Spanning tree protocol (STP)

Overview

The spanning tree protocol (STP) is a standard Ethernet method for eliminating loops and providing alternate routes for service protection. Standard STP depends on information sharing among Ethernet switches/bridges to reconfigure the spanning tree in the event of a failure. The STP algorithm calculates the best loop-free path throughout the network.

STP defines a tree that spans all switches in the network; it e.g. uses the capacity of available bandwidth on a link (path cost) to find the optimum tree. It forces redundant links into a standby (blocked) state. If a link fails or if a STP path cost changes the STP algorithm reconfigures the spanning tree topology and may reestablish previously blocked links. The STP also determines one switch that will be the root switch; all leaves in the spanning tree extend from the root switch.

Maximum bridge diameter

The maximum bridge diameter is the maximum number of bridges between any two hosts on the bridged LAN for any spanning tree configuration.

For TransLAN^® applications the maximum bridge diameter is 25 nodes.

Spanning tree example

The following example network serves to illustrate the principle how a spanning tree is constructed.

Determination of the root

For every switch a priority can be configured. The switch priority is a number between 0 (highest priority) and 61440 (lowest priority) in steps of 4096. The switch with the highest priority will become root.

If there are two or more switches with the same highest priority, then the switch with the lowest number for the MAC address will become root. This rule ensures that there is always exactly one root, as MAC addresses are unique.

Determination of the root ports

Root ports are those ports that will be used to reach the root. For each switch the port with the lowest root path cost is chosen, where the root path cost is determined by adding the path costs to the root. In the example port 2b and 3b are root ports.

For every port a path cost value can be configured. For E/FE TransLAN^® cards, the default value of the path cost is determined by dividing 20,000,000,000 by the bandwidth in kbit/s. For GbE TransLAN^® cards, the path cost is a means to influence the active network topology.

Determination of the designated and blocked ports

The designated port is the one port that is going to be used for a certain LAN. In the example, there are 6 LANs.

The designated ports for LAN 1, LAN 2 and LAN 3 are the ports 1a, 2a and 3a respectively, because these LANs have only one connection to a switch. If there are more connections to a switch, then the port with the lowest root path cost is chosen. Thus the designated ports for LAN 4, LAN 5 and LAN 6 are the ports 1b, 1c and 3c respectively.

Ports that are neither root ports nor designated ports are blocked. In the example port 2c is a blocked port.

Thus the loop free spanning tree is constructed.

Rapid spanning tree protocol (rSTP)

The rapid spanning tree protocol reduces the time that the standard spanning tree protocol needs to reconfigure after network failures. Instead of several tens of seconds, rSTP can reconfigure in less than a second. The actual reconfiguration time depends on several parameters, the two most prominent are the network size and complexity. IEEE802.1w describes the standard implementation for rSTP.

Specific attributes for TransLAN^® STP enhancements:

• Failure Detection - Use SDH-layer failure detection to trigger STP reconfiguration.

• Convergence Time - Key aspects of the message-based IEEE 802.1w/D10 (rSTP) protocol instead of timer-based 802.1D (STP) protocol.

• Support larger network diameter by adjusting the “Maximum Age Timer” parameter and enhanced STP configuration controls and reports.

• Automatic mode detection - The rSTP is supported as an enhancement to STP, it cannot be enabled explicitly. It rather will operate by default and will fall back to STP as soon as it finds peer nodes that do not support rSTP. The STP mode that the bridge elected can be retrieved per port.

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