EDS-405A/408A Series User’s Manual
Featured Functions
3-37
STP Configuration
After all 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 gone down. 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 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 to 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.
Bridge A
LAN Segment 3
LAN Segment 2
LAN Segment 1
Port 2
(Root Bridge)
Port 1
(Designated
Bridge Port)
Bridge B
Port 2
(Designated
Bridge Port)
Port 1
(Root Port)
Cost =100
Bridge X
Port 2
(Blocked Port)
Port 1
(Root Port)
Cost =100
Bridge C
Port 2
(Designated
Bridge Port)
Port 1
(Root Port)
Cost =100
Bridge Y
Port 2
(Blocked Port)
Port 1
(Root Port)
Cost =200
