Allied Telesis AT-GS950/48, User Manual

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Chapter 4: STP and RSTP
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Overview
The performance of an 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 and RSTP prevent 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.
In addition, STP and RSTP can 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 . With the convergence process, 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.
With STP, convergence can take up to a minute or more 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.
RSTP is much faster. It can complete a convergence in seconds, and as
such, greatly diminish the possible impact the process can have on your
network. The STP implementation in the AT-S116 Management software
complies with the IEEE 802.1d standard.
Only one spanning tree at a time can be active on the switch. The default
protocol is RSTP. The RSTP implementation complies with the IEEE
802.1w standard.
The following subsections provide a basic overview on how STP and
RSTP operate and define the different parameters that you can adjust.