25. Spanning Tree
ROX™ v2.2 User Guide
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RuggedBackbone™ RX5000
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.
25.1.5. 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 an unstable network.
There is a relationship between the bridge diameter and the maximum age parameter
2
. 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.
Raise the value of the maximum age parameter if implementing very large bridged
networks or rings.
25.2. MSTP Operation
The Multiple Spanning Tree (MST) algorithm and protocol provide greater control and flexibility than
RSTP and legacy STP. MSTP (Multiple Spanning Tree Protocol) is an extension of RSTP, whereby
multiple spanning trees may be maintained on the same bridged network. Data traffic is allocated to
one or another of several spanning trees by mapping one or more VLANs onto the network.
The sophistication and utility of the Multiple Spanning Tree implementation on a given
bridged network is proportional to the amount of planning and design invested in
configuring MSTP.
If MSTP is activated on some or all of the bridges in a network with no additional configuration, the
result will be a fully and simply connected network, but at best, the result will be the same as a network
using only RSTP. Taking full advantage of the features offered by MSTP requires a potentially large
number of configuration variables to be derived from an analysis of data traffic on the bridged network,
and from requirements for load sharing, redundancy, and path optimization. Once these parameters
have all been derived, it is also critical that they are consistently applied and managed across all bridges
in an MST region.
2
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.
