18-5
Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter18 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 unique bridge identifier (bridge ID), which
controls 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 a different bridge IDs for each configured
VLAN. Each VLAN on the switch has a unique 8-byte bridge ID. The 2 most-significant bytes are used
for the switch priority, and the remaining 6 bytes are derived from the switch MAC address.
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 Table18-1, the 2 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.
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 page18-16, the “Configuring a Secondary Root Switch”
section on page 18-18, and the “Configuring the Switch Priority of a VLAN” section on page18-21 .
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 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.
Table18-1 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