14-4
Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
78-11380-10
Chapter14 Configuring STP
Understanding Spanning-Tree Features
Bridge ID, Switch Priority, and Extended System ID
The IEEE 802.1D standard requires that each switch has an un ique 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 rapid PVST+, 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 Cisco IOS 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 un ique ness o f th e brid ge ID. As shown i n
Table14-1, 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 ea rlier release s, spanning tr ee used one MA C
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 secondar y
root switch, and the switch priority of a VLAN. For example, when you c hange 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. For more information, see the
“Configuring the Root Switch” section on page 14-14, the “Configuring a Secondary Root Switch”
section on page 14-16, and the “Configuring the Switch Priority of a VLAN” section on page 14-20.
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 net work. W he n
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 pr opagate
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 switch 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 determines
that the interface should participate in frame forwarding.
•Learning—The interface prepares to participate in frame forwarding.
Table14-1 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