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To use loopback testing to isolate Fast Ethernet and Gigabit Ethernet interface problems.

Table 1 provides links and commands for using loopback testing for Fast Ethernet and Gigabit Ethernet interfaces.

Table 1: Checklist for Using Loopback Testing for Fast Ethernet and Gigabit Ethernet Interfaces

Tasks	Command or Action
Diagnose a Suspected Hardware Problem with a Fast Ethernet or Gigabit Ethernet Interface
Create a Loopback
Create a Physical Loopback for a Fiber-Optic Interface	Connect the transmit port to the receive port.
Create a Loopback Plug for an RJ-45 Ethernet Interface	Cross pin 1 (TX+) and pin 3 (RX+) together, and pin 2 (TX-) and pin 6 (RX-) together.
Configure a Local Loopback	[edit interfaces interface-name (fastether-options \| gigether-options)] set loopback show commit
Verify That the Fast Ethernet or Gigabit Ethernet Interface Is Up	show interfaces (fe-fpc/pic/port \| ge-fpc/pic/port)
Configure a Static Address Resolution Protocol Table Entry	show interfaces ge-fpc/pic/port [edit interfaces interface-name unit logical-unit-number family inet address address] set arp ip-address mac mac-address show commit run show arp no-resolve
Clear Fast Ethernet or Gigabit Ethernet Interface Statistics	clear interfaces statistics fe-fpc/pic/port \| ge-fpc/pic/port
Ping the Fast Ethernet or Gigabit Ethernet Interface	ping remote-IP-address bypass-routing interface (fe-fpc/pic/port \| ge-fpc/pic/port count 100 rapid
Check for Fast Ethernet or Gigabit Ethernet Interface Error Statistics	show interfaces (fe-fpc/pic/port \| ge-fpc/pic/port ) extensive
Diagnose a Suspected Circuit Problem	Perform Steps 2 through 8 from Diagnose a Suspected Hardware Problem with a Fast Ethernet or Gigabit Ethernet Interface.