Spanning Tree global commands

[lcaruso]

Hi,

If portfast and loopguard have to be enabled on a per interface basis, what is the purpose of the global commands?

For example,

spanning-tree mode rapid-pvst
spanning-tree loopguard default
spanning-tree portfast bpduguard default
spanning-tree extend system-id
udld aggressive
interface GigabitEthernet1/0/1
switchport access vlan 70
switchport mode access
spanning-tree portfast
spanning-tree guard loop

[InayathUlla Sharieff · Accepted solution]

Hi,

You can enable or disable STP PortFast BPDU guard on a global basis, which affects all ports that have PortFast configured.

If you dont want the BPdU guard to be enabled automatically on all the portfast interface then dont enable it globally just enter per interface basis.

More info:-At the global level, you enable BPDU guard on Port Fast-enabled interfaces by using the spanning-tree portfast bpduguard default global configuration command. Spanning tree shuts down interfaces that are in a Port Fast-operational state. In a valid configuration, Port Fast-enabled interfaces do not receive BPDUs. Receiving a BPDU on a Port Fast-enabled interface signals an invalid configuration, such as the connection of an unauthorized device, and the BPDU guard feature puts the interface in the error-disabled state.

http://www.cisco.com/en/US/products/hw/switches/ps5528/products_configuration_guide_chapter09186a00801e85e3.html

HTH

Regards

Inayath

*Plz rate all usefull post.

[lcaruso]

Why is it a best practice to not enable it globally but on a per interface basis?

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/best/practices/recommendations.html#wp1040726

[Bilal Nawaz · Accepted solution]

Hello, Because enabling it globally could cause a loop, say you had three switches connected in a triangle kind of topology, if all of them went to forwarding straight away then this would cause a spanning-tree loop.

It's best to manually configure on access ports just to be on the safe side and avoid anything like this.

You can use 'spanning-tree portfast trunk' for trunk interfaces I believe, but still you must have your layer 2 topology solid foundation.

Hope this helps

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