Filter
Top Products
Chapter
11
11 – Rapid Spanning Tree Protocol
(RSTP)
Create and manage alternate paths to the network
apid Spanning Tree Protocol (RTSP), like STP, was designed to avoid loops in an Ethernet
network. 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.
R
RSTP concepts
The IEEE 802.1d Spanning Tree Protocol (STP) 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. This halt is until all bridges in the network are sure to be aware of the new
topology. Using STP (IEEE 802.1d) recommended values, this period lasts 30 seconds.
j
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
switches (bridges) that guarantees topology information to be rapidly propagated through
the network. RSTP converges in less than one second to six seconds. RSTP also offers a
number of other significant innovations. These include
• 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 switch (bridge), leading to more rapid
propagation of address information
• STP recognizes one state - blocking for ports that should not forward any data or
information. RSTP explicitly recognizes two states or blocking roles - alternate and
backup port including them in computations of when to learn and forward and
when to block
• STP relays configuration messages received on the root port going out of its
designated ports. If an STP switch (bridge) fails to receive a message from its
neighbor it cannot be sure where along the path to the root a failure occurred.
135