5.4 Troubleshooting Tools
Software Tools
📘CompTIA Network+ (N10-009)
Overview
LLDP (Link Layer Discovery Protocol) and CDP (Cisco Discovery Protocol) are network discovery protocols. They are tools that help network devices share information about themselves and learn about their neighboring devices. This is very useful for troubleshooting network issues, planning changes, or mapping the network.
Think of them as “network introduction protocols”: devices tell each other who they are and what they can do.
Key Points
1. CDP (Cisco Discovery Protocol)
- Proprietary Protocol: Only works on Cisco devices.
- Function: Allows a Cisco device to discover other directly connected Cisco devices.
- Protocol Layer: Operates at Layer 2 (Data Link Layer) of the OSI model.
- Information Shared:
- Device ID (hostname)
- IP address
- Software version / IOS version
- Port ID (which port is connected to whom)
- Device capabilities (switch, router, phone, etc.)
- Common Commands:
show cdp neighbors→ Shows connected Cisco devices and their info.show cdp neighbors detail→ Shows detailed information like IP address and IOS version.
Exam Tip:
If the question mentions Cisco devices only, it’s likely referring to CDP.
2. LLDP (Link Layer Discovery Protocol)
- Open Standard Protocol: Works on devices from any vendor, unlike CDP.
- Function: Lets network devices advertise their identity and capabilities to neighbors.
- Protocol Layer: Layer 2 (Data Link Layer), just like CDP.
- Information Shared:
- Device ID (hostname)
- Port ID
- System capabilities (switch, router, access point)
- Management IP address
- VLAN IDs
- Common Commands (varies by vendor):
show lldp neighbors→ Displays connected devices.show lldp neighbors detail→ Shows detailed information.
Exam Tip:
LLDP is used in mixed-vendor environments, while CDP is Cisco-only.
Why They Are Important in IT
- Network Mapping
- Administrators can see what devices are connected to each switch port.
- Example: If a switch has a port connected to an access point, LLDP/CDP will show that connection and the AP’s IP address.
- Troubleshooting Connectivity
- If devices cannot communicate, you can check neighbor information to verify the link.
- Example: If a switch cannot reach a router, CDP/LLDP can confirm whether the router is physically connected.
- Network Inventory & Documentation
- Helps in automatically documenting the network without manually checking each device.
- Configuration Verification
- Ensures devices are on the correct VLAN or subnet. LLDP can show VLAN info advertised by switches.
Differences Between LLDP and CDP
| Feature | CDP | LLDP |
|---|---|---|
| Vendor | Cisco only | Open standard (all vendors) |
| Layer | Data Link (Layer 2) | Data Link (Layer 2) |
| Information Shared | Hostname, IP, port, IOS, capabilities | Hostname, IP, port, VLAN, capabilities |
| Commands | show cdp neighbors | show lldp neighbors |
| Use Case | Cisco environments | Mixed-vendor environments |
Exam Focus
For the CompTIA Network+ exam, you need to know:
- Purpose: Both protocols help discover network devices and provide troubleshooting information.
- Vendor Differences: CDP = Cisco only, LLDP = standard for all vendors.
- Layer: Operate at Layer 2.
- Commands: Be familiar with
show cdp neighborsandshow lldp neighbors. - Uses in IT: Troubleshooting, network mapping, device inventory, verifying network topology.
Quick Memory Tip
Think of it like this for the exam:
- CDP = Cisco-only secret handshake
- LLDP = Universal handshake that all devices understand
Both tell who is next to me on the network and what they can do.
