Install guide

Rapier Layer 3 Gigabit Switch Software Release 2.1.0 39
Rapier Switch Software Release 2.1.0
C613-10200-01 Rev A
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The unique parent of a switch is the LAN, to which the switch is attached,
that is closest to the root bridge.
The spanning tree computation is a continuous, distributed process. The
algorithm uses the following steps to establish the spanning tree:
1. A unique root bridge is elected by the switches in the LAN.
2. A designated bridge is elected for each LAN in the extended LAN by the
switches in the LAN.
3. The logical spanning tree is computed and redundant paths are removed.
Once the spanning tree is established, it is maintained by:
1. Replacing a failed port with a redundant backup port.
2. Detecting and removing loops by declaring a port redundant and removing
it from the logical spanning tree.
3. Maintaining address timers that control the ageing of station map address
entries.
The logical spanning tree, sometimes called the active topology, includes the
root bridge and all designated bridges, i.e. all the ports that are to be used for
communication within the STP. These ports are dynamically marked as
‘Forwarding’. Ports removed from the logical spanning tree are not in the
‘Forwarding’ state. To implement the spanning tree algorithm, switches
communicate with one another using the Spanning Tree Protocol. The primary
protocol data unit (PDU) is the Hello message or Configuration Bridge Protocol
Data Unit (BPDU), which includes the following information:
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The bridge ID of the root bridge.
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The distance (or cost) from this switch to the root bridge.
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The bridge ID of the designated bridge on this LAN.
Hello messages are initiated at regular intervals by the root bridge and
propagate through the extended LAN.
Electing the Root Bridge and Designated Bridge
Each spanning tree has a root bridge, which initiates the propagation of Hello
messages through the extended LAN, and sets the values of parameters that
control the spanning tree computation process. The root bridge is the switch with
the lowest bridge ID and is elected by the exchange of Hello packets. When a
switch receives a Hello packet it compares the value of the root bridge ID in the
message to the value of the root bridge ID parameter in its own spanning tree
database. If the value in the message is better, the switch stores the new value in
its database and sends Hello messages with the new value out on its other ports.
Otherwise, the switch continues to send out Hello messages with the value
currently stored in its spanning tree database. By this process all switches in the
extended LAN will eventually learn the bridge ID of the root bridge.
Each LAN has a single switch, called the designated bridge, that logically
connects the LAN to the next LAN closer to the root bridge. The designated
bridge for a LAN is the switch on the LAN with the lowest root path cost and
bridge ID. The designated bridge is elected by the exchange of Hello messages,
in the same way that the root bridge is elected. The election of a new root
bridge, or a switch becoming unavailable due to a fatal error condition, will
normally result in the election of a new designated bridge in the next few
rounds of Hello messages.