Solved: Why does the 871 use crossover cables?

chrisblaze · ‎06-21-2008

I just purchased a 871 (non-wireless) and during the intial setup I couldnt get the 3 port up. All out of options I created a Vlan with all the ports inside, gave it an ip address and brought it up.

when I connected a straight through cable from the router to the PC the port wouldnt come up, but when I connected a crossover, the port came up.

Why would it not come up on a straight through.

Mark Yeates · ‎06-21-2008

Chris,

Are you sure that you are using a good straight through patch cable? The 871 router supports MDIX auto which means that the switch port will automatically cross over the pairs in hardware.

Mark

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scottmac · ‎06-22-2008

A router and a PC would be peer devices (i.e., both are made to plug into a switch), or you could say they are both DTE devices.

As such, the most appropriate cable would be a crossover cable.

That aside, at least some of the switch modules are auto MDI/MDI-X and should adapt. If the PC NIC is old enough, it may not cooperate with speed/duplex auto-discovery.

There are some other factors, sometimes just hard coding one side screws up discovery for the other side.

What version of IOS are you running?

Have you tried a scratch install?

Good Luck

Scott

View solution in original post

Mark Yeates · ‎06-21-2008

Chris,

Are you sure that you are using a good straight through patch cable? The 871 router supports MDIX auto which means that the switch port will automatically cross over the pairs in hardware.

Mark

chrisblaze · ‎06-21-2008

thanks for the reply Mark, yes we tried many cables but the crossover is the only one that worked. I just found that strange.

Mark Yeates · ‎06-21-2008

Something doesn't seem right. I also have a 871, and use straight through cables just fine. Have you tried a different PC? It should work with both crossover or straight cables.

Mark

scottmac · ‎06-22-2008

A router and a PC would be peer devices (i.e., both are made to plug into a switch), or you could say they are both DTE devices.

As such, the most appropriate cable would be a crossover cable.

That aside, at least some of the switch modules are auto MDI/MDI-X and should adapt. If the PC NIC is old enough, it may not cooperate with speed/duplex auto-discovery.

There are some other factors, sometimes just hard coding one side screws up discovery for the other side.

What version of IOS are you running?

Have you tried a scratch install?

Good Luck

Scott