Rapid Spanning Tree Networks

The most common redundant network topology with managed switches is to form a ring or loop. Typically, the managed switches are connected in series and the last switch is then connected back to the first. In this scheme the RSTP will "block" one of the connections between the switches and maintain it as a backup pathway. If a switch goes down or a connection is lost the managed switch will automatically and quickly re-configure to use the backup pathway(s).

RSTP allows for multiple loops to be formed and interconnected. A common application for this is when you need to connect to managed switches in a remote location of your site. This gives you both local loop redundancy and redundant connections between your loops.

One of the more advanced levels of network redundancy is to have two independent Ethernet networks where each is connected in a loop. This topology is ideal when you have advanced devices with dual Ethernet ports such as the SixTRAK IPm DCS Controller or EtherTRAK I/O Gateway. Use this scheme in combination with redundant controllers, redundant I/O and redundant supervisory HMI/SCADA computers to get the highest level of system redundancy.

In this scheme every "designated" (non-root) switch has a direct connection back to the "root" switch. In addition, there are connections between the "designated" switches creating redundant loops. The advantage to this topology over just a simple loop is that it affords the maximum bandwidth because of the direct connection to the "root". The loop connections are only used as backups. This scheme is tolerant of multiple switch or connection failures.

The Rapid Spanning Tree Protocol may seem complicated but is actually simple to setup and use. In most cases, where all you want is a simple ring of switches, all you have to do is enable RSTP and the rest is plug-and-play. You can leave all the RSTP parameters at their factory defaults and the switch will automatically determine the best primary and backup pathways. For the best RSTP performance here are some basic guidelines to follow:

• Redundancy protocol - Choose the Rapid Spanning Tree Protocol (RSTP) for most applications because it offers the fastest recovery times and is fully compatible with the original Spanning Tree Protocol (STP). The SIXNET managed switch with firmware v2.6 or newer is based on the latest IEEE 802.1D-2004 standard for RSTP and STP.

• Typical recovery time - The SIXNET Managed Switches (firmware v3.1 or higher), with RSTP enabled, typically provide less than 50 mS per hop recovery times when there is a link-loss failure anywhere in the network. A typical ring network of ten managed switches will provide a recovery time of <500 mS.

• Number of switches in a ring - Based on the IEEE 802.1D-2004 standard the maximum number of "hops" in a ring topology is 40. A hop is defined as a link between two switches. This allows for up to 40 switches in a ring but better performance is achieved with less switches.

• Bridge priority - This along with the MAC address determines which managed switch is considered the "root bridge". For simple ring networks (which are most common) you can leave all the switches with the default bridge priority and they will automatically decide which should be the "root" based on the MAC address. You only need to change this value in advanced topologies where you want to force a switch to be the root bridge.

• Port priority - If a switch has redundant direct connections to the root bridge then the port priority determines which will be the primary and backup connections. In most cases you should use the default setting (which is the same for all ports) and let the switch automatically determine which port to make active (port with lowest port number) and which to block.

• Maximum age, hello time, forward delay & transmission limit - These parameters define various aspects of how the RSTP operates. It is recommended that you leave these parameters at the default values for all the managed switches in your network.

• Path cost - This parameter helps the switch decide which port offers the best path to the root bridge. It is recommended that you leave it at the default setting of Auto which will let the switch automatically make the determination.

• Point-to-point MAC - This parameter tells the switch that a port has a direct (one-one-one) connection to another switch which allows it to start forwarding traffic immediately. It is recommended that you leave it at the default setting of Auto which will let the switch automatically make the determination.

• Edge port - If you know a port is directly connected to an end device (that doesn't support RSTP) then set it as an edge port to ensure maximum performance. This will tell the switch to immediately start forwarding traffic on the port and not bother trying to establish a RSTP connection.