Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member

Duplex Mismatch

We recently replaced our T1 and 2600 router with a service from AT&T called Optiman. AT&T has fiber from the outside into a 24 port Catalyst 3550 switch in the MPoE of our building. Cat 5e is running from the MPoE up 5 floors to our server room where it connects to FE1/0 on our 2811 router.

I've enabled QoS on FE1/0 and we cannot seem to get a reliable traffic flow up and down. AT&T has tried to set the interface we're connected to on the 3550 to Auto and Full. Connectivity between our 2811 and the 3550 in the MPoE only seems to work when I hard set FE1/0 to half duplex 100.

Any thoughts on why setting both to 100 full doesn't seem to work?


Re: Duplex Mismatch

Poorly terminated cable, interference somewhere along the line, distance too long,...

New Member

Re: Duplex Mismatch

We are using brand new cables that tested fine using our connectivity test equipment. Isn't the max distance for ethernet 300 meters? 4 normally sized floors is well within that limit. There could be possible interference, but ethernet cable is running along side other cables that are working fine in a separate conduit.

Re: Duplex Mismatch

Ethernet is guaranteed to work properly upto 100m (300 feet) not 300m.

At this length the frame would never be able to detect collisions based on the ethernet slot time.



New Member

Re: Duplex Mismatch

The cable length is quite likely going to cause you problems, either due to timing or signal (noise or attenuation) issues.

You definitely want to be running it at full duplex, you'll get collisions on a link this long.

As to why it works in half/100, I've seen that many times it is the default fallback duplex/rate when the device has no idea what's going on. Something is is not right, but half/100 you get link up. However once your traffic rate goes up, watch the error rate climb too!

It sounds like you have new equipment, perhaps temporarily put something else inline just to rule it out? The link may be ok, you may just have a bad port. As your running long lengths, the port may be normally ok, but not good enough for this extreme case.

CreatePlease to create content