Cisco VIP-4R/4T Installation And Configuration Manual page 67

Configuring CRCs
CRC is an error-checking technique that uses a calculated numeric value to detect errors in
transmitted data. All interfaces use a 16-bit CRC by default, but also support a 32-bit CRC. The
sender of a data frame divides the bits in the frame message 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 that the sender used to
calculate the FCS. 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 designators 16 and 32 indicate 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. CRC-32,
which transmits streams of 16-bit characters, generates a 32-bit FCS. CRC-32 transmits longer
streams at faster rates and, therefore, provides better ongoing error correction with fewer
retransmissions. Both the sender and the receiver must use the same setting.
The default for all serial interfaces is for 16-bit CRC. To enable 32-bit CRC on an interface, specify
the slot and port address of the interface followed by the command crc32. In the example that
follows, the first serial port on an 4T port adapter on a VIP in interface processor slot 3 is configured
for 32-bit CRC:
Router# configure terminal
Enter configuration commands, one per line.
Router(config)# interface serial 3/1/0
Router(config-int)# crc32
Router(config-int)# ^Z
To disable CRC-32 and return to the default CRC-16 setting, specify the slot and port address and
use the no crc32 command. For command descriptions, refer to the related software documentation.
Replacing Port Adapter Cables
The port adapter cable connected to each port determines the electrical interface type and mode of
the port. The default mode of the ports is DCE, which allows you to perform a loopback test on any
port without having to attach a port adapter cable. Although DCE is the default, there is no default
clock rate set on the interfaces. When there is no cable attached to a port, the software actually
identifies the port as Universal, Cable Unattached rather than either a DTE or DCE interface.
Following is an example of the show controller cbus command that shows an interface port (2/1/0)
that has an EIA/TIA-232 DTE cable attached, and a second port (2/1/1) that does not have a cable
attached:
Router# show controller cbus
slot2: VIP, hw 2.1, sw 200.03, ccb 5800FF50, cmdq 48000090, vps 8192
To change the electrical interface type or mode of a port online, you replace the serial adapter cable
and use software commands to restart the interface and, if necessary, reconfigure the port for the new
interface. At system startup or restart, the VIP polls the interfaces and determines the electrical
interface type of each port (according to the type of port adapter cable attached).
software loaded from system
FLASH ROM version 255.255, VPLD version 20.1
4T HW Revision 121, SW Revision 216, Unresponsive 0
Interface 24- Serial2/1/0, electrical interface is RS-232 DTE
31 buffer RX queue threshold, 101 buffer TX queue limit, buffer size 1520
Transmitter delay is 0 microseconds
Interface 24- Serial2/1/1, electrical interface is Universal (cable unattached)
31 buffer RX queue threshold, 101 buffer TX queue limit, buffer size 1520
VIP Port Adapter Functions
End with CNTL/Z.
VIP-4R/4T Installation and Configuration 67