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5: A
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How STP Works
STP Initialization
Initially, the STP system requires the following before
it can configure the network:
■
Communication between all the bridges. This
communication is carried out using Bridge Proto-
col Data Units (BPDUs), which are transmitted in
packets with a known multicast address.
■
One bridge to start as a master or Root Bridge, a
central point from which the network is config-
ured.
The Root Bridge is selected on the basis of it having
the lowest Bridge Identifier value. This is a combina-
tion of the unique MAC address of the bridge and a
priority component defined for the bridge.
The Root Bridge generates BPDUs on all ports at a
regular interval known as the Hello Time. All other
bridges in the network have a Root Port. This is the
port nearest to the Root Bridge, and it is used for
receiving the BPDUs initiated by the Root Bridge.
STP Stabilization
Once the network has stabilized, two rules apply to
the network:
1
Each network segment has one Designated Bridge
Port. All traffic destined to pass in the direction of
or through the Root Bridge flows through this port.
The Designated Bridge Port is the port which has
the lowest Root Path Cost for the segment.
The Root Path Cost consists of the path cost of the
Root Port of the bridge, plus the path costs across
all the Root Ports back to the Root Bridge.
Table 5-1
shows the default path costs for the
Switch 3000 TX.
2
After all the bridges on the network have deter-
mined the configuration of their ports, each bridge
only forwards traffic between the Root Port and the
ports that are the Designated Bridge Ports for each
network segment. All other ports are
blocked
,
which means that they are prevented from forward-
ing traffic.
STP Reconfiguration
In the event of a network failure, such as a segment
going down, the STP system reconfigures the net-
work to cater for the changes. If the topology of
your network changes, the first bridge to detect
the change
sends out an SNMP trap.
Table 5-1
Default path costs
Port Type Duplex Cost
100BASE-TX (VLT) / 100BASE-FX (VLT) Full 5
Half 12
100BASE-TX / 100BASE-FX Full 150
Half 300