Allied Telesis AT-S87 - Manual (Page 94)

Chapter 10: STP and RSTP
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STP Overview
The performance of a Ethernet network can be negatively impacted by the
formation of a data loop in the network topology. A data loop exists when
two or more nodes on a network can transmit data to each other over
more than one data path. The problem that data loops pose is that data
packets can become caught in repeating cycles, referred to as broadcast
storms, that needlessly consume network bandwidth and can significantly
reduce network performance.
STP prevents data loops from forming by ensuring that only one path
exists between the end nodes in your network. Where multiple paths exist,
these protocols place the extra paths in a standby or blocking mode,
leaving only one main active path.
STP can also activate a redundant path if the main path goes down. So
not only do these protocols guard against multiple links between segments
and the risk of broadcast storms, but they can also maintain network
connectivity by activating a backup redundant path in case a main link
fails.
Where the two protocols differ is in the time each takes to complete the
process referred to as convergence. When a change is made to the
network topology, such as the addition of a new bridge, a spanning tree
protocol must determine whether there are redundant paths that must be
blocked to prevent data loops, or activated to maintain communications
between the various network segments. This is the process of
convergence.
With STP, convergence can take up to a minute to complete in a large
network. This can result in the loss of communication between various
parts of the network during the convergence process, and the subsequent
lost of data packets.
The STP implementation on the AT-S87 management software complies
with the IEEE 802.1d standard.
Bridge Priority
and the Root
Bridge
The first task that bridges perform when a spanning tree protocol is
activated on a network is the selection of a root bridge. A root bridge
distributes network topology information to the other network bridges and
is used by the other bridges to determine if there are redundant paths in
the network.
A root bridge is selected by the bridge priority number, also referred to as
the bridge identifier, and sometimes the bridge’s MAC address. The bridge
with the lowest bridge priority number in the network is selected as the
root bridge. If two or more bridges have the same bridge priority number,
of those bridges the one with the lowest MAC address is designated as
the root bridge.