In Figure 3-39, S1 has been assigned as the primary root bridge, using the spanning-tree vlan 1 root primary command, and S2 has been configured as the secondary root bridge, using the spanning-tree vlan 1 root secondary command. This assumes that the rest of the switches in the network have the default 32,768 priority value defined. This ensures that the alternate switch becomes the root bridge if the primary root bridge fails. This command sets the priority for the switch to the predefined value 28,672.
If an alternate root bridge is desired, use the spanning-tree vlan vlan-id root secondary global configuration mode command. The priority for the switch is set to the predefined value of 24,576 or to the highest multiple of 4096 less than the lowest bridge priority detected on the network. To ensure that a switch has the lowest bridge priority value, use the spanning-tree vlan vlan-id root primary command in global configuration mode.
There are two different methods to configure the bridge priority value on a Cisco Catalyst switch. When an administrator wants a specific switch to become a root bridge, the bridge priority value must be adjusted to ensure that it is lower than the bridge priority values of all the other switches on the network. Spanning-tree VLAN port cost (configurable on a per-VLAN basis)Ĭonfiguring and Verifying the Bridge ID (3.3.1.2) Spanning-tree VLAN port priority (configurable on a per-VLAN basis) Spanning-tree port cost (configurable on a per-interface basis) Spanning-tree port priority (configurable on a per-interface basis) PVST+ (Rapid PVST+ and MSTP are disabled.) Table 3-7 Default Switch Configuration Feature Notice that the default spanning-tree mode is PVST+. Table 3-7 shows the default spanning-tree configuration for a Cisco Catalyst 2960 Series switch. Catalyst 2960 Default Configuration (3.3.1.1) The focus of this topic is on how to configure PVST+ in a switched LAN environment.
In this section, you will learn how to implement PVST+ and Rapid PVST+ in a switched LAN environment.
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