Recently we implemented spanning tree and redundant links in our infrastructure. This has resulted in systems on the default VLAN having intermittent connectivity problems when I plug in the backup link.

This is configured between a Cisco 3750 and a couple HP 2900 series switches. The 3750 stack also provides intervlan routing.

A few legacy devices in our environment are still running on a subnet assigned to the default VLAN - mostly printers and a few old servers. These predate any internal subnetting or dedicated network personnel. I'm getting one way routing from the default subnet, accessing reaching only systems plugged into one switch or the other, etc.

Nothing shows up in the switch logs.

I'm thinking that the best solution would be to migrate these devices to a new VLAN. This is something I've been doing in conjunction with a subnet migration for some time - this range is also a bogon net - but I'd rather not need to reconfigure all the attached hardware.

Is there something easier I've overlooked?


These are the spanning tree related settings I'm using. As far as I can tell things are set correctly. The spanning tree does work properly for all the other VLANs listed, just not 1.

Cisco 3750

spanning-tree mode mst
spanning-tree extend system-id
spanning-tree mst configuration
 name mstp
 revision 1
 instance 1 vlan 1, 40, 70, 100, 250
spanning-tree mst 1 priority 0
spanning-tree mst 0 priority 4096

HP Switches

spanning-tree config-name "mstp"
spanning-tree config-revision 1
spanning-tree instance 1 vlan 1 40 70 100 250

It's been a while since I looked at this but make sure you're using the same implementation of STP on both switches. I think HP recommends MSTP if all of your devices support it. I'm sure you've already done this but just in case.

When you plug in your backup link are you sure your switches are not causing a convergence? Double check your switch logs to make sure that bridge you think is the root bridge is still the root bridge. I'm guessing (and I may be completely wrong) that your switches are calculating a different bridge once you connect your backup link and this somehow orphans your machines on the default VLAN from getting to the 3750 where they can get routing.

  • Also check the switch to switch links (forwarding or blocking state) to make sure that a path indeed exists between all switches. It usually helps me to draw a diagram of the switches and the switch interlinks and determine which links are in the forwarding state and which are in the blocking state to make sure that I have a complete path throughout the switching infrastructure, with no loops. – joeqwerty Jan 7 '13 at 16:39
  • @kce - we're using MSTP. I actually have a question about getting the root configured correctly [here][serverfault.com/questions/455731/…. – Tim Brigham Jan 7 '13 at 17:00

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