RuggedCom RS400 manual STP vs. Rstp Costs, Bridge Diameter

Spanning Tree

when the designer is not too concerned with the resultant topology as long as connectivity is assured.

Manual configuration is useful when the exact topology of the network must be predictable under all circumstances. The path cost can be used to establish the topology of the network exactly as the designer intends.

STP vs. RSTP Costs

The IEEE 802.1D-1998 specification limits port costs to values of 1 to 65536. It recommends that a path cost corresponding to the 1x109 / link speed be used. Designed at a time when 9600 bps links were state of the art, this method breaks down in modern use, as the method cannot represent a link speed higher than a gigabit per second.

In order to remedy this problem in future applications the IEEE 802.1w specification limits port costs to values of 1 to 200000, with a path cost corresponding to the 2x1012 / link speed.

RuggedCom bridges support interoperability with legacy STP bridges by selecting the style to use. In practice it makes no difference which style is used as long as it is applied consistently across the network, or if costs are manually assigned.

5.1.5 Bridge Diameter

The bridge diameter is the maximum number of bridges between any two possible points of attachment of end stations to the network.

The bridge diameter reflects the realization that topology information requires time to propagate hop by hop through a network. If configuration messages take too long to propagate end to end through the network, the result will be result an unstable network.

There is a relationship between the bridge diameter and the maximum age parameter3. To achieve extended ring sizes, RuggedCom eRSTP™ uses an age increment of ¼ of a second. The value of the maximum bridge diameter is thus four times the configured maximum age parameter.

Note: Raise the value of the maximum age parameter if implementing very large bridged networks or rings.

3The RSTP algorithm is as follows. STP configuration messages contain “age” information. Messages transmitted by the root bridge have an age of 0. As each subsequent designated bridge transmits the configuration message it must increase the age by at least 1 second. When the age exceeds the value of the maximum age parameter the next bridge to receive the message immediately discards it.

RS400 137 ROS™ v3.5