Troubleshooting RSTP with Wireshark

[David_Purser]

Issue

Need to perform a diagnostic verification as a preventative measure during routine maintenance or reactively in response to cabling that is causing RSTP issues.

Product Line

EcoStruxure Building Operation

Environment

• Multi-purpose Controller (MP-C)
• Multi-purpose VAV (MP-V)
• Room Controller (RPC)
• IP-IO

Cause

Issues can be caused by cabling or cabling terminations between controllers in the RSTP Loop.

Resolution

1. Reference RSTP Troubleshooting for BACnet/IP Controllers - Schneider Electric Community to ensure that:
   1. the managed switch is appropriately configured for RSTP
   2. all SpaceLogic controllers, on the RSTP loop, report RSTP Status = Operational
2. Leave one side connected to the managed switch
3. Plug the other end into a laptop running Wireshark
4. Evaluate based on the information below
5. Ensure both ends are not plugged into the managed switch at the same time by:
   1. Unplug the end from the managed switch
   2. Unplug the end from the laptop and plug it into the manage switch
   3. Take the free end and plug it into the laptop
6. Evaluate based on the information below
7. If it fails either evaluation, check the cables based on Message Age using a cable tester. It is optional but highly recommended to use a cable tester that displays cable length, as they tend to be more accurate, pointing out failures that other testers may report as passing.
8. If it passes the evaluation on both ends, then one can plug in both ends into the managed switch

Evaluation

This capture shows a topology change (TC + Root) in RSTP, then settles out to a consistent Root Bridge Priority and Message Age. Your capture should only show a topology change when Wireshark is capturing on one end and the other end is connecting/disconnecting from the managed switch.

Review the following fields:

• Root Bridge Priority - should be consistent and show the Priority of the Root Bridge. The root bridge can be the switch to which the RSTP loop is connected or a managed switch upstream.
  • If it shows 32768, then another SpaceLogic controller assumed the root because it is not communicating properly back to the root bridge. (See Message Age Note)
• Message Age - should be consistent and show the expected number of controllers away from the root bridge.
  • When Root Bridge Priority is 32768, you can use Message Age to determine how many controllers away the issue is occurring, checking the cabling connected to or close to that controller.