Today, A tech at a remote server site unplugged a single port from one of our fiber converters for our WAN uplink from a Dell PowerConnect Switch. We lost all networking on the 3 stacked 48 port switches for a while, and looking at the log, it appears that removing that one port (which is on a seperate VLAN that only 3 of our ports use) Caused Spanning tree to go crazy.. Apparently, when we plugged the Fiber into the switch 2 days ago, the switch decided to make it the "root port" of the spanning tree config. Once the root port was unhooked, all ports look like they went into a blocking state for a while.

I am more of a server guy, and don't really understand Spanning tree. I thought I set them all to use Rapid STP, but they all went down for more than the few seconds RSTP should take. Is this the expected behavior of the "root port" in spanning tree being removed? Shouldn't the root port be on the vlan that most of the hosts use? (we use the 3 ports in the seperate vlan as a "DMZ" so our two clustered firewalls can both reach the WAN fiber. We have a similar 3 port VLAN setup for our backup internet connection to a cable modem)


Unless something has changed since I last looked at them, Dell switches don't actually support spanning tree on a per-VLAN basis. If there are other switches connected to your stack (on any VLAN) that don't support (or aren't configured for) RPVST then the whole setup will drop back to standard 802.1d timers (...which would account for long convergence times). If the root bridge moves then you'll hit the worst-case scenario.

It's hard to say precisely where you should be setting the priority for the root switch without knowing about the rest of the topology, but generally it should be at a point that is stable and is the destination for most of the L2 traffic. In practice this is typically going to be the L3 default gateway lives. Wherever you choose to put it, set the priorities for the root and secondary root explicitly and unambiguously.

If you -do- want to be configuring spanning tree priorities on a more fine-grained basis then you might want to take a look at configuring multi-instance spanning tree (MST) which will allow you to set up several topologies, each one or more VLAN's assigned.

  • I was thinking that too, however, our gateway is a cluster of Juniper SRX's, and sometimes we down one of them for maintenance, etc. I guess if one went down (because we switched to the other) I would hate to cause a storm.. After reading last night though, it appears that most of my ports need to get setup in PortFast mode, since almost all of them are access ports for end workstations or vmware servers – Brian Sep 13 '12 at 13:49

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