Cisco Discovery Protocol (CDP)

CDP (Cisco Discovery Protocol) is a Layer 2 protocol developed by Cisco. It helps Cisco devices (like switches and routers) discover directly connected neighbors.

Imagine you're connected via console cable to a router (R1), but you have no idea what other devices are connected.

Figure 1 – CDP disabled: R1 cannot see any directly connected neighbors

If CDP is disabled, R1 stays blind it sees no neighbors.
But when CDP is enabled, R1 automatically learns about its directly connected Cisco neighbors such as R2 and R3.

Figure 2 – CDP enabled: neighbors R2 and R3 discovered

CDP is especially useful when you're in an undocumented or unknown network. With just one command, you can start discovering connected devices and mapping the topology.

CDP works in a simple and efficient way:
Cisco devices exchange messages on active interfaces to discover their directly connected Cisco neighbors.

CDP Advertisements

Each Cisco device periodically sends CDP messages on its active interfaces. These messages contain:

Figure 3 – CDP advertises neighbor info on all active interfaces

• Device ID: Hostname of the neighbor (example, R1)

• Local Interface: Port used on your device (example, Gig0/0)

• Port ID: Port used by the neighbor (example, Gig0/1)

• Platform: Neighbor’s model (example, ISR4451)

• Capabilities: Whether it's a router, switch, IP phone, etc.

• IP Address: Management IP (if configured)

These messages are sent as Layer 2 frames and don’t require IP addressing. The information is stored locally in CDP Table.

CDP Table

Each Cisco device builds and maintains a CDP table of its discovered neighbors.

Figure 4 – CDP tables list details of directly connected neighbors

You can view it by using the show cdp neighbors command on a device like SW1:

SW1# show cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
                  D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
R1               Gig 0/0           157              R B             Gig 0/1

This tells us:

• SW1 is connected to R1

• Using local port Gig0/0

• R1 uses port Gig0/1

• R1 is a Router/Bridge

• Its platform is ISR4451

On SW1, we clearly see that it has a directly connected Cisco neighbor which is R1.

CDP uses two timers to manage how long neighbor information is stored and how often it is refreshed:

Figure 5 – CDP sends updates every 60s; hold time is 180s

1. Update Timer

By default, CDP sends advertisements every 60 seconds on all active interfaces.
This regular interval ensures devices consistently share their presence with directly connected neighbors.

2. Hold Timer

Each device applies a hold time of 180 seconds to neighbor entries.
If no new CDP advertisement is received before this timer expires, the neighbor is removed from the CDP table.
This helps prevent outdated or incorrect data from remaining in the network.

Both timers can be adjusted if needed.

Enable CDP Globally

CDP is enabled by default on Cisco devices.
But if, for any reason, it was disabled, you can turn it back on using cdp run command: