The 2 bytes previously used for the device priority are reallocated into a 4-bit priority value and a 12-bit extended system ID value equal to the VLAN ID. Table 1: Device Priority Value and Extended System ID Bit 2 Spanning tree uses the extended system ID, the device priority, and the allocated spanning-tree MAC address to make the bridge ID unique for each VLAN. Support for the extended system ID affects how you manually configure the root switch, the secondary root switch, and the switch priority of a VLAN. For example, when you change the switch priority value, you change the probability that the switch will be elected as the root switch.
Configuring a higher value decreases the probability; a lower value increases the probability. Port Priority Versus Path Cost If a loop occurs, spanning tree uses 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. The spanning-tree path cost default value is derived from the media speed of an interface.
If a loop occurs, spanning tree uses cost when selecting an interface to put in the forwarding state. You can assign lower cost values to interfaces that you want selected first and higher cost values that you want selected last. If all interfaces have the same cost value, spanning tree puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces. If your device is a member of a switch stack, you must assign lower cost values to interfaces that you want selected first and higher cost values that you want selected last instead of adjusting its port priority.
Spanning-Tree Interface States Propagation delays can occur when protocol information passes through a switched LAN. As a result, topology changes can take place at different times and at different places in a switched network. When an interface transitions directly from nonparticipation in the spanning-tree topology to the forwarding state, it can create temporary data loops. Interfaces must wait for new topology information to propagate through the switched LAN before starting to forward frames. They must allow the frame lifetime to expire for forwarded frames that have used the old topology. Each Layer 2 interface on a device using spanning tree exists in one of these states:
• Blocking—The interface does not participate in frame forwarding. • Listening—The first transitional state after the blocking state when the spanning tree decides that the interface should participate in frame forwarding. • Learning—The interface prepares to participate in frame forwarding. • Forwarding—The interface forwards frames. Configuring Spanning Tree Protocol Port Priority Versus Path Cost Priority Value Bit 16 Bit 15 Bit 14 Bit 13 Bit 12 Bit 11 Bit 10 Bit 9 Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 Configuring Spanning Tree Protocol 4