source Guard.
- Protocol violation: A protocol violation is a condition in which a port violates the protocol rules set by the switch. This can happen if any device does not support or conform to the protocol used by the switch, or if any device sends an invalid or malicious packet. Some protocols that can cause a port to become errdisabled are Spanning Tree Protocol (STP), EtherChannel, UDLD, and VTP.
To diagnose the problem that causes the port to become errdisabled, we can use the show interfaces status err-disabled command on the switch. This command will display a list of ports that are errdisabled and the reason why those ports are disabled. The following is an example output of this command:
Switch# show interfaces status err-disabled
Port Name Status Reason
Gi1/0/1 err-disabled link-flap
Gi1/0/2 err-disabled bpduguard
Gi1/0/3 err-disabled psecure-violation
Gi1/0/4 err-disabled udld
Gi1/0/5 err-disabled channel-misconfig
From the output above, we can see that port Gi1/0/1 becomes errdisabled due to link flapping, port Gi1/0/2 becomes errdisabled due to STP violation, port Gi1/0/3 becomes errdisabled due to Port Security violation, port Gi1/0/4 becomes errdisabled due to UDLD violation, and port Gi1/0/5 becomes errdisabled due to EtherChannel misconfiguration.
In addition, we can also use the show logging command to view log messages related to the errdisabled port. This command will display a historical record of switch activity, including error messages generated by errdisabled ports. The following is an example output of this command:
Switch# show logging
...
%PM-4-ERR_DISABLE: link-flap error detected on Gi1/0/1, putting Gi1/0/1 in err-disable state
%PM-4-ERR_DISABLE: bpduguard error detected on Gi1/0/2, putting Gi1/0/2 in err-disable state
%PM-4-ERR_DISABLE: psecure-violation error detected on Gi1/0/3, putting Gi1/0/3 in err-disable state
%PM-4-ERR_DISABLE: udld error detected on Gi1/0/4, putting Gi1/0/4 in err-disable state
%PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Gi1/0/5, putting Gi1/0/5 in err-disable state
...
From the output above, we can see that the log message shows the reason why the port became errdisabled and the time the error occurred.
Using the above commands, we can find out the common causes of errdisabled ports and how to diagnose the problem. The next step is to re-enable errdisabled ports and prevent them from happening in the future.