|  | 4.3 | Router's Network Neighbors
| 4.3.1 | Gaining access to other routers by using Cisco Discovery Protocol (CDP)
Cisco Discovery Protocol (CDP) provides a single proprietary command that enables network administrators to access a summary of what the configurations look like on other directly-connected routers. CDP runs over a data link layer that connects lower physical media and upper network layer protocols, as shown in the Figure. Because it operates at this level, CDP devices that support different network layer protocols can learn about each other. (Remember that a data link address is the same as a MAC address.) When a Cisco device that is running Cisco IOS (Release 10.3 or later) boots up, CDP starts up automatically, which then allows the device to detect neighboring Cisco devices that are also running CDP. Such devices extend beyond those using TCP/IP, and include directly-connected Cisco devices, regardless of which Layer 3 and 4 protocol suite they run.