User Documentation

User Manual Managed Switches
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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.
Differences between STP and RSTP
RSTP is similar to STP, but includes additional information in the BPDUs that allow each bridge to
confirm that it has taken action to prevent loops from forming when it decides to enable a link to a
neighboring bridge. Adjacent bridges connected via point-to-point links will be able to enable a link
without waiting to ensure that all other bridges in the network have had time to react to the change.
The main benefit of RSTP is that the configuration decision is made locally rather than network-wide,
allowing RSTP to carry out automatic configuration and restore a link faster than STP.
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.