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5-26 Cisco 4000 Series Installation Guide
Additional Configuration Tasks
Calculating CRCs on Cisco 4000 Series Serial Interfaces
On Cisco 4000 series routers, all serial interfaces support CRC-ITU-T, a 16-bit cyclic
redundancy check (CRC). CRC is an error-checking technique that uses a calculated
numeric value to detect errors in transmitted data. The sender of a data frame divides the
bits in the frame by a predetermined number to calculate a remainder or frame check
sequence (FCS). Before it sends the frame, the sender appends the FCS value to the
message so that the frame contents are exactly divisible by the predetermined number. The
receiver divides the frame contents by the same predetermined number. If the result is not
0, the receiver assumes that a transmission error occurred and sends a request to the sender
to resend the frame.
The designator 16 indicates the number of check digits per frame that are used to calculate
the FCS. CRC-16, which transmits streams of 8-bit characters, generates a 16-bit FCS.
Both the sender and the receiver must use the same setting of 16.
The default for all serial interfaces is for 16-bit CRC.
Configuring the Asynchronous/Synchronous Serial Interfaces
If you have a NP-2T16S serial network processor module installed, the ports you plan to
use for asynchronous operation must be reconfigured after the initial setup. The following
steps provide an example of how to configure a synchronous serial port to be an
asynchronous serial port.
Step 1 Enter the command config terminal to enter configuration mode:
Router# config terminal
Router(config)#
The router enters global configuration mode, indicated by the Router(config)#
prompt.
Step 2 Select the serial interface to configure:
Router(config)# interface serial 2
Router(config-if)#
The prompt changes again to show that you are in interface configuration mode.
Step 3 Because all serial ports are initially configured as synchronous, you must change
the port to asynchronous operation by entering the physical-layer command: