4.6 Redundancy
4.6.1 Spanning Tree
1. Spanning Tree Protocol
The IEEE 802.1D Spanning Tree Protocol and IEEE 802.1W Rapid Spanning Tree Protocol allow for the
blocking of links between switches that form loops within the network. When multiple links between
switches are detected, a primary link is established. Duplicated links are blocked from use and become
standby links. The protocol allows for the duplicate links to be used in the event of a failure of the primary
link. Once the Spanning Tree Protocol is configured and enabled, primary links are established and
duplicated links are blocked automatically. The reactivation of the blocked links (at the time of a primary
link failure) is also accomplished automatically without operator intervention.
This automatic network reconfiguration provides maximum uptime to network users. However, the
concepts of the Spanning Tree Algorithm and protocol are a complicated and complex subject and must
be fully researched and understood. It is possible to cause serious degradation of the performance of the
network if the Spanning Tree is incorrectly configured. Please read the following before making any
changes from the default values.
The Switch STP performs the following functions:
。
Creates a single spanning tree from any combination of switching or bridging elements.
。
Creates multiple spanning trees – from any combination of ports contained within a single switch, in
user specified groups.
。
Automatically reconfigures the spanning tree to compensate for the failure, addition, or removal of
any element in the tree.
。
Reconfigures the spanning tree without operator intervention.
Bridge Protocol Data Units
For STP to arrive at a stable network topology, the following information is used:
。
The unique switch identifier
。
The path cost to the root associated with each switch port
。
The por tidentifier
STP communicates between switches on the network using Bridge Protocol Data Units (BPDUs). Each
BPDU contains the following information:
。
The unique identifier of the switch that the transmitting switch currently believes is the root switch
。
The path cost to the root from the transmitting port
。
The port identifier of the transmitting port
The switch sends BPDUs to communicate and construct the spanning-tree topology. All switches
connected to the LAN on which the packet is transmitted will receive the BPDU. BPDUs are not directly
forwarded by the switch, but the receiving switch uses the information in the frame to calculate a BPDU,
and, if the topology changes, initiates a BPDU transmission.
