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If the Ethernet interface has a cable connected and is receiving a valid signal, the physical link shows an “up” status and there are no active alarms or defects. If the interface is not connected or is not receiving a valid signal, the output shows a physical link down status and an active LINK alarm and defect.

Source: http://kb.juniper.net/InfoCenter/index?page=content&id=KB26486

What is that 'valid signal'?

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Link Integrity Test pulses.

They are unipolar positive-only electrical pulses of a nominal duration of 100 ns, with a maximum pulse width of 200 ns, generated at a 16 ms time interval (with a timing variation tolerance of 8 ms).

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A device detects the failure of a link if neither a frame nor two of the LIT pulses is received for 50-150 ms.

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  • \$\begingroup\$ Thanks. On router fastethernet interface with status 'link=up, protocol=down' without any cable attached on the interface, how come 'link=up'? \$\endgroup\$
    – user49224
    Commented Aug 14, 2014 at 5:03
  • \$\begingroup\$ Sounds fishy. Maybe a broken MAC. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Aug 14, 2014 at 5:05

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