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Fun with Spanning Tree and Loop Detection

Recently, TBNG Consulting helped a client experiencing some intermittent network connectivity issues.  After an initial check of the usual culprits (namely the ISP and firewall), we determined that a more sinister issue persisted. Upon further analysis, it appeared that we were dealing with a spanning tree issue on one of customers HP switches.

The client’s production network at this location consists of three (3) HP ProCurve 2910 series switches, four VLANs and Layer-3 routing.  All three devices were configured with Multi Spanning Tree Protocol (IEEE 802.1s); a quick check via the command line interface (CLI):

switch # show spanning-tree

displayed the expected root bridge and priority as captured in our network diagrams.  We did notice an abundant amount of topology change notifications (TCNs).  But what was the root cause of this problem?

First, we enabled loop detection via the CLI using the following command:

switch (config)#loop-detection 1-48 receiver-action send-disable

Next, we examined the User VLAN switch via the CLI using the following command:

switch # show spanning-tree

We noticed that an uplink interface was being starved of Bridge Protocol Data Units (BPDUs) on port 48 and therefore elected itself as root.  In this instance, Loop Protection seemingly failed to disable the suspect port on our user VLAN switch.  In particular, this lack of dampening created a broadcast forwarding storm – point of interest, BPDUs are forwarded out all interfaces (even blocking interfaces). Hence, topology changes were being propagated.

Should you have any questions or would like additional information, please contact TBNG Consulting at sales@tbngconsulting.com or at 855-512-4817.

TBNG Blogger

Written by TBNG Blogger

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