Here is my problem. Have an onboard NIC (eth0) which is malfunctioning.
So I got USB->Ethernet adapter (Linksys USB2000). The device is being recognized by Centos OK and I see a new eth1 interface after doing ifconfig -a. However even after I assign IP and netmask to eth1 and ifup I still can't ping it if I disconnect cable from eth0. If cable is connected to eth0 I can ping both devices. Doing route command
shows last line as similar to
default 0.0.0.0 172.23.23.34 eth0

If understand things correctly the setup isn't working because the traffic is still trying to get out through eth0 when going over the default route. So when I unplug the cable and replug it into eth1 the traffic is still trying to go out over unplugged device(eth0).

So my two options are probably to setup new default route
route add default gw 172.23.23.34 eth1. The problem with this is that I can't seem to make this new default route permanent. Every time I reboot the machine route changes to 'eth0'.

Another idea is to somehow disable the eth0 device (the malfunctioning device) and to reassign 'eth0' to the USB->Ethernet adapter(currently at eth1). Not sure how to do this though.

If the device is malfunctioning, just disable it in the BIOS, or blacklist the device in /etc/modprobe.d/blacklist.