3Com 3C17300-US - How STP Works; STP Requirements; STP Calculation

How STP Works 43
How STP Works When enabled, STP determines the most appropriate path for traffic
through a network. It does this as outlined in the sections below.
STP Requirements Before it can configure the network, the STP system requires:
■ Communication between all the bridges. This communication is
carried out using Bridge Protocol Data Units (BPDUs), which are
transmitted in packets with a known multicast address.
■ Each bridge to have a Bridge Identifier. This specifies which bridge acts
as the central reference point, or Root Bridge, for the STP system —
the lower the Bridge Identifier, the more likely the bridge is to become
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 your Switch is 32768.
■ Each port to have a cost. This specifies the efficiency of each link,
usually determined by the bandwidth of the link — the higher the
cost, the less efficient the link. Tabl e 3
shows the default port costs for
a Switch.
Ta b l e 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:
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*
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