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I am working with both older Foundry switches, and Catalyst 3560s, and trying to document the spanning tree topology. The Foundry switches show me the root port and root bridge as you would expect, but the ports that are blocked also show the designated bridge, or in other words, the switch that would take over if the root port failed. The Cisco switches show me the root port and root bridge, and also the alternate/blocked ports, but they don't show what the designated bridge is for those ports. I am trying to figure out how to get this information out of Cisco IOS.

Using the Foundry switch for an example, I am presented with the root port, the designated root (the root switch of the STP domain), and the designated bridge (which is the current STP root for the root port). The alternate blocked port shows the designated root (the root switch of the STP domain), and the designated bridge (which is the "secondary" root or the switch that would take over as the root).

It is the last piece of the puzzle, the designated bridge or rather the switch that would take over and make the alternate port become the root port if there was a failure upstream that I can't seem to figure out from Cisco IOS. I have been through several spanning tree show and debug commands, but I haven't been able to find the answer.

I know how spanning tree works, and I do know the manual process of calculating the different switches and ports, but the issue is that the topology I am working on has many switches, many VLANs, and several different STP root switches. There doesn't seem to be a completely consistent pattern of configuration. I have been assigned the task of documenting the physical and logical topology so we can get things straightened out. There is a pair of Cat6500s acting as the core, with one of the 6500s acting as the STP root for the majority of the VLANs, but not all. The second 6500 is the root for a few VLANs, and then the remaining VLANs have roots on other switches dispersed throughout the network. The reason I can't simply look at each VLAN and simply determine the root and secondary is because nearly all of the Foundry switches are configured to operate only in VLAN1, despite their upstream switches being configured for particular VLANs. In other words, they participate in STP, but not in the VLAN structure.

  • Just 'show spanning tree vlan <inst>' should show you the root bridge and the ports that are in designated state – ajaysdesk Sep 30 '15 at 3:04
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Found the answer:

show spanning-tree interface INT detail

This command goes deeper than the regular "show spanning-tree" and displays the designated bridge as well as root bridge.

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