MNS-BB
Software User Guide
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14.0
Rapid Spanning Tree Protocol (RSTP)
Rapid Spanning Tree Protocol (RSTP) (IEEE 802.1w) is an evolution of the Spanning Tree Protocol
(STP) (802.1D standard) and provides for faster spanning tree convergence after a topology change.
Use spanning tree protocol to ensure that only one active path at a time exists between any two nodes
on the network. In networks where there is more than one physical, active path between any two
nodes, enabling spanning tree ensures a single active path between such nodes by blocking all
redundant paths. Without spanning tree, having more than one active path between a pair of nodes
causes loops in the network, which can result in duplication of messages, leading to a “broadcast
storm” that can bring down the network.
Note
: User should enable spanning tree operation in any switch that is part of a redundant physical
link (loop topology). (It is recommended that you do so on all switches belonging to a loop topology.)
This topic is covered in more detail under
chapter
“Spanning Tree Protocol”.
14.1
How Spanning Tree Operates
The switch automatically senses port identity and type, and automatically defines spanning-tree
parameters for each type, as well as parameters that apply across the switch. You can use the default
values for these parameters, or adjust them as needed.
While allowing only one active path through a network at any time, spanning tree retains any
redundant physical path to serve as a redundant (blocked) path in case the existing active path fails.
Thus, if an active path fails, spanning tree automatically activates (unblocks) an available redundant
to serve as the new active path for as long as the original active path is down.
In the factory default, spanning tree operation is disabled. If a redundant link (loop) exists between
nodes in your network, user should enable the spanning tree operation of their choice.
14.3 RSTP
Concepts
The 802.1d Spanning Tree Protocol was developed to allow the construction of robust networks that
incorporate redundancy while pruning the active topology of the network to prevent loops. While
STP is effective, it requires that frame transfer must halt after a link outage until all bridges in the
network are sure to be aware of the new topology. Using the Spanning Tree Protocol (IEEE 802.1d)
recommended values, this period lasts 30 seconds.
Rapid Spanning Tree Protocol (IEEE 802.1w) is a further evolution of the 802.1d Spanning Tree
Protocol. It replaces the settling period with an active handshake between bridges that guarantees
topology information to be rapidly propagated through the network. RSTP converges in less than one
second. RSTP also offers a number of other significant innovations, including:
• Topology changes in STP must be passed to the root bridge before they can be propagated to the
network. Topology changes in RSTP can be originated from and acted upon by any designated
bridges, leading to more rapid propagation of address information.
• STP recognizes one state, blocking, for ports that should not forward. RSTP explicitly recognizes
two blocking roles, alternate and backup port roles, including them in computations of when to learn
and forward.
• STP relays configuration messages received on the root port out its designated ports. If an STP
bridge fails to receive a message from its neighbor it cannot be sure where along the path to the root a
failure occurred. RSTP bridges generate their own configuration messages, even if they fail to receive
one from the root bridge. This leads to quicker failure detection.
• RSTP offers edge port recognition, allowing ports at the edge of the network to forward frames
immediately after activation while at the same time protecting them against loops.
• An improvement to age configuration messages more quickly to prevent them from “going around
in circles” in the event of a loop.
There are three RSTP states:
Discarding, Learning
and
Forwarding
.
The discarding state is entered when the port is first taken into service. The port does not learn
addresses in this state and does not participate in frame transfer. The port looks for STP traffic in
order to determine its role in the network. When it is determined that the port will play an active part
in the network, the state will change to Learning. The learning state is entered when the port is
preparing to play an active member of the network. The port learns addresses in this state but does not
