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How STP Works
37
cost, the less efficient the link. Table 3 shows the default port costs for
a Switch.
Table 3
Default port costs
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 work out:
■
The identity of the bridge that is to 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 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 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.
Port Speed Link Type
Path Cost
802.1D-1998
Path Cost
802.1w
10 Mbps Half Duplex
Full Duplex
Aggregated Link
100
95
90
2,000,000
1,999,999
1,000,000
*
*This path cost is correct where there are two ports in an aggregated link. However, if there are
more ports in the aggregated link, the path cost will be proportionately lower. For example, if
there are four ports in the aggregated link, the 802.1w path costs will be: 500,000 for
10 Mbps, 50,000 for 100 Mbps, and 5,000 for 1000 Mbps. The 802.1D-1998 path cost values
are not affected by the number of ports in an aggregated link.
100 Mbps Half Duplex
Full Duplex
Aggregated Link
19
18
15
200,000
199,999
100,000*
1000 Mbps Full Duplex
Aggregated Link
4
3
20,000
10,000*