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H3C Loopback Detection Feature Manual 
Release 2107 

Table of Contents

i 

Table of Contents 

Chapter 1 Loopback Detection Configuration............................................................................ 1-1

1.1 Loopback Detection Configuration..................................................................................... 1-1

1.1.1 Loopback Detection Overview ................................................................................ 1-1

1.1.2 Configuring Loopback Detection for Ethernet Port(s) ............................................. 1-2

Chapter 2 Loopback Detection Configuration Commands ....................................................... 2-1

2.1 Loopback Detection Configuration Commands ................................................................. 2-1

2.1.1 loopback-detection control enable .......................................................................... 2-1

2.1.2 loopback-detection enable ...................................................................................... 2-2

2.1.3 loopback-detection 

interface-list

 enable.................................................................. 2-4

2.1.4 loopback-detection interval-time ............................................................................. 2-5

2.1.5 loopback-detection per-vlan enable ........................................................................ 2-5

2.1.6 loopback-detection shutdown enable...................................................................... 2-6

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Chapter 1  Loopback Detection Configuration

1-1 

Chapter 1  Loopback Detection Configuration 

  Note: 

Two functions are added to release 2107: configuring loopback detection for a list of 

ports in bulk, and enabling auto-shutdown of loopback ports. 

1.1  Loopback Detection Configuration 

1.1.1  Loopback Detection Overview 

Loopback detection is used to monitor if a port of a switch is looped back. 

After you enable loopback detection on Ethernet ports, the switch can monitor if an 

external loopback occurs on them. If there is a loopback port found, the switch will deal 

with the loopback port according to your configuration.  

1) 

If a loop is found on an access port, the system will set the port to the 

block

 state 

(ports in this state cannot forward data packets), send log messages to the 

terminal, and remove the corresponding MAC forwarding entry.  

z

If you have additionally enabled the loopback port auto-shutdown function on the 

port, the system will shut down the port, and send log messages to the terminal. 

After the loop is removed, you need to use the 

undo shutdown

 command to bring 

up the port. 

z

If you have not enabled the loopback port auto-shutdown function on the port, the 

port will automatically resume the normal forwarding state after the loop is 

removed. 

2) 

If a loop is found on a trunk or hybrid port, the system sends log messages to the 

terminal. If you have additionally enabled the loopback port control function or the 

loopback port auto-shutdown function, the system will deal with the port 

accordingly: 

z

If the loopback port control function is enabled on the port, the system will set the 

port to the 

block

 state (ports in this state cannot forward data packets), send log 

messages to the terminal, and remove the corresponding MAC forwarding entry. 

After the loop is removed, the port will automatically resume the normal forwarding 

state. 

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z

If the loopback port auto-shutdown function is enabled on the port, the system will 

shut down the port and send log messages to the terminal. After the loop is 

removed, you need to use the 

undo shutdown

 command to bring up the port. 

  Note: 

You cannot enable both the loopback port control function and the loopback port 

auto-shutdown function on a port. If you do so, the function configured later will take 

effect. 

1.1.2  Configuring Loopback Detection for Ethernet Port(s) 

Table 1-1 

Configure loopback detection for Ethernet port(s) 

Operation 

Command 

Remarks 

Enter system view  

system-view 

— 

Enable loopback 
detection globally  

loopback-detection 
enable 

Optional 

By default, the global loopback 
detection function is enabled if 
the device boots with the default 
configuration file (config.def); if 
the device boots with null 
configuration, this function is 
disabled.  

Enable loopback 
detection on the 
specified ports in 
bulk 

loopback-detection 
interface-list enable 

Optional 

By default, the loopback 
detection function is enabled on 
ports if the device boots with the 
default configuration file 
(config.def); if the device boots 
with null configuration, this 
function is disabled. 

At present, you can configure the 
loopback detection for ports in 
bulk on the S3100-EI series 
(except the S3100-EPON-EI 
series) switches and the E126A 
switch only. These switches are 
all shipped with a default 
configuration file (config.def). 

Set the interval for 
performing port 
loopback detection  

loopback-detection 
interval-time time 

Optional 

The default is 30 seconds.  

Enter Ethernet port 
view  

interface

interface-type 

interface-number

— 

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Operation 

Command 

Remarks 

Enable loopback 
detection on a 
specified port  

loopback-detection 
enable 

Optional 

By default, the loopback 
detection function is enabled on 
ports if the device boots with the 
default configuration file 
(config.def); if the device boots 
with null configuration, this 
function is disabled. 

Enable loopback 
port control on the 
trunk or hybrid port  

loopback-detection 
control enable 

Optional 

By default, the loopback port 
control function is enabled on 
ports if the device boots with the 
default configuration file 
(config.def); if the device boots 
with null configuration, this 
function is disabled. 

