Allied Telesis AT-S62 - Manual (Page 330)

Chapter 18: STP and RSTP
Section IV: Spanning Tree Protocols 330
STP and RSTP Overview
The performance of a Ethernet network can be severely impaired by the
existence 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
Ethernet 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. They maintain network connectivity by activating a backup
redundant path in the event a main link fails or is taken off-line.
The principal different between the two protocols lies 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 network traffic.
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.
The AT-S62 management software features both spanning tree
protocols. Only one spanning tree protocol can be active on a switch at a
time. The default active spanning tree is RSTP.
The STP implementation on the AT-S62 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.