SHDSL traffic throughput - traffic loss- inbound crc erros on atm interface

Unanswered Question
Apr 14th, 2008

Hello,

I use Cisco 1841 with WIC-1SHDSL-V3 and IOS c1841-ipbase-mz.124-15.T4.bin as CPE to connect via PPPoE to a BRAS. The connection between CPE and the BRAS is made via L2TP. Further I use a GRE tunnel to bridge layer 2 traffic between remote and central side. (I know this is an unsupported feature, but I've no other choice.)

The SHDSL line itself has a bandwidth of 2,3 MBit/s. But measurements on the link only confirms that for the upstream direction of the CPE ( 1841 -> BRAS). The downstream shows packet loss and (massive) inbound CRC errors at the atm interface of the 1841. The maxium bandwidth varies between 1 and 1,2 MBit/s.

When I apply traffic shaping on the tunnel interface at the BRAS, e.g. "traffic-shape rate 1900000 45000 45000 4096", I can get some kind of improvement. The inbound CRC errors will reduce to zero and the throughput rises to nearly 1,6 MBit/s, but only for large packets (1280 bytes). With smaller packets there are still a lot of CRC errors and packet losses.

Does someone here has an explanation for this behavior? Or knows how to get the full badwidth in both directions?

Any hints are welcome.

Thanks in advance an kind regards

Mark

I have this problem too.
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smahbub Fri, 04/18/2008 - 06:14

CRC errors are caused by one of the following reasons and you can check for these issues on youe setup.

1) Duplex Mismatch: When we have an interface that has a duplex mismatch it will generate

CRC errors, having combinations like Full---Half, Auto---Full, Auto---Half could increase

the CRC counter to increase, and the way to solve this problem is having the same duplex

on both ends.

2) Bad Cable: When a cable goes bad or we have something interrupting the signal we can

experience CRC errors, the solution for this is clear, just change the cables and remove

any device/machine that could be affecting the signal.

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