Stoping STP

tarun_cisco · ‎09-16-2011

Hi,

can somoen please explain how can i stop STP processs between two switches ?

ameya_oke · ‎09-16-2011

Hi Tarun,

You can stop spanning tree by issuing below command.

no spanning-tree vlan

Note: This is not advisable.

https://supportforums.cisco.com/thread/2046805

Please rate if helpful.

Ameya

Joseph W. Doherty · ‎09-17-2011

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The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

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Posting

As Ameya notes, this isn't really advisable; curious why you want to stop STP.

Somasundaram Jayaraman · ‎09-17-2011

Hi,

If you are sure about your network topology, ie. there is no redundant paths which leads to loops then you can disable spanning tree.

++ By default, spanning tree runs on every port. Kindly note that the spanning tree feature cannot be turned off in switches on a per-port basis.

++ you can turn off STP on a per-VLAN basis, or globally on the switch. Extreme care should be taken whenever you disable spanning tree because this creates Layer 2 loops within the network.

++ Use the no spanning-tree vlan 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.

Please refer the below link for more details

http://www.cisco.com/en/US/customer/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml

Hope this answers your question.

Cheers

Somu

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tarun_cisco · ‎09-17-2011

Thanks everyone.

How about putting the ports in port fast mode.

Say I have as simple as 2 switches with backup link and if I put ports in portfast mode then still STP process will occur?

Sent from Cisco Technical Support iPhone App

Somasundaram Jayaraman · ‎09-17-2011

Hi,

Port-fast is configured only on access ports. It should not be configured on a trunk link where the two switches getting connected.

SW2(config)#int f0/6

SW2(config-if)#

spann portfast

%Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc… to this interface when portfast is enabled, can cause temporary bridging loops.

Use with CAUTION

If you are sure that enabling port-fast will not cause any loops you can use that.

Here’s what happens with the ‘trunk’ option enabled:

SW2(config)#int fa0/6
SW2(config-if)#spann portfast trunk
%Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc… to this interface when portfast is enabled, can cause temporary bridging loops.
Use with CAUTION

SW2#sh spanning-tree int f0/6 portfast
VLAN0001         enabled
VLAN0005         enabled
VLAN0007         enabled
VLAN0008         enabled
VLAN0009         enabled
VLAN0010         enabled
VLAN0022         enabled
VLAN0043         enabled
VLAN0058         enabled
VLAN0067         enabled
VLAN0079         enabled
VLAN0146         enabled

Hope this helps.

Cheers

Somu

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ameya_oke · ‎09-17-2011

Hi Tarun,

It is a wrong notion that portfast stops STP.

Yes it is true that portfast directly takes a switchport in forwarding state but underlying theory is that just the STP states are bypassed.

Now consider there are 3 swithces A,B and C.

Each is connected to one another back to back.

Now even though you configure all uplinks as portfast STP will work.

Reason is that portfast do not stop BPDU transmission.

Now with back to back connectivity in A,B and C we have a Layer 2 loop and thus one port will transition in blocking state.

Only way by which you can ensure STP doesnt work is via no spanning-tree vlan abc cmd.

Please rate the helpful posts.

Ameya

cadet alain · ‎09-17-2011

Hi,

Yes it is true that portfast directly takes a switchport in forwarding state but underlying theory is that just the STP states are bypassed

And no TCNs are issued when the port goes up or down.

Regards.

Alain.

Don't forget to rate helpful posts.

Joseph W. Doherty · ‎09-18-2011

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

And no TCNs are issued when the port goes up or down.

Unsure that's true if a portfast port is "seeing" BPDUs.

There was another recent posting where Peter Paluch noted change in behavior of portfast ports when they see a BPDU, see: https://supportforums.cisco.com/message/3219909#3219909

Peter didn't address whether portfast behavior change impacts TCN too.