unexplained "duplex mismatch discovered"

[network770]

I have a wan between 2 sites, one site has a 3750 facing the wan and the other side has a router, both cisco of course

They both have a layer3 interface within the same segmet and i can ping across.

the router's interface has 100\full hard coded and the switch has auto\auto, I am noticing that the router negotiates at 100\full but the switch at 100\half and i get this message:

%CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on GigabitEthernet9/0/2 (not full duplex), with RTR1 FastEthernet0/1 (full duplex). (C3750-SW)

when i try to hard code the switch with 100\full i loose the wan connectivity and the interface shows as 'not connected'

any idea what's happening here?

[Leo Laohoo]

Your WAN link goes to an ISP right?

Can you try the command "speed auto 100"?

[lgijssel]

What happens here is that the cdp messages are sent over the provider network. (Provider is not Cisco.)

This leads to wrong information because the speed/duplex settings must match with the provider settings on each side.

Although not optimal, it is not required to have the settings for both sides matching.

Best is to turn off cdp as it serves no real purpose here and delivers incorrect information.

regards,

Leo

[jyoung]

If the ISP is providing you with a 802.1Q tunnel for connectivity than both the switch and router should be set manually or auto, not both.  Auto negotiation of speed and duplex only works correctly when both sides are set to auto.  When one side is set manually, it ignores the auto negotiation process of the other side.  I would set both sides to manual and go from there

[network770]

When I set both switch & router to auto\auto I loose the wan... when I go back to the original config, interesting enough, one side of the wan (which is hardcoded to 100\full) negotiates at 100\full but the auto\auto side negotiates at 100\half, how could that be?

I am thinking maybe the ISP equipment has half duplex hard coded... possbile?

otherwise there is no way to explain this

[jyoung]

Oh yea, you should contact the ISP and see what the speed/duplex settings are on each side and match them. The two sides are independant of eachother because the speed/duplex settings need to match up with the ISP equipment.  Sorry, I had a brain fart there before.

[Leo Laohoo]

I am thinking maybe the ISP equipment has half duplex hard coded... possbile?

That's why I'm asking a silly question (just to be sure).

Was involved in an upgrade of 24 sites from Frame Relay to IP-based network.  The provider put their PE (non-Cisco) to  auto/auto and we see nothing but 100/Half.  So we asked them to hard-code their side to 100/Full and we did the same in our side to fix the issue.