Specifications

8-34 Rapier Switch Software Reference
Software Release 2.7.3
C613-03098-00 REV A
A spanning tree running in standard mode can take up to one minute to rebuild
after a topology or configuration change. The Rapid Spanning Tree algorithm
provides for a more rapid recovery of connectivity following the failure of a
bridge, bridge port, or a LAN.
RSTP provides rapid recovery by including port
roles in the computation of port states, and by allowing neighbouring bridges
to explicitly acknowledge signals on a point-to-point link that indicate that a
port wants to enter the forwarding mode.
In rapid mode, the rapid transition of a port to the forwarding state is possible
when the port is considered to be part of a Point-to-Point link, or when the port
is considered to be an Edge port. An edge port is a port that attaches to a LAN
that is known to have no other bridges attached.
To ensure that rapid transitions take place on an edge port, the port must be
explicitly configured with the set stp port= {port-list|all} edgeport=true
command.
Rapid Mode Spanning Tree Types
The RSTP algorithm has two types of operation: normal and stp compatible. If
normal is specified, the algorithm uses rapid port role transitions and transmits
and receives RST BPDUs. If STP compatible is specified, then rapid transitions
are disabled and RST BPDUs are discarded. The default is normal. Setting the
RSTP type to be STP compatible allows RSTP to support applications and
protocols that may be sensitive to frame duplication and misordering, for
example NetBeui.
Setting rstptype to normal, when normal has already been set, sets all ports to
the “sending RSTP” state. This is referred to in IEEE Standard 802.1w as
mCheck, and is useful for restoring full rapid mode operation when one or more
ports on the switch has entered the “sending STP” state. RSTP capable devices
operating with RSTP set to normal that receive the RST BPDUs enter the
“sending RSTP” state. After the mCheck operation, if an STP BPDU is received,
either as a result of a device operating in rapid mode with rstptype set to
stpcompatible, or as a result of a device operating in standard mode, the ports
that received the STP BPDUs reverts to the “sending STP” state.
mCheck is most effective on switches acting as designated bridges for LANs because they
regularly propagate BPDUs. Other bridges in the LAN do not transmit BPDUs as
frequently.
Spanning Tree and Rapid Spanning Tree Port
States
If STP is running in standard mode, then each port can be in one of five
spanning tree states, and one of two switch states. If STP is running in rapid
mode, then each port can be in one of four states. The state of a switch port is
taken into account by STP. To be involved in STP negotiations, STP must be
enabled on the switch, the port must be enabled on the switch, and enabled for
the STP it belongs to.
The Spanning Tree port states (Table 8-7 on page 8-35 and Table 8-8 on
page 8-35) affect the behaviour of ports whose switch state is enabled.