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Collision on ethernet between 837 and 1700

We have a WAN setup where a Cisco 837 (provided by ISP Carrier) sits in bridge mode acting as DSL Modem and then we connect using a crossover to our Router (Cisco 1760). Ethernet0 on 1760 and Ethernet 0/0 on the 837. fast eth 0/0 on 1760 then connects to a Cisco 2950 Switch towards the LAN.

OK... the problem is-

1. High amount of Input CRC errors on the Ethernet of 1760.

2. From the Cisco 1760, CDP sees the the 837 ethernet port as Full Duplex (this is impossible due to hardware limitation) We have diabled CDP Duplex Mismatch logging.

2. We have changed duplex to half on the 1760 ethernet, this fix the CRC errors, but now we have increasing collision on this interface.

3. 1760 does not have capability to set speed to auto-neg..

4. Increasing collison causes interface resets.. causing intermittent outages through the day..

Any ideas on how I can go about this problem.



Super Bronze

Re: Collision on ethernet between 837 and 1700


This is a common mistake with 837s- they are actually full-duplex capable.

The 'ethernet' port is actually a virtual port which is kind of software-bridged to the four physical fastethernet ports.

So you can't set the duplex on the ethernet port, but you can on whichever fastethernet port you have the cable connected to.

Set that to 10 full or 100 full, set your 1760 to match, and that will sort out your port errors.

Unfortunately there is what I'd call a bug, but Cisco would probably call 'by design' where CDP will keep telling you that there is a duplex mismatch.

You can either ignore it or disable CDP on the port.

You'll be able to verify the config by looking at the error/crc/collision counters on the ports ( the fastethernet1/2/3/4 physical port and the 1760 port being the important ones).



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