As discussed on the page, Network Manager, the Network Manager makes setup and configuration of network connections very easy for non-technical users. It can be very useful in a mobile environment where a laptop may move from being connected by hard wired Ethernet cable, to a connection via wireless, to being connected over a WiMax connection. Those types of dynamic transitions are made extremely easy by the Network Manager.
That easy transition can cause issues when using multiple NICs in a more static environment. I have found that Network Manager cannot always determine the correct NICs to bring up on a couple of my systems that I use for routers and which have four NICs each. In fact, sometimes no connectivity at all is extant after a reboot. This is not such a good thing in the event that a remote reboot is required. Under such conditions it may be desirable to disable the Network Manager and return control of network initialization to the older network init scripts.
Both the Network Manager and the older network init scripts are controlled by the chkconfig command. Be sure to both turn off the Network Manager and turn on network using the commands:
chkconfig NetworkManager off service NetworkManager stop chkconfig network on service network restart
These commands transfer network startup control back to the older init scripts and turn off the dynamic Network Manager. And yes, the commands for NetworkManager do contain capital letters.
You must also be sure that the network configuration files, /etc/sysconfig/network-scripts/ifcfg-ethX are properly configured. See the sample on the Network Manager page and a more complete explanation of these files on the Configuring Network Interface Cards (NIC) page.