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Configuring STP
Information About Configuring STP
A root port is selected for each switch (except the root switch). This port provides the best path (lowest cost) when
the switch forwards packets to the root switch.
The shortest distance to the root switch is calculated for each switch based on the path cost.
A designated switch for each LAN segment is selected. The designated switch incurs the lowest path cost when
forwarding packets from that LAN to the root switch. The port through which the designated switch is attached to
the LAN is called the designated port.
All paths that are not needed to reach the root switch from anywhere in the switched network are placed in the
spanning-tree blocking mode.
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 Table 37
on page 317, 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 Configuring the Root Switch, page 328, the Configuring a Secondary Root
Switch, page 328, and the Configuring Optional STP Parameters, page 329.
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.
Table 37 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 8Bit 7Bit 6Bit 5Bit 4Bit 3Bit 2Bit 1
32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1
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