Enable the loopback 
port auto-shutdown 
function 

loopback-detection 
shutdown enable 

Optional 

By default, the loopback port 
auto-shutdown function is 
enabled on ports if the device 
boots with the default 
configuration file (config.def); if 
the device boots with null 
configuration, this function is 
disabled. 

Configure the 
system to run 
loopback detection 
on all VLANs of the 
current trunk or 
hybrid port  

loopback-detection 
per-vlan enable 

Optional 

By default, the system runs 
loopback detection only on the 
default VLAN of the current trunk 
or hybrid port. 

  Caution: 

z

To enable loopback detection on a specific port, you must use the 

loopback-detection enable

 command in both system view and the specific port 

view.  

z

After you use the 

undo loopback-detection enable

 command in system view, 

loopback detection will be disabled on all ports.  

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Release 2107 

Chapter 2  Loopback Detection Configuration

 Commands

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Chapter 2  Loopback Detection Configuration 

Commands 

  Note: 

Two functions are added to release 2107: configuring loopback detection for a list of 

ports in bulk, and enabling auto-shutdown of loopback ports. For detailed description of 

the two functions, refer to loopback-detection 

interface-list

 enable and 

loopback-detection shutdown enable. 

2.1  Loopback Detection Configuration Commands 

2.1.1  loopback-detection control enable 

Syntax 

loopback-detection control enable 

undo loopback-detection control enable 

View 

Ethernet port view 

Parameter 

None 

Description 

Use the 

loopback-detection control enable

 command to enable the loopback port 

control function on the current trunk or hybrid port.  

Use the 

undo loopback-detection control enable

 command to disable the loopback 

port control function on the trunk or hybrid port. 

The loopback port control function works in conjunction with the loopback detection 

function (refer to loopback-detection enable). If a loop is found on a VLAN of the trunk 

or hybrid port: 

z

With the function enabled on the trunk or hybrid port, the system will set the port to 

the 

block

 state (ports in this state cannot forward data packets), send log 

messages to the terminal, and remove the corresponding MAC forwarding entry. 

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After the loop is removed, the port will automatically resume the normal forwarding 

state. 

z

With the function disabled on the trunk or hybrid port, the system will only send log 

messages to the terminal, and the port is still in the normal forwarding state. 

By default, the loopback port control function is enabled on ports if the device boots with 

the default configuration file (config.def); if the device boots with null configuration, this 

function is disabled. 

Note that this command is invalid for an access port.  

Related command: 

loopback-detection enable

.  

Example 

# Enable the loopback port control function on Ethernet 1/0/1.  

<H3C> system-view 

System View: return to User View with Ctrl+Z. 

[H3C] interface ethernet 1/0/1 

[H3C-Ethernet1/0/1] port link-type trunk 

[H3C-Ethernet1/0/1] loopback-detection control enable 

2.1.2  loopback-detection enable 

Syntax 

loopback-detection enable 

undo loopback-detection enable

View 

System view or Ethernet port view 

Parameter 

None 

Description 

Use the 

loopback-detection enable

 command to enable the loopback detection 

function on the current port to detect whether an external loopback occurs on the port.  

Use the 

undo loopback-detection enable

 command to disable the loopback 

detection function on the port.  

1) 

If a loop is found on an access port, the system will set the port to the 

block

 state 

(ports in this state cannot forward data packets), send log messages to the 

terminal, and remove the corresponding MAC forwarding entry.  

z

If you have enabled the loopback port auto-shutdown function (with the 

loopback-detection shutdown enable command) on the access port, the system 

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will shut down the port, and send log messages to the terminal. After the loop is 

removed, you need to use the 

undo shutdown

 command to bring up the port. 

z

If you have not enabled the loopback port auto-shutdown function on the access 

port, the port will automatically resume the normal forwarding state after the loop is 

removed. 

2) 

If a loop is found on a trunk or hybrid port, the system sends log messages to the 

terminal. If you have additionally enabled the loopback port control function or the 

loopback port auto-shutdown function, the system will deal with the port 

accordingly: 

z

If the loopback port control function is enabled on the port (with the 

loopback-detection control enable command), the system will set the port to the 

block

 state (ports in this state cannot forward data packets), send log messages 

to the terminal, and remove the corresponding MAC forwarding entry. After the 

loop is removed, the port will automatically resume the normal forwarding state. 

z

If the loopback port auto-shutdown function is enabled on the port (with the 

loopback-detection shutdown enable command), the system will shut down the 

port and send log messages to the terminal. After the loop is removed, you need to 

use the 

undo shutdown

 command to bring up the port. 

  Note: 

The loopback detection function on a specific port can take effect only after you enable 

the loopback detection function globally (in system view) and on the port (in the 

specified port view). 

By default, the loopback detection function is enabled globally and on ports if the device 

boots with the default configuration file (config.def); if the device boots with null 

configuration, this function is disabled. 

Related command: 

loopback-detection control enable

, 

loopback-detection 

shutdown enable

. 

Example 

# Enable the loopback detection function on Ethernet 1/0/1.  

<H3C> system-view 

System View: return to User View with Ctrl+Z. 

