25. Spanning Tree
ROX™ v2.2 User Guide
231
RuggedBackbone™ RX5000
25. Spanning Tree
ROX™ provides 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 (802.1Q-2005), 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 bridge and port status displays provide a rich set of tools for performance monitoring and
debugging.
Historically, a device implementing STP on its ports has been referred to as a bridge.
RuggedCom uses the terms "bridge" and "switch" synonymously.
• SNMP-manageable including newRoot and topologyChange traps.
25.1. RSTP Operation
The 802.1D Spanning Tree Protocol (STP) was developed to enable 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 halt after a link outage until all bridges in the network are
guaranteed to be aware of the new topology. Using the values recommended by 802.1D, this period
lasts 30 seconds.
The Rapid Spanning Tree Protocol (RSTP, 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 throughout 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 - which are included in
computations of when to learn and forward. STP, however, recognizes only 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.
• 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.
While providing much better performance than STP, IEEE 802.1w RSTP still required up to several
seconds to restore network connectivity when a topology change occurred.
