Allied Telesis AT-S62 - Manual (Page 456)

Chapter 23: STP and RSTP
Section IV: Spanning Tree Protocols 456
STP and RSTP Overview
The performance of a Ethernet network can be severely impaired by the
existence of a physical loop in the network topology. A loop exists when
two or more nodes on a network can transmit data to each other over
more than one traffic path. The problem that 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 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 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 physical 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.