Lessons · Network switches · duplex and speed
The link that works badly rather than failing
Auto-negotiation needs a partner. Fix one end and leave the other on auto, and the auto end falls back to half duplex: the link comes up and runs badly.
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
It is the hardest kind of fault, because nothing is down. Throughput is a fraction of what it should be, errors climb, and everything reports as connected.
How to think about it
Both ends auto, or both ends fixed to the same thing. Never one of each. `show interfaces status` prints `a-` in front of anything it negotiated, and that prefix is the diagnosis.
Worked example
enableThe # prompt.
configure terminalInto configuration mode.
interface gi0/3The suspect port.
duplex halfForced, the way somebody 'fixed' it once.
speed 100And forced again.
endOut.
show interfaces statusGi0/3 reads half and 100 with no a- in front. No prefix means forced, and forced against an auto partner is the fault.
configure terminalBack in.
interface gi0/3The same port.
no duplexBack to auto.
no speedAnd auto.
endOut.
show interfaces statusa-full and a-1000 now. The prefix says it negotiated, which is what both ends should be doing.
Your turn
Put the duplex setting back to automatic.
duplex
Console into a switch
The trap
Fixing one end to 'make it stable'. That guarantees the mismatch on the other end unless somebody remembers to go and fix it too, and nobody writes that down.