Why does a gigabit port sometimes link up at only 100 Mbps, and how do the four pairs get used?
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Say this first
10BASE-T and 100BASE-TX use only two pairs: pins 1-2 and 3-6, one pair for each direction. 1000BASE-T uses all four pairs, each carrying traffic in both directions at once with echo cancellation, PAM-5 coding at 125 megasymbols per second, about 250 Mbps per pair. If pair 4-5 or 7-8 is broken or missing, the link cannot run at 1 Gbps, and many PHYs downshift to 100 Mbps. A gigabit port stuck at 100 is a cabling clue.
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Diagnosing it
SW1# show interfaces GigabitEthernet1/0/14 status
Port Name Status Vlan Duplex Speed Type
Gi1/0/14 Printer-2F connected 20 a-full a-100 10/100/1000BaseTXThe device supports gigabit, yet negotiation settled at a-100. Run the built-in cable test (it may bounce
the link on some platforms, so do it in a window):
SW1# test cable-diagnostics tdr interface GigabitEthernet1/0/14
SW1# show cable-diagnostics tdr interface GigabitEthernet1/0/14The TDR result shows each pair's status (Normal, Open, Short) and approximate distance to a fault.
Other causes
- A patch cord or wall plate wired with only two pairs.
- The far device only advertises 100 Mbps, or someone set
speed 100on one end. - An old Cat5 cable with poor crosstalk performance at gigabit.
They will ask next
Why does 1000BASE-T need autonegotiation even if you want to fix the speed?
The two PHYs must agree which one is the clock master and which is the slave, and that is resolved during autonegotiation. On many switches,
speed 1000on a copper port only limits what is advertised rather than turning negotiation off.
Traps
- Blaming the NIC driver first. A clean a-100 on a gigabit device usually points at the cable.