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EDS-518A Series User’s Manual Featured Functions
3-24
y 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 be calculated:
y Which bridge should be the Root Bridge. The Root Bridge is the central reference point from
which the network is configured.
y The Root Path Costs for each bridge. This is the cost of the paths from each bridge to the Root
Bridge.
y 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, the port connected to the Root
Bridge via the path with the lowest Root Path Cost. The Root Bridge, however, does not have
a Root Port.
y 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 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.