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Webcast-Catalyst9k
New Member

Identifying spanning-tree root switch

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?

4 REPLIES

Re: Identifying spanning-tree root switch

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

New Member

Re: Identifying spanning-tree root switch

Thanks Martin, appreciate your swift response!

The 'show spanning-tree' output on the 6509 and the access switches does not contain the line 'This bridge is the root', but the port in the Root ID section is a trunk to one of the access switches.

Am I then to presume that the access switch is the root switch for that spanning tree instance?

If I wanted to manually change the root switch to the 6509 (which seems sensible to me) how would I go about it?

Purple

Re: Identifying spanning-tree root switch

It should tell what the root port is if the 6509 is not the root and whatever is on the other end of that is probably the root , if it is a access switch go there and do the same command . If is the access switch then you should change it to the 6509 . If it a IOS native box it will be

spanning-tree vlan XXX root primary

New Member

Re: Identifying spanning-tree root switch

Thanks Glen, this was very helpful.

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