Specifications

8-32 Rapier Switch Software Reference
Software Release 2.7.3
C613-03098-00 REV A
the frame is transmitted on the port specified by the entry in the forwarding
database.
Spanning Tree Protocol (STP)
The Spanning Tree Protocol (STP) automatically disables redundant paths in a
network to avoid loops, and enables them when a fault in the network means
they are needed to keep traffic flowing.
A sequence of LANs and switches may be connected together in an arbitrary
physical topology resulting in more than one path between any two switches.
If a loop exists, frames transmitted onto the extended LAN would circulate
around the loop indefinitely, decreasing the performance of the extended LAN.
On the other hand, multiple paths through the extended LAN provide the
opportunity for redundancy and backup in the event of a bridge experiencing a
fatal error condition. The spanning tree is created through the exchange of
Bridge Protocol Data Units (BPDUs) between the bridges in the LAN when
they start up, or when a change in the configuration of the network is detected.
The spanning tree algorithm ensures that the extended LAN contains no loops
and that all LANs are connected by:
■ Detecting the presence of loops and automatically computing a logical
loop-free portion of the topology, called a spanning tree. The topology is
dynamically pruned to a spanning tree by declaring the ports on a switch
redundant, and placing the ports into a ‘Blocking’ state.
■ Automatically recovering from a switch failure that would partition the
extended LAN by reconfiguring the spanning tree to use redundant paths,
if available.
The logical tree computed by the spanning tree algorithm has the following
properties:
■ A single switch, called the root bridge, forms a unique root to the tree. The
root bridge is the bridge with the lowest Bridge ID. Each switch in an
extended LAN is uniquely identified by its Bridge ID, which comprises the
switch’s root priority (a spanning tree parameter) and its MAC address.
■ Each switch or LAN in the tree, except the root bridge, has a unique parent,
known as the designated bridge. Each LAN has a single switch, called the
designated bridge, that logically connects the LAN to which the switch is
attached, to the next LAN closer to the root bridge.
■ Each port connecting a switch to a LAN has an associated cost. The root path
cost is the sum of the costs for each port between the switch and the root
bridge. The designated bridge for a LAN is the switch on the LAN with the
lowest root path cost, and therefore logically closer to the root bridge. If
two switches on the same LAN have the same lowest root path cost, the
switch with the lowest bridge ID is elected the designated bridge.
The spanning tree computation is a continuous, distributed process. The
algorithm uses the following process 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.