Allied Telesis AT-S39 - Manual (Page 97)

AT-S39 User’s Guide
97
STP and RSTP Overview
A significant danger to Ethernet network performance is the existence of
a data loop in a 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 link. 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 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 commonly 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 intercommunications between the various network segments.
This process is referred to as convergence.
With STP, convergence for a large network can take up to a minute to
complete. This can result in lost data packets and the loss of
intercommunication between various parts of the network during the
convergence process.
RSTP is much faster. RSTP can complete a convergence in seconds, and
so greatly diminish the possible impact the process can have on your
network.
Note
RSTP is activated by default on the switch.
The STP implementation on the AT-8000 Series Switch 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.