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Networking, Iris Scenarios: Ports disabled by Spanning Tree BPDUs
After patching a network jack to a switch, IT Pros may notice that the switchport shows as "Disabled" or "Error" in Iris. Even after re-enabling the port, the port will go back into the disabled state almost immediately. This may be due to a network loop or connecting a device that is trying to participate in Spanning Tree.
Spanning Tree and Bridge Protocol Data Units (BPDUs)
Technology Services - Networking utilizes a network protocol called the Spanning Tree Protocol (STP). STP is used to prevent loops on the network which could constantly transmit more and more copies of the same network traffic and would eventually fill the uplinks of building switches and routers and eventually cause a service impact. On the UIUC campus, network switches are configured to send STP Bridge Protocol Data Units (BPDUs) on all customer-facing ports. The switches are also configured to disable a port if a BPDU is received as this indicates the presence of a loop. The most common causes of a port being disabled by STP are:
- Two switchports accidentally patched to each other by:
- patching two wall jacks directly to each other
- patching two wall jacks into the same mini switch or IP phone
- patching two interfaces on a mini switch to each other
- Connecting a device to the network that is trying to participate in Spanning Tree and is transmitting BPDUs (This can cause issues if the device steals the STP "root" which will cause an outage each time the device is connected or disconnected to/from the network.)
In all of the cases above, the interface that receives a BPDU will automatically disable itself. Re-enabling the interface through Iris, without resolving the network loop, will result in the switchport immediately disabling itself again.
Will Iris notify me if a switchport has been disabled due to a loop?
Fundamentally, Iris strives to display the state of the switch as accurately as possible. This means that, on older switches (Aruba ProCurve line), Iris will show the port as disabled with no indication that a BPDU disabled the port. There is no way to display that the port was disabled by a STP BPDU on older switches.
Newer switches (Aruba CX line) have the ability to record the state of the switchport as "Disabled due to STP". In this case, Iris will show the port as being in "Error" and will highlight that port in yellow. (See below)
What can I do to recover from this scenario?
If you notice that a switchport is consistently disabling itself, check to see what's plugged into that port. You may need to trace the cable to see what jack that switchport is connected to and then go to the other end of that jack to see what it's patched to.
If you find that the wall jack is patched into another wall jack, remove that cable. If you find that the wall jack is patched to a mini switch or IP phone, ensure that no other wall jacks are patched to that equipment or look for looped ports on the mini switch. If you find that the wall jack is patched to a device, ensure that that device does not have STP enabled.
Once the loop has been broken, you should be able to re-enable the interface in Iris and it will remain enabled. (See Iris Scenarios: Enabling and Disabling Ports)
If you have checked the path and are not finding a loop, please contact Technology Services Networking.