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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 13 Configuring STP
Understanding Spanning-Tree Features
In Cisco IOS Release 12.1(9)EA1 and later, Catalyst 2950 and Catalyst 2955 switches support the IEEE
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 13-1, 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 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. For more information, see the
“Configuring the Root Switch” section on page 13-14, the “Configuring a Secondary Root Switch”
section on page 13-16, and the “Configuring the Switch Priority of a VLAN” section on page 13-19.
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 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.
• Forwarding—The interface forwards frames.
• Disabled—The interface is not participating in spanning tree because of a shutdown port, no link on
the port, or no spanning-tree instance running on the port.
An interface moves through these states:
• From initialization to blocking
• From blocking to listening or to disabled
• From listening to learning or to disabled
• From learning to forwarding or to disabled
Tab l e 13-1 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