I have 4 ports.

3 are like this, Gi 0/1-3

switchport access vlan xxx
switchport mode access
power inline never
speed 100
spanning-tree portfast edge

One Trunk, Gi 0/8

switchport mode trunk
power inline never

Gi 0/2 keep disconnecting / reconnecting with tons of errors codes %LINEPROTO-5 UPDOWN and %LINK-3-UPDOWN if I remove the line speed 100

Why ?

  • 1
    What is in the log? Also, what is plugged into the port? Does it happen if you move devices around?
    – Ron Trunk
    May 17, 2021 at 16:44
  • These 2 lines are the only lines in the log. They keep coming if I used 1000 instead of 100. If I change port, the new port create the same error. Gi 0/1 is computer with VNC Server, Gi 0/1 and 0/2 are both VNC client
    – PowerCat
    May 18, 2021 at 15:22
  • Most likely, @Zac67 has the correct answer -- it's a cable problem
    – Ron Trunk
    May 18, 2021 at 16:30
  • 1
    On a side note (now that the actual problem is solved): If ever, on this and similar product families by Cisco, I suggest to use speed auto 100 10 instead of speed 100. It keeps speed & duplex negotiation intact while only offering 100 or 10 Mbits/sec. Jul 22, 2021 at 14:49

1 Answer 1


The speed 100 line configures the interface down to 100BASE-TX. If that works correctly and 1000BASE-T doesn't (the default, fully autonegotiated mode), the likely cause is a cable fault.

100BASE-TX and 10BASE-T use only two pairs of the four-pair cable. These pairs are also the ones used for autonegotiation. So, if one of the other pairs is at fault (or both), 1000BASE-T is still autonegotiated, but it can't complete the link due to the PCS sublayer failing to 'up' each lane/pair separately. This causes the port to fall back to autonegotiation, ad nauseam.

Some devices fall back to 100BASE-TX in such a case but that is not a given. Try a known-good cable.

  • We're going to change all the cable, I will make a feedback once it's done. Thank you !
    – PowerCat
    May 19, 2021 at 11:57
  • And you were right. The cable were dead !
    – PowerCat
    Jul 22, 2021 at 12:29

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