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Chapter 40: STP, RSTP and MSTP Protocols
562
Overview
The Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol
(RSTP) and Multiple Spanning Tree Protocol (MSTP) guard against the
formation of loops in an Ethernet network topology. A topology has a loop
when two or more nodes can transmit packets to each other over more
than one data path. The problem that data loops pose is that packets can
become caught in repeating cycles, referred to as broadcast storms, that
needlessly consume network bandwidth and that can significantly reduce
network performance.
Spanning tree prevents 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.
Spanning tree can also activate redundant paths if primary paths go 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 backup redundant paths.
One of the primary differences 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 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 and is the default spanning tree mode. It can
complete a convergence in seconds to greatly diminish the possible
impact the process can have on your network.
MSTP is similar to RSTP in its efficiency of convergence. It also allows
more than one instance of spanning tree to be active at a time. See
“Multiple Spanning Tree Protocol” on page 641 for more information about
how MSTP operates in an environment of multiple spanning tree
instances.
The STP implementation on the switch complies with the IEEE 802.1d
standard. The RSTP implementation complies with the IEEE 802.1w
standard. The MSTP feature complies with the IEEE 802.1s standard. The
following subsections provide an overview the basic features of STP,
RSTP and MSTP, and define the different parameters that you can adjust.