Configuring Spanning Tree Parameters Using RRSTP
OmniSwitch 6800/6850/9000 Network Configuration Guide March 2008 page 7-39
Using RRSTP
The Ring Rapid Spanning Tree Protocol (RRSTP) is complimentary to both the Spanning Tree Protocol
(STP) as well as the Multiple Spanning Tree Protocol (MSTP). It is designed to provide faster conver-
gence time when switches are connected point to point in a ring topology. RRSTP can only be configured
on an OmniSwitch running in flat mode.
RRSTP reduces convergence time by finding the bridge that hosts the alternate (ALT) port and immedi-
ately changing the ALT port state to forwarding without altering the MSTP port state. This process
quickly enables the data path. The RRSTP frame travels from the point of failure to the ALT port in both
directions. The MAC addresses corresponding to the ports in the ring are flushed to make the data path
convergence time much faster than the normal MSTP.
While RRSTP is already reacting to the loss of connectivity, the standard MSTP BPDU carrying the link
down information is processed in normal fashion at each hop. When this MSTP BPDU reaches the bridge
whose ALT port is now in the "ALT FWD" state, due to RRSTP frame processing, it updates the MSTP
state of the two ports in the ring as per the MSTP standard.
The following limitations should be noted when using RRSTP:
• A port on a bridge can only be part of one RRSTP ring at any given instance.
• All bridges, which need to be made part of a ring, can be configured only statically.
• Fast convergence will not occur if an RRSTP frame is lost. However, MSTP convergence will still take
place at a later time because there is no way of knowing about the RRSTP frame loss.
• RRSTP convergence may not happen when changes in configuration result in an unstable topology.
• If either of the two ports of the RRSTP ring on a bridge goes down or if one of the bridges in the ring
goes down, the RRSTP convergence may not happen. However, MSTP convergence will continue
without interruption.
• Maximum of 128 rings can participate on a switch.
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