Understanding Default Speed and Duplex, CDP, Interface Range, and LLDP in Cisco Devices

1. Default Speed and Duplex in Cisco

In Cisco devices, Speed and Duplex define the link speed and the method of data transmission on a network interface.

What Is Speed?

Speed determines the rate of data transfer on an interface and is typically measured in Mbps or Gbps.

What Is Duplex?

Duplex defines how data is transmitted and received:

• Half-Duplex: Data is sent and received alternately
• Full-Duplex: Data is sent and received simultaneously

Default Behavior:

By default, Cisco interfaces operate in Auto-Negotiation mode.

speed auto
duplex auto

In this mode, connected devices automatically negotiate and select the highest common speed and duplex setting.

2. Cisco Discovery Protocol (CDP)

CDP or Cisco Discovery Protocol is a Cisco-proprietary protocol used to discover and share information with directly connected neighboring devices.

Information Provided by CDP:

• Neighbor device name
• Device type
• Connected interface
• IP address

Enable or Disable CDP:

cdp run
no cdp run

View CDP Neighbors:

show cdp neighbors
show cdp neighbors detail

3. Interface Range in Cisco

The Interface Range command allows administrators to apply configuration changes to multiple interfaces simultaneously, significantly improving efficiency.

Entering Interface Range Mode:

interface range GigabitEthernet0/1 - 4

Applying Configuration to Multiple Interfaces:

switchport mode access
speed auto
duplex auto
description Access Ports

All commands above are applied to every selected interface.

4. Link Layer Discovery Protocol (LLDP)

LLDP or Link Layer Discovery Protocol is a vendor-neutral discovery protocol defined by IEEE, used to identify neighboring network devices.

Unlike CDP, LLDP works across devices from different vendors.

Enable LLDP:

lldp run

View LLDP Neighbors:

show lldp neighbors
show lldp neighbors detail

5. CDP vs LLDP Comparison

• CDP is Cisco-proprietary
• LLDP is vendor-neutral
• CDP is enabled by default on Cisco devices
• LLDP usually requires manual activation

Conclusion

Understanding Default Speed and Duplex, effectively using CDP and LLDP, and mastering the Interface Range command are essential skills for managing Cisco devices. These concepts enhance network performance, simplify configuration, and improve troubleshooting efficiency.