As discussed on the page, NetworkManager, the NetworkManager 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 NetworkManager.
That easy transition can cause issues when using multiple NICs in a more static environment. I have found that NetworkManager 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 NetworkManager and the older network init scripts are controlled by the chkconfig command. Be sure to both turn off the NetworkManager 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 NetworkManager. 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-<NIC Name> are properly configured. See the sample on the NetworkManager page and a more complete explanation of these files on the Configuring Network Interface Cards (NIC) page.