Bridge Id, Switch Priority, And Extended System Id; Spanning-Tree Interface States - Cisco Catalyst 3750 Software Configuration Manual

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 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 switch supports the 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
reallocated into a 4-bit priority value and a 12-bit extended system ID value equal to the VLAN ID.
Table 14-1 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.
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:
•
•
•
•
•
Catalyst 3750 Metro Switch Software Configuration Guide
14-4
Extended System ID (Set Equal to the VLAN ID)
Bit 13 Bit 12 Bit 11
4096 2048 1024
14-16, and the
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.
Table 14-1, the two bytes previously used for the switch priority are
Bit 10 Bit 9 Bit 8 Bit 7
512 256 128 64
14-14, the
"Configuring the Switch Priority of a VLAN" section on page
Chapter 14
Bit 6 Bit 5 Bit 4 Bit 3
32 16 8 4
"Configuring a Secondary Root Switch"
Configuring STP
Bit 2 Bit 1
2 1
14-19.
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