Cisco Cisco IOS Software Release 12.0(19)S

To test the interface, use the loopback interface configuration command. 

Router(config)# clock redundancy mode manual internal

or

Router(config)# clock redundancy mode auto [revertive 

| non-revertive]

Option 1: Sets the clock mode in manual mode to 
use an internal source for the system clock. This is 
the default setting for the system clock.

or

Option 2: Sets the clock mode in auto selection 
mode. Indicate if the auto mode should be 
revertive or non-revertive:

revertive: after a clock source failure, the 
system will revert back to a higher-priority 
source if the (previously failed) higher 
priority clock source has recovered.

non-revertive: will not revert to a higher 
priority clock source. The next available clock 
source in descending order will be chosen.

The default clock redundancy mode is:
clock redundancy mode manual internal

While a card is in “auto non-revertive” 
mode, the system clock can be manually 
switched to the highest priority clock 
currently available with the command: 
sysclock switch slot.

Router(config)# end

Exits configuration mode.

Router# copy running-config startup-config

To write the new configuration to nonvolatile 
random access memory (NVRAM).

Router# configure terminal

Enters configuration mode. 

Router(config)# interface POS slot/port

Selects the interface.

Router(config-if)# [no] loopback [internal | line]

Enables or disables a loopback on the interface.

internal specifies a local loopback.

line specifies a network loopback.

The default is no loopback (loopbacks on the 
interface are disabled).