RuggedCom RS400 manual Rstp Operation, Spanning Tree

Spanning Tree

5 Spanning Tree

The RuggedSwitch™ family of Ethernet switches provide the latest in IEEE standard Spanning Tree functionality, including:

•Industry standard support of Rapid Spanning Tree (802.1D-2004), which features a compatibility mode with legacy STP (802.1D-1998)

•Industry standard support of Multiple Spanning Trees (802.1s and 802.1Q-2003), which is interoperable with both RSTP and legacy STP.

•RuggedCom RSTP feature enhancements (eRSTP™)

•Superior performance - RSTP will recognize a link failure and put an alternate port into forwarding within milliseconds.

•RSTP may be enabled on a per-port basis.

•Ports may be configured as edge ports, which allow rapid transitioning to the forwarding state for non-STP hosts.

•Path costs may be hard-configured or determined by port speed negotiation, in either the STP or RSTP style.

•Full bridge1 and port status provide a rich set of tools for performance monitoring and debugging.

•SNMP manageable including newRoot and topologyChange traps

5.1RSTP Operation

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 802.1D recommended values, this period lasts 30 seconds.

The Rapid Spanning Tree Protocol (IEEE 802.1w) was a further evolution of the 802.1D Spanning Tree Protocol. It replaced the settling period with an active handshake between bridges that guarantees the rapid propagation of topology information through the network. RSTP also offers a number of other significant innovations, including:

•Topology changes in RSTP can originate from and be acted upon by any designated bridges, leading to more rapid propagation of address information, unlike topology changes in STP, which must be passed to the root bridge before they can be propagated to the network.

•RSTP explicitly recognizes two blocking roles, alternate and backup port roles, including them in computations of when to learn and forward while STP recognizes one state, blocking, for ports that should not forward.

•RSTP bridges generate their own configuration messages, even if they fail to receive any from the root bridge. This leads to quicker failure detection. STP, by contrast, must relay 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.

1Historically, a device implementing STP on its ports has been referred to as a bridge. RuggedCom uses the term bridge and switch synonymously.

RS400 133 ROS™ v3.5