Identifying spanning-tree root switch

Question

Looking at a network with a 6509 at the core running in pvst mode. I think the 6509 is the root switch but need to confirm this.
Show spanning-tree gives a bridge id and a root id. My understanding is that the root id should be the MAC address of the root switch.
However I can't find the MAC address given as the root id in the 6509s mac address table, nor in the access switches mac address tables.
I'm sure I'm missing something here - any ideas?

Answer 1

Hi,
in the output of "show spanning-tree" you should look for a line "This bridge is the root". The output will give you the root id and the bridge id of the switch, where you execute the command.
The output looks like this:
Router# show spanning-tree vlan 200
VLAN0200
Spanning tree enabled protocol ieee
Root ID Priority 32768
Address 00d0.00b8.14c8
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768
Address 00d0.00b8.14c8
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
------------- snip -----------
The MAC used for creating the bridge id is not used for forwarding BPDUs and thus does not show up in the CAM table afaik.
To find the root in a switch network, follow the root ports for a given spanning tree instance.
Hope this helps! Please rate all posts.
Regards, Martin