14-3
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-06
Chapter 14 Configuring STP Understanding Spanning-Tree Features
Note The switch sends keepalive messages (to ensure the connection is up) only on interfaces that do not have
small form-factor pluggable (SFP) modules.
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 STP-enabled
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, or on the Cisco ME switch, only through the STP-enabled
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 14-1 on page 14-4.
• 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.