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Duluth, GA

Cisco 8x7 CRCs on logical interface only

Morning all, Please consider the following two CLI commands:

show interface
show dsl interface

IOS version: 12.3(11)T10 on a K9 router. CRC counter is clean on the dsl interface, but not on the logical interface. No other errors are observed on either interface.

ISP is Windstream. They are not seeing or acknowledging a problem. Trying to interpret these two forms of CRC counters.

Originally, I thought the CRC counters represented instances in which the actual ATM cells are failing check, but I didn't know if this was just the header, header+payload, or just payload in the 53-byte cell. HEC counter is zero.

Also, I wasn't sure if the logical interface CRC counter presented instanced in which the SAR reassembled datagram was failing check.



1 edit
Hi GhostDoggy,

I'm pretty new to Cisco and trying to get my CCENT cert by this year. Do correct me if I'm wrong, i'm still learning hehe..

From what I understand, CRC errors on the "show DSL interface" would mean physical cabling (RJ11), port issues with the WAN port on the router, or the local media provider's issue. However if you see CRC errors on the WAN interface (ATMx or otherwise), most likely it could be overusage of the advertised speed of the DSL line. Since DSL lines have overhead, their usage may exceed more than advertised.

That's what I understand of the 2 CRC error differences. Anybody else can comment?

drop the crantini and move it, sister
Morristown, NJ
I am not 100% sure, but I think that on the logical interface you're looking at errors end-to-end, which would include errors beyond your DSL line...


Duluth, GA
reply to GhostDoggy
Turned out to be the AAL5 PDU frame failing CRC check. This means the Ethernet frame built by the SAR function based on ATM cells received was missing 48-byte expected payloads.

So, while those ATM cells that showed up were 100% kosher, not all of the ATM cells were showing up.