[H3C] loopback-detection enable 

[H3C] interface ethernet 1/0/1 

[H3C-Ethernet1/0/1] loopback-detection enable 

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2.1.3  loopback-detection interface-list enable 

Syntax 

loopback-detection interface-list enable 

undo loopback-detection interface-list enable

View 

System view 

Parameter 

interface-list

: Ethernet port list, in the form of 

interface-list 

= { 

interface-type 

interface-number

 [ 

to

interface-type interface-number

 ] } &<1-10>, where 

z

interface-type

 is the port type, and 

interface-number

 is the port number. 

z

Keyword 

to

 is used to specify a range of ports. The port number after 

to

 must be 

equal to or greater than that before 

to

. 

z

&<1-10> means that you can specify up to 10 ports or port ranges. 

Description 

Use the 

loopback-detection  interface-list enable

 command to enable the loopback 

detection function on a range of ports.  

Use the 

undo loopback-detection interface-list enable 

command to disable the 

loopback detection function on a range of ports.  

By default, the loopback detection function is enabled on ports if the device boots with 

the default configuration file (config.def); if the device boots with null configuration, this 

function is disabled. 

At present, you can configure the loopback detection for ports in bulk on the S3100-EI 

series (except the S3100-EPON-EI series) switches and the E126A switch only. These 

switches are all shipped with a default configuration file (config.def). 

Example 

# Enable the loopback detection function on ports Ethernet 1/0/1 through Ethernet 

1/0/4. 

<H3C> system-view 

System View: return to User View with Ctrl+Z. 

[H3C] loopback-detection enable 

[H3C] loopback-detection Ethernet 1/0/1 to Ethernet 1/0/4 enable 

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2.1.4  loopback-detection interval-time 

Syntax 

loopback-detection interval-time time 

undo loopback-detection interval-time 

View 

System view 

Parameter 

time

: Time interval for loopback detection, in the range of 5 to 300 (in seconds). It is 30 

seconds by default. 

Description 

Use the 

loopback-detection interval-time

 command to set time interval for loopback 

detection.  

Use the 

undo loopback-detection interval-time

 command to restore the default time 

interval.  

Example 

# Set time interval for loopback detection to 10 seconds. 

<H3C> system-view 

System View: return to User View with Ctrl+Z. 

[H3C] loopback-detection interval-time 10 

2.1.5  loopback-detection per-vlan enable 

Syntax 

loopback-detection per-vlan enable 

undo loopback-detection per-vlan enable 

View 

Ethernet port view 

Parameter 

None 

Description 

Use the 

loopback-detection per-vlan enable

 command to configure the system to 

run loopback detection on all VLANs of the current trunk or hybrid port.  

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Use the 

undo loopback-detection per-vlan enable

 command to restore the default 

setting.  

By default, the system runs loopback detection only on the default VLAN of the trunk or 

hybrid port.  

Note that the command is invalid for any access port. 

Example 

# Configure the system to run loopback detection on all VLANs of the trunk port 

Ethernet1/0/1. 

<H3C> system-view 

System View: return to User View with Ctrl+Z. 

[H3C] interface ethernet 1/0/1 

[H3C-Ethernet1/0/1] port link-type trunk 

[H3C-Ethernet1/0/1] loopback-detection per-vlan enable 

2.1.6  loopback-detection shutdown enable 

Syntax 

loopback-detection shutdown enable 

undo loopback-detection shutdown enable 

View 

Ethernet port view 

Parameter 

None 

Description 

Use the 

loopback-detection shutdown enable

 command to enable the loopback port 

auto-shutdown function. 

Use the 

undo loopback-detection shutdown enable

 command to disable the 

function.  

The loopback port auto-shutdown function works in conjunction with the loopback 

detection function (refer to loopback-detection enable). If a loop is found at a port: 

z

With the function enabled on the port, the system will shut down the port, and send 

log messages to the terminal. After the loop is removed, you need to use the 

undo 

shutdown

 command to bring up the port. 

z

With the function disabled on the port, the system will only send log messages to 

the terminal, and the port is still in the normal forwarding state. 

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 Commands

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By default, the loopback port auto-shutdown function is enabled on ports if the device 

boots with the default configuration file (config.def); if the device boots with null 

configuration, this function is disabled.  

  Note: 

You cannot enable both the loopback port control function (with the loopback-detection 

control enable command) and the loopback port auto-shutdown function on a port. If 

you do so, the function configured later will take effect. 

Related command: 

loopback-detection enable

, 

loopback-detection control 

enable

. 

Example 

# Enable the loopback port auto-shutdown function on port Ethernet 1/0/1.  

<H3C> system-view 

System View: return to User View with Ctrl+Z. 

[H3C] loopback-detection enable 

[H3C] interface ethernet 1/0/1 

[H3C-Ethernet1/0/1] loopback-detection shutdown enable