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Cisco Discovery Protocol (CDP) and Link Layer Discovery Protocol (LLDP) in Data Link Layer

Layer 2 or the Datalink layer provides physical addressing and access to media. It defines how data is to be formatted for transmission and how access to the network is to be controlled. It also provides error detection, ensuring data on higher layers is formatted correctly for transmission.

Cisco Discovery Protocol (CDP) and Link Layer Discovery Protocol (LLDP) in Data Link Layer are layer 2 (Datalink layer) protocols. They both help to discover how devices are connected to each other in a network. They both run independently of protocols IPv4/IPv6. They also help to verify and create documentation.

Cisco Discovery Protocol (CDP):
CDP is a Cisco proprietary protocol developed by Cisco Systems. It is used to discover other Cisco devices and can be used to share information such as OS version IP address, etc. CDP uses multicast frames for transmission and doesn’t rely on higher layer protocols. It can also be used for On Demand Routing to include routing information in CDP announcements so that dynamic routing protocols do not need to be used in simple networks.

Link Layer Discovery Protocol (LLDP):
LLDP on the other hand is the industry standard version of CDP. It is used by network devices for advertising themselves and their neighbors on a local area network based on IEEE 802 technology.

Consider the following diagram:

In the above diagram: connection 1 is possible as the connection is between 2 Cisco devices and both would use CDP to discover their neighbors and share their information. The same goes for connection 2 which is between 2 non Cisco devices. It will be using LLDP instead of CDP to discover its neighbors. A Cisco device cannot locate a non Cisco device using CDP and vice versa. Therefore, we can say that connection 3 is not possible.

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