User guide

There are several types of loopback testing supported by the Junos OS, as follows:

•

DCE local—Loops packets back on the local DCE.

•

DCE remote—Loops packets back on the remote DCE.

•

Local—Useful for troubleshooting physical PIC errors. Configuring local loopback on

an interface allows transmission of packets to the channel service unit (CSU) and then

to the circuit toward the far-end device. The interface receives its own transmission,

which includes data and timing information, on the local router's PIC. The data received

from the CSU is ignored. To test a local loopback, issue the show interfaces

interface-name command. If PPP keepalives transmitted on the interface are received

by the PIC, the Device Flags field contains the output Loop-Detected.

•

Payload—Useful for troubleshooting the physical circuit problems between the local

router and the remote router. A payload loopback loops data only (without clocking

information) on the remote router’s PIC. With payload loopback, overhead is

recalculated.

•

Remote—Useful for troubleshooting the physical circuit problems between the local

router and the remote router. A remote loopback loops packets, including both data

and timing information, back on the remote router’s interface card. A router at one end

of the circuit initiates a remote loopback toward its remote partner. When you configure

a remote loopback, the packets received from the physical circuit and CSU are received

by the interface. Those packets are then retransmitted by the PIC back toward the

CSU and the circuit. This loopback tests all the intermediate transmission segments.

Table 4 on page 86 shows the loopback modes supported on the various interface types.

Table 4: Loopback Modes by Interface Type

Usage GuidelinesLoopback ModesInterface

Configuring Ethernet Loopback CapabilityLocalAggregated Ethernet,

Fast Ethernet, Gigabit

Ethernet

“Configuring E1 Loopback Capability” on page 10Local and remoteCircuit Emulation E1

Configuring T1 Loopback CapabilityLocal and remoteCircuit Emulation T1

“Configuring E1 Loopback Capability” on page 10

and Configuring E3 Loopback Capability

Local and remoteE1 and E3

Configuring Channelized E1 IQ and IQE Interfaces,

Configuring T1 and NxDS0 Interfaces, Configuring

Channelized OC12/STM4 IQ and IQE Interfaces

(SONET Mode), Configuring Channelized STM1

IQ and IQE Interfaces, and Configuring

Channelized T3 IQ Interfaces

PayloadNxDS0

Configuring Serial Loopback CapabilityLocal and remoteSerial (V.35 and X.21)

E1 Interfaces Feature Guide for Routing Devices