User`s manual

TN-5516/5518 Series Featured Functions
3-34
STP will determine which path between each bridged segment is most efficient, and then assign a specific
reference point on the network. When the most efficient path has been identified, the other paths are blocked.
In the above 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 method is
described below:
STP Requirements
Before STP can configure the network, the system must satisfy the following requirements:
• Communication must be established between all 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 TN-5500 is 32768.
• Each port has a cost that specifies the efficiency of each link. The efficiency 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
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 then be calculated:
• The bridge that will act as 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, this port connects to the Root Bridge via the path with the
lowest Root Path Cost. The Root Bridge itself 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.