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RS400 137 ROS™ v3.5
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 1x10
9
/ 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 2x10
12
/ 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 parameter
3
. 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.
3
The 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.