Lessons · Network switches · industrial ring topologies
Why the plant floor is wired in a ring
A ring gives every device two paths. One port is blocked so the ring is not a loop, and it is unblocked in milliseconds when a cable breaks.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
A stopped line costs more per minute than the extra cable costs in total. Ring recovery is fast enough that machinery does not fault, which spanning tree is not.
How to think about it
Read a ring as a line that can heal. In normal operation it IS a line, because one port is blocked; the ring only exists as a ring at the moment something breaks.
Worked example
enableThe # prompt.
configure terminalInto configuration mode.
interface gi0/1One of the two ring ports.
description Ring A to CELL8-SWSay which way it goes. In a ring, a mislabelled port is a fault nobody can trace.
switchport mode trunkThe ring carries the cell's VLANs between switches, so it is a trunk.
exitOut.
interface gi0/2The other ring port.
description Ring B to CELL6-SWLabelled the same way.
switchport mode trunkA trunk too. Between them the ring is closed.
endOut.
show interfaces statusTwo trunks and their descriptions. Which one is blocked is decided by the ring protocol, not by anything typed here.
Your turn
Make the second ring port carry the cell's VLANs.
switchport mode
Console into a switch
The trap
Closing a ring with no ring protocol and no spanning tree running. That is not redundancy, it is a loop, and it takes the whole network down in seconds.