This file discusses the two Cyclic Redundancy check (CRC) alternatives on Packet over Synchronous Optical Network (POS) router interfaces.



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contents Used

This document is not restricted to certain software and hardware versions.

The details in this file was produced from the gadgets in a details lab environment. All of the devices used in this record started through a cleared (default) configuration. If her network is live, make certain that you recognize the potential impact of any command.


Refer to technological Tips Conventions for an ext information on record conventions.

Cyclic Redundancy examine

The CRC is a an approach used to examine errors. The CRC uses a calculation numeric value to recognize errors in transmitted data. The sender that a data framework calculates the Frame check Sequence (FCS). The sender appends the FCS value to outgoing messages. The recipient recalculates the FCS, and also compares the value with the FCS native the sender. If a difference exists, the receiver assumes that a infection error occurred, and sends a request to the sender come re-send the frame. The retention of the true worth of a framework is important to ensure the the destination correctly interprets the data the you communicate.

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CRC-16 and also CRC-32

request for comment (RFC) 2615

defines the usage of the Point-to-Point Protocol (PPP) end SONET/ Synchronous Digital power structure (SDH). Below is just how this RFC specifies when a POS interface can use the 16-bit CRC (CRC-16) and also when it can use the 32-bit CRC (CRC-32):

"Regarding the FCS length, through one exception, the 32-bit FCS should be offered for every SONET/SDH rates. For Synchronous carry Signal (STS)-3c- Systems procedure Engineering (SPE)/VC-4 only, the 16-bit FCS may be used, although the 32-bit FCS is recommended. The FCS length is collection by provisioning and also is not negotiated."

RFC 2615 calls for (and recommends) the 32-bit CRC. The 32-bit CRC is much superior in the detection that certain species of errors than a 16-bit CRC. The much less robust CRC-16 deserve to fail come detect a bit error on links that deserve to transmit Gigabits the data every second.

You have the right to do the really CRC computation in hardware through no power implication because that either CRC length. Therefore, back the 32-bit CRC adds much more overhead, recommends this size of CRC on Optical Carrier-3 (OC-3) interfaces.

number 1 indicates the bespeak of operation on a POS interface, and when the user interface generates the CRC:

figure 1 – stimulate of procedure on a POS Interface


Configure the CRC size

Ensure that both router ends of a POS link use the very same CRC. Mismatched CRC setups are one construction parameter to inspect when a POS interface stays up/down. Use the show interface command to confirm your settings. In order to comply v RFC 2615, every POS interfaces support CRC-32. Higher-rate interfaces use CRC-32 as the default.

Here is the calculation from a 4xOC12 POS line card for the Gigabit switch Router (GSR):

RTR12410-2#show interface pos 8/0 POS8/0 is up, heat protocol is increase (looped) Hardware is Packet over SONET MTU 4470 bytes, BW 622000 Kbit, DLY 100 usec, depend 255/255, load 1/255 Encapsulation HDLC, crc 32, loopback collection (internal) Keepalive collection (10 sec) Scramble disabled critical input 00:00:00, output 00:00:00, output hang never Last clearing of "show interface" counters never ever Queueing strategy: fifo calculation queue 0/40, 0 drops; intake queue 0/75, 0 autumn 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 101418 packets input, 7853571 bytes, 0 no buffer obtained 0 broadcasts, 31 runts, 0 giants, 0 throttles 0 same 213 entry errors, 128 CRC, 0 frame, 0 overrun, 54 ignored, 0 abort 101414 packets output, 7853571 bytes, 0 underruns 0 output errors, 0 applique, 0 interface resets 0 calculation buffer failures, 0 output buffers swapped the end 3 transport transitions usage the crc command come configure a non-default value, as presented here:

RTR12410-2(config)#interface pos 8/0RTR12410-2(config-if)#crc ? 16 crc word-size32 crc word-size

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