low transfer rate with NAT/Duplex/Speed settings

Answered Question
Nov 6th, 2007

Hi all, I'm having this problem and I cannot find any bug or TAC case in the collection. I'm using PAT between two ethernet interfaces on a c3745 or c3845(I tried both in a lab).

If I keep the duplex and speed negociation to auto, everything works fine and it autonegotiates to full/100, but as soon as I force the duplex to full and speed to 100, the transfer rate of my FTP drops considerably. When I change the speed back to auto everything goes back to normal. I've tried the latest IOS but the problem persist.

Anyone experienced the same problem? Any ideas?

Thanks in advance,


I have this problem too.
0 votes
Correct Answer by lgijssel about 9 years 2 months ago

Fixing speed and duplex must be done on both sides. If one end remains in auto-mode the connection will default to half duplex.

This will result in a duplex mismatch between the devices.




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Overall Rating: 5 (1 ratings)
marianodt Tue, 11/06/2007 - 07:16

Thank you for your answer. I was confused because both sides reported a full/100 negotiation, but forcing caused the problem.

I guess I'll stay in the safe side by forcing everything. Anyway, thanks again.


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