Chapter 9: Rapid Spanning Tree Protocol (RSTP)
108 Section I: Using the Menus Interface
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.
RSTP prevents data loops from forming by ensuring that only one path
exists between the end nodes in your network. Where multiple paths exist,
this protocol places the extra paths in a standby or blocking mode, leaving
only one main active path.
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.
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.
RSTP can complete a convergence in seconds, and so greatly diminishes
the possible impact the process can have on your network.
At this time, only RSTP is available on the switch.
The RSTP implementation complies with the IEEE 802.1w standard. The
following subsections provide a basic overview on how RSTP operates
and define the different parameters that you can adjust.

Bridge Priority

and the Root

Bridge

The first task that bridges perform when a spanning tree protocol is
activated on a network is the selection of a root bridge. A root bridge
distributes network topology information to the other network bridges and
is used by the other bridges to determine if there are redundant paths in
the network.
A root bridge is selected by the bridge priority number, and sometimes the
bridge’s MAC address, also referred to as the bridge identifier. The bridge
with the lowest bridge priority number in the network is selected as the
root bridge. If two or more bridges have the same bridge priority number,
of those bridges the one with the lowest MAC address is designated as
the root bridge.