Rstp (Ieee 802.1D-2004) And Daisy Chain; Rstp Description; Rstp Concepts - GE F650 User Manual

B.2 RSTP (IEEE 802.1D-2004) AND DAISY CHAIN
B.2 RSTP (IEEE 802.1D-2004) and daisy chain
B.2.1 RSTP description
The Rapid 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.
B.2.2 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 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. The 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. 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. RSTP
switches (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 in RSTP allows configuration messages to age more quickly preventing them from "going around in
circles" in the event of a loop RSTP has three states. They are 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 plays an active part in the network, the state changes 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
participate in frame transfer. In a network of RSTP switches (bridges) the time spent in this state is usually quite short. RSTP
switches (bridges) operating in STP compatibility mode spend between 6 to 40 seconds in this state. After 'learning' the
bridge places the port in the forwarding state. While in this state the port both learn addresses and participates in frame
transfer while in this state. The result of these enhanced states is that the IEEE 802.1d version of spanning tree (STP) can
take a fairly long time to resolve all the possible paths and to select the most efficient path through the network. The IEEE
802.1w Rapid reconfiguration of Spanning Tree significantly reduces the amount of time it takes to establish the network
path. The result is reduced network downtime and improved network robustness. In addition to faster network
reconfiguration, RSTP also implements greater ranges for port path costs to accommodate the higher connection speeds
that are being implemented.
Proper implementations of RSTP (by switch vendors) is designed to be compatible with IEEE 802.1d STP. GE recommends
that you employ RSTP or STP in your network.
B-6 F650 DIGITAL BAY CONTROLLER
APPENDIX B:
GEK-113000-AF