TCP and UDP

TCP and UDP are more important protocols in transport layer.

TCP is used by many services and UDP is used by few services.

TCP provides  reliability and UDP doesn't.

TCP provides error recovery.

Cisco IOS Command Line Interface(CLI)

The Cisco IOS command line interface (CLI) is the user interface used for configuring, troubleshooting, and monitoring Cisco devices like routers and switches. The iOS command s are issued on the CLI . The information needed from the router and switch is also displayed on the CLI using the show commands.
To access the CLI of the router or switch you need to install software on your computer. Some Softwares are putty, SecureCRT.

DHCP Snooping

The job of a DHCP server is to assign the IP addresses automatically to the hosts which request the IP address on the enterprise network.
But if an attacker connects a rogue DHCP server to your network and starts assigning an IP address to the hosts which request them, then there would be a problem as explained below.