Maintaining and upgrading: When upgrading the current networks, the operators need to manually replace images or
upgrade network device hardware. Often the involved systems need to be reloaded. As control, forwarding, and management functions are coupled together in a single network device, the network upgrades and function expansion involve a wide range of network components, and the upgrade cycle is long. The omission of a network device upgrade or improper
configuration may result in poor performance over the network, or even make the network completely unavailable