You are configuring the link between a Cisco 2950 series switch and a Cisco 2611 router. You have physically connected the router’s Ethernet port to the switch using a straight-through cable. The switch has not been configured, except for a hostname. The router’s hostname has also been configured, and the Ethernet port has been enabled. However, you forgot to assign an IP address to the Ethernet port.
You issue the show cdp neighbors command and get the following output:
If you did not configure IP addresses, how is this information being passed between the two devices?
A. The devices established a connection using default IP addresses.
B. The ip unnumbered command has been issued, which means the interface does not require an IP address to be configured.
C. CDP is a Layer 2 protocol and does not require IP addresses to be configured.
D. CDP uses its own IP addressing system.
Correct Answer: C
Explanation:
CDP is a Layer 2 protocol and does not require IP addresses to be configured. The structure of the OSI model requires that the upper-layer protocols rely on the lower-layer protocols for operation. Protocols at Layer 3 cannot be operational unless Layers 1 and 2 are operational. Conversely, lower-layer protocols do not rely on upper-layer protocols for their operation. Because CDP operates at Layer 2 of the OSI model, it does not require an IP address to be active, since IP addresses are a function of Layer 3.
The ip unnumbered command has not been issued in this scenario. This command can only be used on serial interfaces, not Ethernet interfaces. It allows a serial interface to use an address that is already applied to an Ethernet interface.
Information is not being passed between the devices through default IP addresses. There is no such thing as default IP addresses on Ethernet interfaces for Cisco routers.
Information is not being passed between the devices through CDP’s IP addressing system. CDP does not have its own IP addressing system because it does not use IP addresses for its operation.
