Cisco IE-4000 Software Configuration Manual page 320

Configuring STP
Information About Configuring STP
Spanning tree forces redundant data paths into a standby (blocked) state. If a network segment in the spanning tree fails
and a redundant path exists, the spanning-tree algorithm recalculates the spanning-tree topology and activates the
standby path. Switches send and receive spanning-tree frames, called bridge protocol data units (BPDUs), at regular
intervals. The switches do not forward these frames but use them to construct a loop-free path. BPDUs contain
information about the sending switch and its ports, including switch and MAC addresses, switch priority, port priority,
and path cost. Spanning tree uses this information to elect the root switch and root port for the switched network and
the root port and designated port for each switched segment.
When two ports on a switch are part of a loop, the spanning-tree port priority and path cost settings control which port
is put in the forwarding state and which is put in the blocking state. The spanning-tree port priority value represents the
location of a port in the network topology and how well it is located to pass traffic. The path cost value represents the
media speed.
Note: The default is for the switch to send keepalive messages (to ensure the connection is up) only on interfaces that
do not have small form-factor pluggable (SFP) modules. You can use the [no] keepalive interface configuration
command to change the default for an interface.
Spanning-Tree Topology and BPDUs
The stable, active spanning-tree topology of a switched network is controlled by these elements:

The unique bridge ID (switch priority and MAC address) associated with each VLAN on each switch.

The spanning-tree path cost to the root switch.

The port identifier (port priority and MAC address) associated with each Layer 2 interface.
When the switches in a network are powered up, each functions as the root switch. Each switch sends a configuration
BPDU through all of its ports. The BPDUs communicate and compute the spanning-tree topology. Each configuration
BPDU contains this information:

The unique bridge ID of the switch that the sending switch identifies as the root switch

The spanning-tree path cost to the root

The bridge ID of the sending switch

Message age

The identifier of the sending interface

Values for the hello, forward delay, and max-age protocol timers
When a switch receives a configuration BPDU that contains superior information (lower bridge ID, lower path cost, and
so forth), it stores the information for that port. If this BPDU is received on the root port of the switch, the switch also
forwards it with an updated message to all attached LANs for which it is the designated switch.
If a switch receives a configuration BPDU that contains inferior information to that currently stored for that port, it discards
the BPDU. If the switch is a designated switch for the LAN from which the inferior BPDU was received, it sends that LAN
a BPDU containing the up-to-date information stored for that port. In this way, inferior information is discarded, and
superior information is propagated on the network.
A BPDU exchange results in these actions:

One switch in the network is elected as the root switch (the logical center of the spanning-tree topology in a
switched network).
For each VLAN, the switch with the highest switch priority (the lowest numerical priority value) is elected as the root
switch. If all switches are configured with the default priority (32768), the switch with the lowest MAC address in the
VLAN becomes the root switch. The switch priority value occupies the most significant bits of the bridge ID, as shown
in Table 37 on page 317.
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