The line keyword, the default setting, specifies that the clock source is derived from the active line. The internal keyword specifies the free-running internal clock.
These rules apply to clock sourcing:
When both ports are set to line with no primary specification, port 0 is the default primary clock source and port 1 is the default secondary clock source.
When both ports are set to line and one port is set as the primary clock source, the other port is, by default, the backup or secondary source and is loop-timed.
If one port is set to line or to line primary and the other port is set to internal, the internal port recovers the clock from the line port if the line port is up. If it is down, the internal port generates its own clock.
If both ports are set to internal, there is only one clock source internal.
The independent keyword expands on the clock source internal and clock source line to specify that the port can operate on an independent clocking domain (NMSI mode). Currently, on a 2-port multiflex truck voice WAN interface card (VWIC-MFT), if both ports are configured as clock source line, the 2-port is really looped, which means that it gets the clock from the first port. With NMSI mode, this dependency no longer exists, so the keyword independent means that both ports can be independently clocked. Refer to Clock Configuration for Cisco 1751/60 Routers for more information.