Cisco Catalyst 2960 Software Configuration Manual page 441

Chapter 16 Configuring STP
The switch supports the IEEE 802.1t spanning-tree extensions, and 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
are reallocated into a 4-bit priority value and a 12-bit extended system ID value equal to the VLAN ID.
Table 16-1 Switch Priority Value and Extended System ID
Switch Priority Value
Bit 16 Bit 15 Bit 14
32768 16384 8192
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. Because the switch stack appears as a single
switch to the rest of the network, all switches in the stack use the same bridge ID for a given spanning
tree. If the stack master fails, the stack members recalculate their bridge IDs of all running spanning trees
based on the new MAC address of the new stack master.
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
section on page
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:
•
•
•
•
•
An interface moves through these states:
•
•
•
OL-26520-01
Extended System ID (Set Equal to the VLAN ID)
Bit 13 Bit 12 Bit 11
4096 2048 1024
16-18, and the
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.
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.
From initialization to blocking
From blocking to listening or to disabled
From listening to learning or to disabled
Catalyst 2960 and 2960-S Switches Software Configuration Guide, Release 15.0(1)SE
Table 16-1, the 2 bytes previously used for the switch priority
Bit 10 Bit 9 Bit 8 Bit 7
512 256 128 64
16-16, the
"Configuring the Switch Priority of a VLAN" section on page
Understanding Spanning-Tree Features
Bit 6 Bit 5 Bit 4 Bit 3
32 16 8 4
"Configuring a Secondary Root Switch"
Bit 2 Bit 1
2 1
16-21.
16-5