How do I force Time Capsule to use Ethernet port rather than wireless for backup?

What linksys router do you have? Actual model number helps.

Is the macbook connecting to the TC or the Linksys?

Is the TC connected to the linksys by ethernet?

Is the TC in bridge?

We might be able to figure out the fundamental problem or bypass it.

It is easy to force the Laptop to use ethernet.. you do this on the MBP.

Open the network preferences

Click on the * symbol at the bottom of the list of devices.. go to set service order.. and ensure that ethernet is above wireless settings.. that will force the MBP to use ethernet whenever it is available rather than wireless.

I tried connecting MBP and TC via Ethernet - after verifying the settings that you suggested - and it does not work: if I turn Airport off, Time Machine says it can't find the TC

Any other sugestions?

Ok, then ethernet is not getting an ip address from the TC and is plugged in wrongly..

You still didn't give me any idea of what actual model the linksys is.. and why is it in the network.. Is it a modem router?? If you have cable internet just throw it away and use the TC.

If you have the linksys as modem, then bridge it.. turn off wireless and run pppoe client on the TC.

If the ISP does not allow pppoe client mode then simply use the TC in bridge. Turn off all wireless in the linksys and just use the TC.

Still need info on how everything is setup to analyse the problem.