Cisco Catalyst 2000 Configuration Handbook page 139

118 Cisco LAN Switching Configuration Handbook
root bridge's priority. You can use the secondary keyword to place a secondary or
backup root bridge, in case of a primary root failure. Here, the bridge priority is set
to 16384. (For MST, the root priority is set to 24576, and the secondary priority to
28672.)
The diameter keyword specifies the diameter or the maximum number of bridges or
switches between two endpoints across the network (1 to 7, default 7). The BPDU
Hello Time interval can also be set (default 2 seconds). Setting the network diameter
causes other STP timer values to be automatically calculated and changed. You can
adjust the timers explicitly with other commands, but adjusting the diameter hides
the complexity of the timer calculations.
(Optional) Adjust the bridge priority:
5.
(global) spanning-tree vlan vlan priority priority
You can also directly modify the bridge priority to achieve other values than the
automatic root or secondary priorities. The priority can be set on a per-VLAN or
instance basis. Instances can be given as an instance-list, as one or more instance
numbers separated by commas, or a hyphenated range of numbers.
To force a switch to become the root, the priority should be chosen so that the root
bridge has a lower priority than all other switches on that VLAN or STP instance.
The bridge priority ranges from 0 to 65535 (default 32768) for PVST+, or one of the
values 0 (highest), 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, 36864,
40960, 45056, 49152, 53248, 57344, and 61440 (lowest) when enabling extended
VLAN support.
(Optional) Prevent a port from becoming a STP root port:
6.
(interface) spanning-tree rootguard
STP Root Guard will be enabled on the port or interface. If another bridge connect-
ed to that port reports a lower bridge ID, becoming the root, the port will be moved
to root-inconsistent (listening) STP state. When the BPDUs of a root bridge are no
longer detected on the port, it will be moved back into normal operation.
(Optional) Tune the root path cost.
7.
a. (Optional) Set the port cost scale
b. Set the port cost for all VLANs or instances:
(global) spanning-tree pathcost defaultcost-method {long | short}
By default, PVST+ switches use the short (16-bit) port cost values. If you have
any ports that are 10 gbps or greater, you should set the port cost scale to long
(32-bit) values on every switch in your network. MISTP, MISTP-PVST+, and
MST modes use long mode by default.
(interface) spanning-tree cost cost