CDP
The Cisco Discovery Protocol is a data link layer protocol that shares configuration information between locally connected Cisco devices including routers and switches.
CDP works on the same LAN segment or over serial interfaces and can be used to discover the network topology.
The basic command is the show cdp neighbor command. This command displays the router interface of the neighbor, the hold-down timer, the type of device it is (listed under capacity), the platform (series number), and the local port that the neighbor is attached to.
The show cdp neighbor details or just sho cdp neigh det command displays the information displayed in the sho cdp neigh command plus some IOS information.