Alcatel-Lucent 6850-48 Guida Di Rete
Using RRSTP
Configuring Spanning Tree Parameters
page 13-38
OmniSwitch AOS Release 6 Network Configuration Guide
September 2009
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.
(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 direc-
tions. The MAC addresses corresponding to the ports in the ring are flushed to make the data path conver-
gence time much faster than the normal MSTP.
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 direc-
tions. The MAC addresses corresponding to the ports in the ring are flushed to make the data path conver-
gence 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.
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:
• There can be no alternate connections for the same instance between any two switches within an
RRSTP ring topology.
• 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.
without interruption.
• A single switch can participate in up to 128 RRSTP rings.