Allied Telesis AT-S63 - Manual (Page 250)

Chapter 16: STP and RSTP
250 Section II: Advanced Features
STP and RSTP 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 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.
STP and RSTP 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.
RSTP is much faster. It can complete a convergence in seconds, and so
greatly diminish the possible impact the process can have on your
network.
Only one spanning tree can be active on the switch at a time. The default
is RSTP.
The STP implementation on the AT-S63 management software complies
with the IEEE 802.1d standard. 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.