If you ever worked in an environment where you have to deal with leased / dedicated lines provider by your SP (service provider) then you know that whenever it’s a problem on the line they request, if possible, to put a loop on the line from one end toward the other end, so they can do some measurements. Usually from my experience with SP, this is a standard approach in case that they don’t know exactly what problem is with your line or where the issue occurred, especially if they use sub-providers of their own.

Lately I saw some questions on Cisco support forums regarding the usage of software loops on Ethernet interfaces so I’ve decided to write a small how-to about the basic configuration of a soft loop on different interfaces.

Controller (E3, T3) soft loop on all channels

configure terminal
! Apply the loop on the controller interface to loop the entire (e.g.) T3 interface (all 28 x T1 channels)
controller t3 3/0
loopback [local | network | remote]

Mandatory parameter:

– loopback – place the loop

Optional:

local –  Loops the data back toward the router and sends an AIS signal out toward the network

network –  Loops the data toward the network at the T1 framer

remote – Sends a far-end alarm control (FEAC) request to the remote end requesting that it enter into a network line loopback. FEAC requests (and therefore remote loopbacks) are only possible when the T3 is configured for C-bit framing.

Controller (T3, E3) soft loop on one channel (T1, E1)

If your controller is channelized for T1, E1, you can avoid to loop the entire controller, but choose to apply the soft loop on only one channel:

configure terminal
! Apply the soft loop under interface configuration rather than controller
interface  Serial3/0:1
loopback [local | network {line | payload} | remote {line {fdl {ansi | bellcore} | inband} | payload [fdl] [ansi]}]

Mandatory:

loopback – applies the soft loop

Optional:

local –  Loops the router output data back toward the router at the T1 framer and sends an AIS signal out toward the network.

network – Loops the data back toward the network before the T1 framer and automatically sets a local loopback at the HDLC controllers (line) or loops the payload data back toward the network at the T1 framer and automatically sets a local loopback at the HDLC controllers (payload

remote line fdl –  Sends a repeating, 16-bit ESF data link code word; ansi—Places the CSU into loopback, per the ANSI T1.403 Specification; bellcore—Places the SmartJack into loopback, per the TR-TSY-000312 Specification

remote line inband –  Sends a repeating, 5-bit inband pattern (00001) to the remote end requesting that it enter into a network line loopback.

payload – Sends a repeating, 16-bit ESF data link code word to the remote end requesting that it enter into a network payload loopback. Enables the remote payload Facility Data Link (FDL) ANSI bit loopback on the T1 channel. Rarely it’s necessary to specify fdl or ansi keywords

To be honest I never used here more than local or network parameters. The other ones I add them here with explanation, but never use them.

Serial interfaces (PA-E3 or a PA-T3 port adapter)

configure terminal
! Apply the soft loop on the serial interface
interface Serial3/0
! If the interface is a port on a PA-E3
loopback [dte | local | network {line | payload}]
! If the interface is a port on a PA-T3
loopback [dte | local | network {line | payload} | remote]

Mandatory:

loopback – apply the soft loop

Optional:

dte – Sets the loopback after the LIU toward the terminal.

local – Sets the loopback after going through the framer toward the terminal.

network – Sets the loopback toward the network before going through the framer (line) or after going through the framer (payload).

remote (only T3) – Sends a far-end alarm control (FEAC) to set the remote framer in loopback.

Ethernet interfaces

configure terminal
! Apply the soft loop on a Ethernet interface
interface GigabitEthernet
loopback [driver | mac] 

Mandatory:

loopback – apply the loop

Optional (only on Gigabit Interfaces):

– driver – apply the loop at the transceiver level

– mac – apply the loop at the MAC controller level

You can use the loopback driver and loopback mac interface configuration commands with the 2-Port 10/100/1000 Gigabit Ethernet SPA. These commands do not apply to the 4-Port 10/100 Fast Ethernet SPA.To properly enable internal loopback, you must disable autonegotiation (under interface configuration, you have to apply no negotiation auto)
Due to different card/router models, IOS versions and specific SP configuration  not all the commands will fit exactly how described above, but at least this is a starting point to check when you need to enable a soft loop. If you are a beginner you may wonder why I’m calling it soft loop. This is because is a software loop, opposite to a hardware loop which implies that wires are physically looped.

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