Wireshark-users: Re: [Wireshark-users] CRC errors at router
From: Alex Lindberg <[email protected]>
Date: Thu, 7 May 2009 13:12:43 -0700 (PDT)
CRC errors are all most always the result of a mismatch in port duplex settings.

If a device or port is configured with auto-negotiation disabled and is statically configure to 100MB/Full Duplex, and the connected port is enabled for auto-negotiation, then the second port will set itself to 100MB/Half Duplex.  The result being that the second port will see CRC errors if it observes a collision.  This can cause a host of problems from problems viewing web pages, control issues of TCP devices, to poor/choppy voice quailtiy for VoIP sessions.

The port that detects CRC errors will not accept the packet into its switching/routing engine because the packet is in error, hence this packet will not showup if the port is mirrored to allow packet capture.

The "fix" is to set both ports the same.  One note - many devices allow the ports to determine if the connection is MDI-MDI or MDI-MDIX and will set the port as necessary to allow a straight though cable to work when connecting MDI (Switch) ports together.  In many cases this will not work if auto-negotiation is disabled.

Wireshark will not see any errors, except perhaps TCP retransmissions.

--- On Wed, 5/6/09, Craig OBrien <[email protected]> wrote:
From: Craig OBrien <[email protected]>
Subject: [Wireshark-users] CRC errors at router
To: [email protected]
Date: Wednesday, May 6, 2009, 12:14 PM

The FastEthernet interface  on our cisco router is gathering input errors.   
Generally at the rate that the CRC's errors are incrementing, it indicates
either a faulty application or speed negotiation issues with an internal server
or computer. But in not sure what,

Using wireshark, what would be the best method of capturing data on that port
so I can at least get a handle of what's doing these CRC errors?


Craig O'Brien
Steen Outdoor Advertising / Big Mountain Imaging
(215) 334-1700 x166

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