User Manual Managed Switches
64
STP Calculation
The first step of the STP process is to perform calculations. During this stage, each bridge on the
network transmits BPDUs. The following items will be calculated:
•
Which bridge should be the
Root Bridge
. The Root Bridge is the central reference point from
which the network is configured.
•
The
Root Path Costs
for each bridge. This is the cost of the paths from each bridge to the Root
Bridge.
•
The identity of each bridge’s
Root Port
. The Root Port is the port on the bridge that connects to
the Root Bridge via the most efficient path. In other words, the port connected to the Root Bridge
via the path with the lowest Root Path Cost. The Root Bridge, however, does not have a Root
Port.
•
The identity of the
Designated Bridge
for each LAN segment. The Designated Bridge is the
bridge with the lowest Root Path Cost from that segment. If several bridges have the same Root
Path Cost, the one with the lowest Bridge Identifier becomes the Designated Bridge. Traffic
transmitted in the direction of the Root Bridge will flow through the Designated Bridge. The port
on this bridge that connects to the segment is called the
Designated Bridge Port
.
STP Configuration
After all of the bridges on the network agree on the identity of the Root Bridge, and all other relevant
parameters have been established, each bridge is configured to forward traffic only between its Root
Port and the Designated Bridge Ports for the respective network segments. All other ports are
blocked, which means that they will not be allowed to receive or forward traffic.
STP Reconfiguration
Once the network topology has stabilized, each bridge listens for Hello BPDUs transmitted from the
Root Bridge at regular intervals. If a bridge does not receive a Hello BPDU after a certain interval (the
Max Age time), the bridge assumes that the Root Bridge, or a link between itself and the Root Bridge,
has ceased to function. This will trigger the bridge to reconfigure the network to account for the
change. If you have configured an SNMP trap destination, when the topology of your network
changes, the first bridge to detect the change will send out an SNMP trap.
STP Example
The LAN shown in the following figure has three segments, with adjacent segments connected using
two possible links. The various STP factors, such as Cost, Root Port, Designated Bridge Port, and
Blocked Port are shown in the figure.
