Specifications

12-4
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-07
Chapter 12 Configuring STP
Understanding Spanning-Tree Features
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.
The root switch is the logical center of the spanning-tree topology in a switched network. All paths that
are not needed to reach the root switch from anywhere in the switched network are placed in the
spanning-tree blocking mode.
BPDUs contain information about the sending switch and its ports, including switch and MAC
addresses, switch priority, port priority, and path cost. Spanning tree uses this information to elect the
root switch and root port for the switched network and the root port and designated port for each
switched segment.
Bridge ID, Switch Priority, and Extended System ID
The IEEE 802.1D standard requires that each switch has an unique bridge identifier (bridge ID), which
determines the selection of the root switch. Because each VLAN is considered as a different logical
bridge with PVST and PVRST, the same switch must have as many different bridge IDs as VLANs
configured on it. Each VLAN on the switch has a unique 8-byte bridge ID; the two most-significant bytes
are used for the switch priority, and the remaining six bytes are derived from the switch MAC address.
In Release 12.1(9)EA1 and later, Catalyst 2950 and Catalyst 2955 switches support the 802.1T
spanning-tree extensions. Some of the bits previously used for the switch priority are now used as the
VLAN identifier. The result is that fewer MAC addresses are reserved for the switch, and a larger range
of VLAN IDs can be supported, all while maintaining the uniqueness of the bridge ID. As shown in
Table 12-1, the two bytes previously used for the switch priority are reallocated into a 4-bit priority value
and a 12-bit extended system ID value equal to the VLAN ID. In earlier releases, the switch priority is
a 16-bit value.
Spanning tree uses the extended system ID, the switch priority, and the allocated spanning-tree MAC
address to make the bridge ID unique for each VLAN. With earlier releases, spanning tree used one MAC
address per VLAN 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 more information, see the “Configuring the Root
Switch” section on page 12-12, “Configuring a Secondary Root Switch” section on page 12-14, and
“Configuring the Switch Priority of a VLAN” section on page 12-18.
Table 12-1 Switch Priority Value and Extended System ID
Switch Priority Value Extended System ID (Set Equal to the VLAN ID)
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 Bit 2 Bit 1
32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1