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Community Member

cannot communicate

hi everybody,

i need help again. i connect a 1720 router from the ethernet port to one of the 2820 switch port using a cross cable. the ip address i set for the router is and the ip address i set for the switch is the subnet mask is i configure rip for the router. but when i key in the command show interface fastethernet 0 it shows line is up line protocol is down. how should i solve the problem? please advice. thanks a lot in advance.


Re: cannot communicate

The cable connecting the router LAN interface to the switch should be a straight-through cable, not a crossover cable pinout. The router patches into a switch just like a computer does.

Also check the configuration of the switch port and the router LAN, to make sure there is not a speed or duplex mismatch. Either set both ends to autodetect speed and duplex (easier), or manally set them to run at the desired speed and duplex (recommended).

Hope this helps.

Community Member

Re: cannot communicate

Rule of thumb

If you are connecting devices at the same protocol layer, use a cross-over cable.

At different protocol layer, use straight cable

e.g. A switch is a layer 2 (L2) device, connecting to another L2 device such as a switch or hub, you will use cross-over

A switch is a layer 2 (L2) device, connecting to a router at layer 3 (L3), you will need a straight through.

A PC NIC to another PC NIC, you will also need a cross over.



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