Ethernet port collisions

[peterd51]

Hi,

I need a pointer on this as I'm not exactly sure if it's 'normal' or not...

we have a Cisco switch directly connected to a Cisco router and the only available port on the router is an Ethernet 10 Meg.

The config on each port is set to 10 Meg 1/2 duplex and we're seeing massive ammounts of collisions at the router interface (but not at the switch port).

Big Brother/MRTG shows the link at 80% for long periods.

I know that 80% on a broadcast network would be a problem but on a 'serial' link, port to port, would the high usage be causing excess collisions?

I don't know why it's limited to half duplex but as the gear is on a remote site with no qualified network staff I don't want to start changing things remotely.

I suggested to the boss that someone needs to visit to check out the state of the patch leads, etc, but could it be the amount of traffic on a half-duplex link that's causing the excess collisions?

Regards
Peter

 

[Grenage]

If possible, set the ports on each end to be Half Duplex, or Full duplex. You don't want one to be automatic and the other manually set, you really want to be sure they match.

If they are different, you can get some horrible collisions.

Carlsberg don't run I.T departments, but if they did they'd probably be more fun.

 

[NettableWalker]

Collisions would ususally be a sign of mis-matched duplex settings.

Change them on one side and see if it cures it.

HTH

 

[watchguardmonkey]

you could also check and make sure that the router is not running cisco cdp, and ensure that the statement "no ip directed-broadcast" is in the router eth int statement.


WGM

 

[peterd51]

Hi,

both ports are set for 10 meg half duplex...

we are using CDP but that won't affect the traffic level much as it's hitting 80% regularly.

Regards
Peter

 

[jneiberger]

Collisions are perfectly normal and expected on Ethernet connections. This is not a problem. Don't worry about it. If you are running a 10/half connection with high utilization then you would expect to see a pretty high percentage of collisions with two-way traffic.

 

[peterd51]

Hi,

it looks like we need to do some work to try to get this particular link upgraded to 100 Full...

Thanks
Peter

 

[vipergg]

Anything over 30-40% on a half duplex link is problematic and you will see a lot of collisions, crc's etc...

 

[jneiberger]

Yes, upgrading to Fast Ethernet would be a good idea. As vipergg said, anything over about 40% two-way traffic and the link will really bog down. If you do a "show controller ethernet" on the switch, you'll probably see a lot of excessive collisions.