EDS-405A/408A Series User’s Manual
Featured Functions
3-36
STP determines which path between each bridged segment is most efficient, and then assigns a
specific reference point on the network. When the most efficient path has been identified, the other
paths are blocked. In the previous 3 figures, STP first determined that the path through Bridge C
was the most efficient, and as a result, 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. The way it
does this is outlined in the following sections.
STP Requirements
Before STP can configure the network, the system must satisfy the following requirements:
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 must have a Bridge Identifier that specifies which bridge acts as the central
reference point, or Root Bridge, for the STP system—bridges with a lower Bridge Identifier
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. The efficieny cost is usually
determined by the bandwidth of the link, with less efficient links assigned a higher cost. 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 step of the STP process is to perform calculations. During this stage, each bridge on the
network transmits BPDUs. The following items will be calculated:
The bridge that 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. This is the cost of the paths from each bridge to the Root
Bridge.
The identity of each bridge’s Root Port. The Root Port is the port on the bridge that connects
to the Root Bridge via the most efficient path; in other words, the port connected to the Root
Bridge via the path with the lowest Root Path Cost. The Root Bridge, however, does not have
a Root Port.
The identity of the Designated Bridge for each LAN segment. The Designated Bridge is the
bridge with the lowest Root Path Cost from that segment. If several bridges have the same
Root Path Cost, the one with the lowest Bridge Identifier becomes the Designated Bridge.
Traffic transmitted in the direction of the Root Bridge will flow through the Designated
Bridge. The port on this bridge that connects to the segment is called the Designated Bridge
Port.
