Interoperability between Classic Spanning Tree Protocol (802.1D) and Rapid Spanning Tree (802.1w)

• Inherent fast convergence benefit of 802.1w are lost when it interacts with legacy bridges.
• Each port maintains a variable that defines the protocol to run on that segment.
• A migration delay timer of three seconds starts when the port comes up.
• Migration delay timer resets if port changes its mode of operation.
• Classic 802.1D timers (forward delay and max_age) are only used as backup and should not be necessary if point-to-point links and edge ports are properly identified and set by the administrator.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

Etherchannel Mode On and Spanning Tree Loop

A common issue during EtherChannel configuration is that the interfaces go into err-disable mode. This can be seen when Etherchannel is switched to the ON mode in one switch, and the other switch is not configured immediately. If left in this state for a minute or so, STP on the switch where EtherChannel is enabled thinks there is a loop. This causes the channeling ports to be put in err-disable state.


n order to resolve the issue, set the channel mode to desirable on both sides of the connection, and then re-enable the interfaces.


http://www.cisco.com/en/US/tech/tk389/tk213/technologies_configuration_example09186a0080094647.shtml

BPDU Guard

• Prevents a switch to be added to a port by mistake
• If any BPDU is received port will be put in err-disable
• Used with portfast ports
• Can't be used with root guard
• Blocks the whole port

Switch(config)# spanning-tree portfast bpduguard default

Switch(config-if)# spanning-tree bpdugard enable

MST Configuration

For MST to work between two switches the following has to be the same:

1. Name
2. Revision Number
3. Instance to VLAN Mapping

Switch(config)# spanning-tree mode mst

Switch(config)# spanning-tree mst configuration

Switch(config-mst)# name <instance-name>

Switch(config-mst)# revision <revision-number>

Switch(config-mst)# instance <instance-id> vlan <vlan-list>

Switch(config)# spanning-tree mst <instance-id> root [primary | secondary] diameter <diameter>

Switch(config)# spanning-tree mst <instance-id> priority <bridge-priority>

Switch(config-if)# spanning-tree mst <instance-id> cost <cost>

Switch(config-if)# spanning-tree mst <instance-id> port-priority <port-priority>

Switch(config)# spanning-tree mst <instance-id> mst hello-time <seconds>

Switch(config)# spanning-tree mst <instance-id> mst forward-time <seconds>

Switch(config)# spanning-tree mst <instance-id> mst max-age <seconds>