STRIDE SE-MC2U-SC, User Manual

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Redundancy Settings
Another benefit of using managed switches over unmanaged switches is their redundancy
capabilities. This allows you to have an Ethernet network with extra connections, so if one
path between two points on the network fails, another path can be used to deliver messages.
If one link or switch fails, another link or switch can take over transparently to prevent
unnecessary down time. So why not just physically connect each of the switches in your
network in various loop configurations such that there are always at least two paths going to
and from each switch? That would create a broadcast loop that will bring a network to its
knees very quickly.
In an unmanaged Ethernet network there can be only one path between any two ports on the
network. If there is more than one path from one switch to another a broadcast message (and
in some cases other messages) sent by the network will be forwarded until it completes a loop
by returning on the second path. Since the switches forward all broadcasts and do not keep
track of the messages they have sent, the returning message will be sent around the loop again
and again. A single message circulating forever around a loop at high speed is clearly not a
good thing, so no loops are allowed.
The limitations of having only one path are even simpler to see. If the one and only path
fails for any reason, such as a broken cable or power failure at one of the switches, there are
no paths left and no network traffic can get through. We need a way to add alternate paths
without creating loops. A redundancy protocol such as RSTP, a loop prevention protocol, is
used such that switches can communicate with each other to discover and prevent loops.
There are four methods of accomplishing redundancy in the Stride managed switches:
•
Spanning Tree Protocol (STP)
• Rapid Spanning Tree Protocol (RSTP)
• Multiple Spanning Tree Protocol (MSTP)
• Real-Time Ring
The Spanning Tree Protocols (STP, RSTP and MSTP) are an industry standard and are thus
compatible with other manufacturer’s managed switches for situations where both need to
coexist and communicate. The recovery time, however, is slower with the Spanning Tree
Protocols than with the proprietary Real-Time Ring protocol. The merits of both will be
discussed in more detail below.
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Chapter 4 - Managed Switch Software Setup
Stride Industrial Ethernet Switches User Manual 2nd Ed. Rev. A