Cisco Catalyst 3550 series Software Configuration Manual page 355

Chapter 16 Configuring STP
Before Cisco IOS Release 12.1(8)EA1, entering the spanning-tree vlan vlan-id root global
configuration command on a Catalyst 3550 switch (no extended system ID) caused it to set its own
switch priority for the specified VLAN to 8192 if this value caused this switch to become the root for
the specified VLAN. If any root switch for the specified VLAN has a switch priority lower than 8192,
the switch sets its own priority for the specified VLAN to 1 less than the lowest switch priority.
These examples show the effect of the spanning-tree vlan vlan-id root command with and without the
extended system ID support:
For Catalyst 3550 switches with the extended system ID (Cisco IOS Release 12.1(8)EA1 and later),
•
if all network devices in VLAN 20 have the default priority of 32768, entering the spanning-tree
vlan 20 root primary command on the switch sets the switch priority to 24576, which causes this
switch to become the root switch for VLAN 20.
• For Catalyst 3550 switches without the extended system ID (software earlier than
Cisco IOS Release 12.1(8)EA1), if all network devices in VLAN 100 have the default priority of
32768, entering the spanning-tree vlan 100 root primary command on the switch sets the switch
priority for VLAN 100 to 8192, which causes this switch to become the root switch for VLAN 100.
Note If your network consists of switches that both do and do not support the extended system 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 VLAN number is greater than the priority of the
connected switches running older software.
The root switch for each spanning-tree instance should be a backbone or distribution switch. Do not
Note
configure an access switch as the spanning-tree primary root.
Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of
switch hops between any two end stations in the Layer 2 network). When you specify the network
diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age
time for a network of that diameter, which can significantly reduce the convergence time. You can use
the hello keyword to override the automatically calculated hello time.
Note After configuring the switch as the root switch, we recommend that you avoid manually configuring the
hello time, forward-delay time, and maximum-age time by using the spanning-tree vlan vlan-id
hello-time, spanning-tree vlan vlan-id forward-time, and the spanning-tree vlan vlan-id max-age
global configuration commands.
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Catalyst 3550 Multilayer Switch Software Configuration Guide
Configuring Spanning-Tree Features
16-15