Cisco 851W Duplex Issue

Posted on 2008-06-19
Last Modified: 2013-12-14
ISP = Wireless Highspeed Provider
ISP Router = Microtik

My Router = Cisco 851W
    WAN = FastEthernet4
    LAN = FastEthernet0-3
Server = SBS 2k3 R2 (DHCP Server)
Access Point = Cisco 1121G
LAN = 3 PCs and 2 Laptops

I installed this Cisco in place of a Linksys WRT54GS.  My ISP claims that his Gateway is getting a bunch of errors pertaining to duplex mode configuration.  I set the WAN on my router to half duplex mode and had Internet but my ISP was getting errors.  I then set the WAN to full duplex mode which resulted in no Internet for us and no errors for the ISP. Lastly I tried to set  the duplex mode to auto.  We gained Internet access but the ISP started to receive errors again.  What are the default settings in a Cisco 851W?  Can it be a link speed issue?  ANy help would be appreciated.

BTW, I'm not onsite today to test but I sure would like to get some info before I go over there.

Thanks in advance for your help.

Question by:dcsdave
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Expert Comment

ID: 21825509
'duplex' settings should be sam in both site to get the best performance and stability.

for example, if you set your WAN to 'full-duplex', then your ISP's port needs to be exact 'full-duplex', not auto.

please contact your isp to get the 'duplex' settings of the port that connected to your router.
try to avoid 'auto'.

hope it helps.

Author Comment

ID: 21825555
I will call them and get the settings.

Accepted Solution

dcsdave earned 0 total points
ID: 21978521
The ISP called me finally and said they had the settings manually configured to Full Duplex and Speed of 100.  I tried these settings and no Internet.  I switched back to Auto on both, which gives ue Internet, and I will wait for the ISP to come up with a better idea.

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