CRC Errors & Fragments

[TomBaltimore]

I just installed a managed switch and now see some errors that I am unfamiliar with:

CRC Errors
Fragments
Port In Discards

I'm mainly concerned about the CRC errors. I've done some basic research but I'm unsure on how to track down the source of the problem.

My router guy is telling me that the LAN errors are causing the router to go down.

Do I need a sniffer program? Do I need cable testers? How can I tell if the error is NIC card, Wall cable, patch cable, electromagnetic interference?
---------------------------------------
Tom Berger
Formix Solutions, Corp.
Baltimore, MD

http://www.formixsolutions.com

 

[pansophic]

The CRC errors are probably the source of your "port in discards." Your switch shouldn't be discarding fragments if they are legitimate and not the result of a collision.

Does your switch identify which port is the source of the CRC errors? If so, you can narrow your search down to the hosts on that port. If not, you may have to monitor each port individually with a protocol analyzer. A sniffer might do the job, but I've yet to see a CRC error on Ethereal, and I have a hard time believing that every network that I have sniffed on is running completely clean.

If you can capture an errored packet, then the MAC and IP address should be right in the header. You should be able to go directly to that host and decide for yourself whether or not this is a cable, NIC or power issue.

pansophic

 

[jimbopalmer]

CRC is a Cyclic Redundancy Check (like a checksum) put on every packet so the receiver can tell if it is garbled, sadly sometimes the driver that inserts them is corrupt, so they fail, if you track down a computer, reinstall the TCP/IP stack. if it IS being garbled enroute, suspect the wiring, slow down the connection.

Fragment should be packets with no trailer at all, but your equipment may also count any packet shorter than 64 bytes

A port in discard should be when the switch sees obviously wrong packets made by the other end I tried to remain child-like, all I acheived was childish.