36 CHAPTER 4: USING RESILIENCE FEATURES
Figure7 Traffic flowing through Bridge B
STP determines which is the most efficient path between each bridged
segment and a specifically assigned reference point on the network. Once
the most efficient path has been determined, all other paths are blocked.
Therefore, in Figure5, Figure 6, and Figure 7, STP initially determined that
the path through Bridge C was the most efficient, and so blocked the
path through Bridge B. After the failure of Bridge C, STP re-evaluated the
situation and opened the path through Bridge B.
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 t he 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 defin ed 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 li nk — the higher the