Uplink Fast with STP Turned Off?

Answered Question
Dec 29th, 2008

I inherited a network and see some odd configurations that I don't normally do and can't think of a reason for it. One of them has the following statements

spanning-tree uplinkfast

no spanning-tree vlan 20

no spanning-tree vlan 27

no spanning-tree vlan 90

Is there a reason anyone could think of for this?

I have this problem too.
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Correct Answer by Giuseppe Larosa about 7 years 11 months ago

Hello Roland,

in this case I strongly recommend to enable STP on all Vlans in use even if they are at the moment confined in a single switch.

I agree that this configuration is not logical and unsafe.

Hope to help

Giuseppe

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Giuseppe Larosa Mon, 12/29/2008 - 09:09

Hello Roland,

be aware that in low end Cisco Switches there is a limit in the number of STP instances that can run on the device.

There is a limit in the maximum number of vlans that can be defined but it can be higher.

To make an example I used some 29xx there was a max STP instances of 64 and a max vlans of 128.

Vlans in excess of first number can be defined but their STP instance is automatically disabled so you can see a configuration like that even without a human configuration.

You can check how many vlans are defined on the switch with

sh vlan

and status of STP with

sh spanning-tree summary

There was a similar thread some mounths ago with the same scenario: someone starting to manage a network with this kind of strange commands combination

Hope to help

Giuseppe

rshum Mon, 12/29/2008 - 11:42

Thanks Giuseppe but that's not the case here, we have 15 VLAN's on these switches and the ones with STP turned off are actually being used.

I can't think of any real reason my predecessor would do something like this other than misunderstanding the UplinkFast functionality.

Correct Answer
Giuseppe Larosa Mon, 12/29/2008 - 12:25

Hello Roland,

in this case I strongly recommend to enable STP on all Vlans in use even if they are at the moment confined in a single switch.

I agree that this configuration is not logical and unsafe.

Hope to help

Giuseppe

rshum Mon, 12/29/2008 - 12:38

Thanks for the confirmation. I thought I might be missing something but it's nice to get a sanity check.

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