Gigabit ethernet uses all 8 wires on the cable. If you have no or bad signal on one of them then it would explain the negotiation to 100Mbps. Do you have a cable tester?
this post was submitted on 13 Nov 2023
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Home Networking
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Best to test it and verify that it does so with other known 1gbps connections. If it always is 100mbps , then reterminate and test again. Most likely one of the wires didn't connect the you are only using 2 of the 4 pairs.
10/100Mb network connections use only 2 pairs, the Orange and Green pairs.
1Gb connections need all 4 pairs (8 wires).
If the termination failed, and it resulted in lack of connectivity with one or more wires belonging to the Blue or Brown pairs, then the switch would only negotiate a 100Mb link.
If you're crimping on plastic plugs - I recommend against it. They are prone to failure. Best to punch down to jacks, and then use patch cords from there.
You have a problem with the blue and/or brown pair in one or more of your terminations. Even if the wire is properly seated in the groove, look for nicks or breaks in the conductor where you stripped the outer jacket off, or maybe you accidently had the tool reversed when you punched the wire and it's cut on the inside, not the outside, or if you reused an end, check that the original wire didn't leave some of its plastic behind in the groove preventing a proper punch down.
Bad termination, either in cable alloxation, or to pins.
Re-punch.
You can even get a block fail, yep spent a whole day re-punching, fault tracing (can't justify a fluke or borrow one anymore from work), to find it was the damn socket!