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Ethernet Card Software Feature and Configuration Guide, R7.2
January 2009
Chapter 7 Configuring STP and RSTP
Configuring the Root Switch
Beginning in privileged EXEC mode, follow these steps to disable STP or RSTP on a per-VLAN basis:
To reenable STP, use the no bridge-group bridge-group-number spanning disabled interface-level
configuration command.
Configuring the Root Switch
The switch maintains a separate spanning-tree instance for each active VLAN configured on it. A
bridge ID, consisting of the switch priority and the switch MAC address, is associated with each
instance. For each VLAN, the switch with the lowest bridge ID becomes the root switch for that VLAN.
Note If your network consists of switches that both do and do not support the extended sy stem ID, it is unlikely
that the switch with the extended system ID support will become the root switch. The extended system
ID increases the switch priority value every time the bridge ID is greater than the priority of the
connected switches that are running older software.
Configuring the Port Priority
If a loop occurs, spanning tree uses the port priority when selecting an interface to put into the
forwarding state. You can assign higher priority values (lower numerical values) to interfaces that you
want selected first, and lower priority values (higher numerical values) that you want selected last. If all
interfaces have the same priority value, spanning tree puts the interface with the lowest interface number
in the forwarding state and blocks the other interfaces.
Beginning in privileged EXEC mode, follow these steps to configure the port priority of an interface:
Command Purpose
Step 1 Router# configure terminal Enters the global configuration mode.
Step 2 Router(config)# interface
interface-id
Enters the interface configuration mode.
Step 3 Router(config-if)# bridge-group
bridge-group-number
spanning disabled
Disables STP or RSTP on a per-interface basis.
Step 4 Router(config-if)# end Returns to privileged EXEC mode.
Command Purpose
Step 1 Router# configure terminal Enters the global configuration mode.
Step 2 Router(config)# interface
interface-id
Enters the interface configuration mode, and specifies an
interface to configure.
Valid interfaces include physical interfaces and
port-channel logical interfaces (port-channel
port-channel-number).