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EDS-508 Series User’s Manual Featured Functions
3-25
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
EDS is 32768.
# Each port has a cost that specifies the efficiency of each link, usually determined by the
bandwidth of the link—the higher the cost, the less efficient the link. 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 stage in the STP process is the calculation stage. During this stage, each bridge on the
network transmits BPDUs that allow the system to calculate:
# Which bridge should be the Root Bridge. The Root Bridge is 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 Root Port is the port
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
Designated Bridge is 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.
STP Configuration
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.
STP Reconfiguration
Once the network topology is stable, all the bridges listen 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. The bridge then reconfigures 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.
How RSTP Differs from STP
RSTP works in a similar way to STP, but it includes additional information in the BPDUs. This
information allows each bridge to confirm that it has taken action to prevent loops from forming
when it wants to enable a link to a neighboring bridge. This allows adjacent bridges connected via
point-to-point links to enable a link without needing to wait to ensure that all other bridges in the
network have had time to react to the change. So the main benefit of RSTP is that the
configuration decision is made locally rather than network-wide, which is why RSTP can carry out
automatic configuration and restore a link faster than STP.