Cat3750 - Stacking & Disabling Spanning-Tree

[Robert Ho]

hey all, anyone run a stack of 3750's with spanning-tree disabled?

our vlan count (plus trunk ports) are running high and do not want to surpass any type of limit for spanning-tree instances.

all trunks have been configured to only allow the necessary vlans.

there is no loop (two-port etherchannel connection to core with backup etherchannel shutdown).

the only thing i can think is that there is a cross-stack etherchannel from the stack to the core.

any help is appreciated, thanks!

[Shashank Singh]

Hi Robert,

The spanning tree feature cannot be turned off in switches on a per port basis. Although it is not recommended, you can turn off Spanning Tree Protocol (STP) on a per-VLAN basis, or globally on the switch. Use the no spanning-tree vlan vlan-id command in order to disable STP on a per virtual LAN (VLAN) basis.

Also ensure that all switches and bridges in the VLAN have spanning tree disabled. You cannot disable spanning tree on some switches and bridges in a VLAN and leave it enabled on other switches and bridges in the VLAN. If you do this, you can have unexpected results because switches and bridges with spanning tree enabled have incomplete information in regards to the physical topology of the network.

I would suggest you try VTP pruning on the client switches before you decide to turn off spanning tree.

Note: Extreme care needs to be taken whenever you disable spanning tree because this creates Layer 2 loops within the network. Do not disable spanning tree in a VLAN, and make sure that there are no physical loops present in the VLAN.

Refer to Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches for more information.

Cheers,

Shashank

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[Robert Ho]

Thanks Shashank. I think VTP pruning does not reduce the # of STP instances.

Instead, it prunes the VLANs off trunks to reduce unnecessary flooding.

I'd have to check back on that though.

Thank you for your response!