User`s manual

IKS-6726 Featured Functions
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• 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 the bridges on the network agree on the identity of the Root Bridge and all relevant parameters have
been established, each bridge is configured to forward traffic only between its Root Port and the Designated
Bridge Ports for their 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 that are 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 gone
down. This will trigger the bridge to reconfigure the network to account for the change. If you have configured
an SNMP trap destination, the first bridge to detect a topology change in your network sends out an SNMP trap.
Differences between RSTP and STP
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 can carry out automatic
configuration and restore a link faster than STP.
STP Example
The LAN shown below 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.