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New Member

High number of ethernet collisions when plugging into a 3com switch

I am seeing extremely high numbers of collisions on my Router interfaces when I plug into a 3com switch. Has anyone seen this, and if so, what was the work around?

6 REPLIES
Bronze

Re: High number of ethernet collisions when plugging into a 3com

I’d guess this is an auto detection issue. You might want to try locking your ports down to 100 full or 10baseTX.

Good Luck

New Member

Re: High number of ethernet collisions when plugging into a 3com

I've seen the exact same thing happen with both routers and switches. We had success by manually setting ALL the port speeds and duplex's on the 3COM switch, don't let it autonegotiate.

TJ

New Member

Re: High number of ethernet collisions when plugging into a 3com

In common with the other two replies you have received, this is almost certainly an "auto-negotiation" issue at the hardware level. Research across our own networks within the UK shows it to be an endemic problem across a range of manufactured switch/router combinations. The simple answer is, do NOT use auto-negotiation. Ever. Always lock ports to the desired speed/bandwidth. This equally applies to Servers<>Hubs/Switches too.

New Member

Re: High number of ethernet collisions when plugging into a 3com

could u kindly provide more information about the most important thing in combination of 3COM and Cisco products except the speed specification .\

\

New Member

Re: High number of ethernet collisions when plugging into a 3com

upgrade to all cisco... j/k.

New Member

Re: High number of ethernet collisions when plugging into a 3com

I found this explanation from Net3 Group's website. I thought it explained very well how auto-negotiation can cause problems:

Assume for instance, that a server has a dedicated

point-to-point connection to a switch port and that the server's NIC is set to auto negotiate while the switch is set to 100 Mbps full duplex. If the auto negotiation fails to detect the proper duplex, a server may default to half duplex, causing throughput to be a fraction of 100 Mbps, due to "false collision detection."

While operating in half duplex mode, the receive side of the server's Ethernet connection (connected to one pair of Cat 5 wire) looks for activity while the send side (connected to a different pair) is transmitting a packet. If activity is sensed on the receiver, then the NIC assumes that there is a collision, since on a hub-based Ethernet, you can only have one sender active at any given time. The NIC will back off the transmission and subsequently attempt to retransmit the packet. The NIC's tranmission will be interrupted everytime it receives a packet even though physically, there is no collision or problem with the transmission!

The simple solution is to "hard configure" the server's NIC for full duplex operation. This turns off the collision detection, allowing packets to be

received during transmission. The downside is that you would not want to reconnect the server to a standard hub port since collision detection has

been disabled! In this instance, be sure to change the port back to half duplex.

Hope this helps.

Greg

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