User`s manual

NPort S8000 Series Switch Featured Functions
6-16
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 sections below.
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 the NPort S8000 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
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:
• Which bridge 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.
STP Configuration
After all the bridges on the network agree on the identity of the Root Bridge, and all other relevant parameters
have been established, each bridge is configured to forward traffic only between its Root Port and the
Designated Bridge Ports for the respective network segments. All other ports are blocked, which means that
they will not be allowed to receive or forward traffic.
STP Reconfiguration
Once the network topology has stabilized, each bridge listens for Hello BPDUs transmitted from the Root Bridge
at regular intervals. If a bridge does not receive a Hello BPDU after a certain interval (the Max Age time), the
bridge assumes that the Root Bridge, or a link between itself and the Root Bridge, has gone down. This will