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Cisco Systems IntelligentGigabit Ethernet Switch Modules for the IBMBladeCenter, Software Configuration Guide
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Chapter9 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.
The Cisco Systems Intelligent Gigabit Ethernet Switch Module 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 br idge ID. As shown in Tabl e 9-1, 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.
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.
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 9-15, the “Configuring a Secondary Root Switch”
section on page 9-17, and the “Configuring the Switch Priority of a VLAN” section on page 9-20.
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. 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 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.
•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 po rt.
Table9-1 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 2 048 1024 512 256 128 64 32 16 8 4 2 1