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Excessive CRC/Alignment errors

Occasional Visitor

Excessive CRC/Alignment errors

l am a networking virgin and l would be grateful of some help.

May two firewalls create the following.. Excessive CRC/Alignment errors messages.. back to my main 2848switch. is this normal or should l worry
My main 2848 also send out excessive to the other lower level broadcasts when l have removed all the broadcasting hubs in my network. could This be excessive server traffic if so how do l prove it is there also a manual for this level of work on the 2848
Esteemed Contributor

Re: Excessive CRC/Alignment errors

Hello Tim,

Welcome to the world of networking. The CRC errors could be caused by a speed/duplex missmatch or bad cable or etc...

What you want to do first is to check the portsettings on both sides (switch and firewall) see to it the are equal. The 2848 is on autonegotiation by default. If the firewalls are on a fixed speed set them to auto or the switch port to the same fixed speed.

How have you connected these two firewalls? (point to point or is there a patch panel inbetween? What kind of cable did you use?

Manuals can be found at:



Les Ligetfalvy
Esteemed Contributor

Re: Excessive CRC/Alignment errors

It is not uncommon to see some CRC errors when a link first comes up. Is this a one time error at link up, or a recurring one?

If you fix the port on one side you should also fix the port settings on the other side. don't leave one side auto and the other side fixed.
Shiraz Malik

Re: Excessive CRC/Alignment errors

Hi Tim,
CRC errors with Firewall and hubs usually occur due to the port speed/duplex mismatch. Best thing would be to check the configuration on firewall and then make the necessary changes on the 2848. Most probably, it would just be the speed that needs to be match. In case of hub, configure the port with static speed / duplex settings.

Please feel free to ask if you are still not clear with anything.

Esteemed Contributor

Re: Excessive CRC/Alignment errors


At least now you have 3 people (one claims to be an expert in this area, one is a graduate and one a pro) giving you the same answers.


rick jones
Honored Contributor

Re: Excessive CRC/Alignment errors

How Autoneg is supposed to work:

When both sides of the link are set to autoneg, they will "negotiate"
the duplex setting and select full duplex if both sides can do

If one side is hardcoded and not using autoneg, the autoneg process
will "fail" and the side trying to autoneg is required by spec to use
half-duplex mode.

If one side is using half-duplex, and the other is using full-duplex,
sorrow and woe is the usual result.

So, the following table shows what will happen given various settings
on each side:

Auto Half Full

Auto Happiness Lucky Sorrow

Half Lucky Happiness Sorrow

Full Sorrow Sorrow Happiness

Happiness means that there is a good shot of everything going well.
Lucky means that things will likely go well, but not because you did
anything correctly :) Sorrow means that there _will_ be a duplex

When there is a duplex mismatch, on the side running half-duplex you
will see various errors and probably a number of late collisions. On
the side running full-duplex you will see things like FCS errors.
Note that those errors are not necessarily conclusive, they are simply
there is no rest for the wicked yet the virtuous have no pillows