Spanning-Tree Interface States - Cisco Catalyst 3550 series Software Configuration Manual

Understanding Spanning-Tree Features
In Cisco IOS Release 12.1(8)EA1 and later, Catalyst 3550 switches support the 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 bridge ID. As shown in
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. In earlier releases, the switch priority is a 16-bit value.
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. With earlier releases, spanning tree used one MAC
address per VLAN 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:
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•
•
•
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An interface moves through these states:
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Catalyst 3550 Multilayer Switch Software Configuration Guide
16-4
Extended System ID (Set Equal to the VLAN ID)
Bit 13 Bit 12 Bit 11
4096 2048 1024
16-16, 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.
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
Bit 10 Bit 9 Bit 8 Bit 7
512 256 128 64
16-14, the
"Configuring the Switch Priority of a VLAN" section on page
Chapter 16
Table
Bit 6 Bit 5 Bit 4 Bit 3
32 16 8 4
"Configuring a Secondary Root Switch"
Configuring STP
16-1, the two
Bit 2 Bit 1
2 1
16-20.
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