PT-7828 User’s Manual
Featured Functions
3-37
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Each bridge must have a Bridge Identifier that specifies which bridge acts as the central
reference point, or Root Bridge, for the STP system. Bridges with a lower Bridge Identifier
are more likely to be designated as the Root Bridge. The Bridge Identifier is calculated using
the MAC address of the bridge and a priority defined for the bridge. The default priority of
PT-7828 is 32768.
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Each port has a cost that specifies the efficiency of each link. The efficiency cost is usually
determined by the bandwidth of the link, with less efficient links assigned a higher cost. The
following table
shows the default port costs for a switch:
Port Speed
Path Cost 802.1D,
1998 Edition
Path Cost
802.1w-2001
10 Mbps
100
2,000,000
100 Mbps
19
200,000
1000 Mbps
4
20,000
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 then be calculated:
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The bridge that will act as the Root Bridge. The Root Bridge is the central reference point
from which the network is configured.
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The Root Path Costs for each bridge. This is the cost of the paths from each bridge to the Root
Bridge.
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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, this port connects to the Root
Bridge via the path with the lowest Root Path Cost. The Root Bridge itself does not have a
Root Port.
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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
