16-4
Catalyst 2928 Switch Software Configuration Guide
OL-23389-01
Chapter 16 Configuring STP
Understanding Spanning-Tree Features
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
, 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.
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 16-14
“Configuring a Secondary Root Switch”
“Configuring the Switch Priority of a VLAN” section on page 16-19
.
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 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.
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
•
From forwarding to disabled
Table 16-1
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
