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RUGGEDCOM ROX II
CLI User Guide
STP vs. RSTP Costs
The STP specification limits port costs to values of 1 to 65536. 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 10 Gbit/s.
To remedy this problem in future applications, the RSTP specification limits port costs to values of 1 to 20000000,
and a link speed up to 10 Tbit/s can be represented with a value of 2.
Section 5.36.1.5
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.
NOTE
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.
To achieve extended ring sizes, Siemens's 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.
IMPORTANT!
Raise the value of the maximum age parameter if implementing very large bridged networks or rings.
Section 5.36.1.6
eRSTP
Siemens's enhanced Rapid Spanning Tree Protocol (eRSTP) improves the performance of RSTP in two ways:
• Improves the fault recovery time performance (< 5 ms per hop)
• Improves performance for large ring network topologies (up to 160 switches)
eRSTP is also compatible with standard RSTP for interoperability with commercial switches.
For example, in a network comprised of 15 RUGGEDCOM hardened Ethernet switches in a ring topology, the
expected fault recovery time would be less than 75 ms (i.e. 5 ms x 15). However, with eRSTP, the worst case
fault recovery time is less than 26 ms.
Bridge Diameter
Chapter 5
Setup and Configuration
517
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Troubleshooting
