ETHERNET TROUBLESHOOTING

Why a Gigabit Ethernet Link Runs at 100 Mbps

A practical installer workflow for a device that should negotiate at 1 Gbps but links at Fast Ethernet speed.

Why a pair fault is a strong suspect

Cisco's Catalyst 1000 data sheet identifies 100BASE-TX as a two-pair interface and 1000BASE-T as a four-pair interface. A cabling fault affecting the additional pairs can therefore leave a usable Fast Ethernet path while preventing Gigabit Ethernet operation.

But do not assume the cable is always the cause

Also verify both endpoints actually support 1000BASE-T, that speed/duplex settings allow correct autonegotiation, and that the switch port has not been manually forced to 100 Mbps.

Installer sequence

Record the negotiated speed and duplex on both ends.
Check switch configuration and endpoint capability.
Replace the device patch lead with a known-good four-pair lead.
Replace the rack patch lead if present.
Run wiremap on the permanent link.
If wiremap passes but the problem remains, certify the link or inspect performance results.
Check interface errors after the link returns to 1 Gbps.

DIAGNOSTIC CLUE: a link that consistently falls to 100 Mbps after a cabling change should make the installer inspect all four pairs and both terminations, but endpoint configuration must also be checked.

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Technical references

These sources support the standards, test-method and manufacturer-specific statements on this page. Project specifications and current manufacturer documentation take precedence for a real installation.