Stp Calculation; Stp Configuration; Stp Reconfiguration - 3Com SuperStack II Manual

STP Calculation

STP Configuration

STP Reconfiguration

The first stage in the STP process is the calculation stage. During this
stage, each bridge on the network transmits BPDUs that allow the system
to work out:
The identity of the bridge that is to be the Root Bridge — the
central reference point from which the network is configured.
The Root Path Costs for each bridge — that is, the cost of the paths
from each bridge to the Root Bridge.
The identity of the port on each bridge that is to be the Root Port
— the one that is connected to the Root Bridge using the most
efficient path, that is, the one that has the lowest Root Path Cost.
Note that the Root Bridge does not have a Root Port.
The identity of the bridge that is to be the Designated Bridge of
each LAN segment — the one that has the lowest Root Path Cost
from that segment. Note that if several bridges have the same Root
Path Cost, the one with the lowest Bridge Identifier becomes the
Designated Bridge.
All traffic destined to pass in the direction of the Root Bridge flows
through the Designated Bridge. The port on this bridge that connects
to the segment is called the Designated Bridge Port.
After all the bridges on the network have agreed on the identity of the
Root Bridge, and have established the other relevant parameters, 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 are prevented from
receiving or forwarding traffic.
Once the network topology is stable, all the bridges listen for special
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. The bridge then reconfigures the
network to cater for the change. If the topology of your network
changes, the first bridge to detect the change sends out an SNMP trap.
CAUTION: To ensure that the bridges can communicate after a
reconfiguration, all potential Designated Bridge ports and Root Ports
must belong to the same VLANs. For more information about VLANs, see
"Virtual LANs (VLANs)" on page 163.
How STP Works 197