User manual
21
wEb-bASED bROwSER MANAGEMENT
RSTP
The Gigabit Web-Smart Switch provides these RSTP-related features:
• Industry-standard support of the Rapid Spanning Tree Protocol, which features
a compatibility mode with legacy STP.
• Superior performance, as RSTP will recognize a link failure and put an alternate
port into forwarding mode within milliseconds.
• RSTP may be enabled on a per-port basis.
• Ports may be congured as edge ports, which allows rapid transitioning to the
forwarding state for non-STP hosts.
• Path costs may be hard-congured or determined by port speed negotiation, in
either the STP or RSTP style.
• Full bridge (historically, a device implementing STP on its ports has been referred
to as a bridge, and the device uses the terms bridge and switch synonymously)
and port status provide a rich set of tools for performance monitoring and
debugging.
RSTP 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 802.1D-recommended values, this period lasts 30 seconds.
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 topology information to be rapidly progagated
through the network. RSTP also offers a number of other signicant innovations:
• Topology changes in RSTP can be originated from and 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
r ecognizes one state — blocking — for ports that should not forward.
• RSTP bridges generate their own conguration messages, even if they fail to
receive one from the root bridge. This leads to quicker failure detection, but STP
relays conguration 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.