36. Layer 2 Discovery Protocols CDP and LLDP

Layer 2 Discovery Protocols: CDP and LLDP

Layer 2 Discovery Protocols, such as CDP and LLDP, are used to share and discover information about neighboring (connected) devices in a network.

Overview

Shared Information:
- Hostname
- IP Address
- Device Type
- And more
CDP (Cisco Discovery Protocol):
- A Cisco proprietary protocol
- Enabled by default on Cisco devices
LLDP (Link Layer Discovery Protocol):
- An industry standard protocol (IEEE 802.1AB)
- Usually disabled by default on Cisco devices

Because they share device information, these protocols can pose security risks and are often disabled depending on network security policies.

Layer 2 Discovery Protocols

Cisco Discovery Protocol (CDP)

CDP is a Cisco proprietary protocol used for discovering neighboring Cisco devices.

Key Features

Default Behavior:
- Enabled by default on Cisco devices, including routers, switches, firewalls, and IP phones.
- CDP messages are sent periodically to the multicast MAC address 0100.0CCC.CCCC.
- Messages are sent every 60 seconds by default.
- The hold-time for CDP is 180 seconds. If a message isn’t received from a neighbor within this time, the neighbor is removed from the CDP Neighbor Table.
- CDPv2 is enabled by default.

CDP Overview CDP Operation

CDP Neighbor Table

The CDP Neighbor Table provides information about discovered devices.

Device ID: The identifier of discovered devices.
Local Interface: The local interface connected to the neighbor.
Holdtime: The countdown in seconds (0 means the device is removed from the table).
Capabilities: The capabilities of the neighbor device.
Platform: The model of the neighbor device.
Port ID: The port on the neighbor device connected to the local device.

CDP Neighbor Table

Detailed CDP Output

For more detailed information, including the Cisco IOS version running on the device:

Detailed CDP Output

Show Specific CDP Neighbor Entry

To display specific information about a CDP neighbor:

Show CDP Neighbor Entry

CDP Configuration Commands

Enable/Disable CDP Globally:
```
R1(config)# [no] cdp run
```
Enable/Disable CDP on Specific Interfaces:
```
R1(config-if)# [no] cdp enable
```
Configure CDP Timer:
```
R1(config)# cdp timer <seconds>
```
Configure CDP Holdtime:
```
R1(config)# cdp holdtime <seconds>
```
Enable/Disable CDPv2:
```
R1(config)# [no] cdp advertise-v2
```

CDP Configuration

Link Layer Discovery Protocol (LLDP)

LLDP is an industry-standard protocol used for discovering neighboring devices, similar to CDP.

Key Features

Default Behavior:
- LLDP is usually disabled by default on Cisco devices and must be manually enabled.
- Devices can run both CDP and LLDP simultaneously.
- LLDP messages are sent periodically to the multicast MAC address 0180.c200.000E.
- Messages are sent every 30 seconds by default.
- LLDP Holdtime is 120 seconds.
- An additional timer, the reinitialization delay, defaults to 2 seconds and delays the initialization of LLDP.

LLDP Configuration Commands

Enable LLDP Globally:
```
R1(config)# lldp run
```
Enable LLDP on Specific Interfaces:
- To enable transmission:
```
R1(config-if)# lldp transmit
```
- To enable reception:
```
R1(config-if)# lldp receive
```
Configure LLDP Timer:
```
R1(config)# lldp timer <seconds>
```
Configure LLDP Holdtime:
```
R1(config)# lldp holdtime <seconds>
```
Configure LLDP Reinit Timer:
```
R1(config)# lldp reinit <seconds>
```