Specifications

Configure an Interface for Line Loopback
Configuring Serial Interfaces IC-99
To enable or disable internal loopback on the interface, use one of the following commands in
interface configuration mode:
Local loopback is useful for checking that the POS is working. Packets from the router are looped
back in the framer.
Configure an Interface for Line Loopback
Line loopback is used primarily for debugging purposes.
To enable or disable an interface for line loopback, use one of the following commands in interface
configuration mode:
The receive fiber (RX) is logically connected to the transmit fiber (TX) so that packets from the
remote router are looped back to it.
Set the Source of the Transmit Clock
By default, the Packet OC-3 interface uses the recovered receive clock to provide transmit clocking.
To change the transmit clock source, use one of the following commands in interface
configuration mode:
Enable Payload Scrambling
SONET payload scrambling applies a self-synchronous scrambler (x^43+1) to the Synchronous
Payload Envelope (SPE) of the interface to ensure sufficient bit transition density. Both ends of the
connection must use the same scrambling algorithm. When enabling POS scrambling on a VIP2
POS on the Cisco 7500 series that has a hardware revision of 1.5 or higher, you can specify CRC 16
only (that is, CRC 32 is currently not supported).
To enable SONET payload scrambling on a POS interface, use the following command in interface
configuration mode.
Command Purpose
loop internal Enable internal loopback.
no loop internal Disable internal loopback.
Command Purpose
loop line Enable line loopback.
no loop line Disable line loopback.
Command Purpose
clock source Set the internal clock as the transmit clock source.
no clock source Set the recovered receive clock to provide transmit
clocking.
Command Purpose
pos scramble-atm Enable SONET payload scrambling.