Spanning-Tree Topology And Bpdus - Cisco ME 3400G-2CS - Ethernet Access Switch Software Configuration Manual

Chapter 14 Configuring STP
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:
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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 NNIs. The
BPDUs communicate and compute the spanning-tree topology. Each configuration BPDU contains this
information:
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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:
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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 (or with
each Layer 2 NNI in the Cisco ME switch).
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
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
A root port is selected for each switch (except the root switch). On the Cisco ME switch, this port
is always an NNI. 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.
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
Understanding Spanning-Tree Features
Table 14-1 on page 14-4.
14-3