CDP & LLDP

Question 1

What is CDP ?

Answer 1

CDP is a Cisco proprietary protocol for Layer 2 discovery. It’s enabled on Cisco devices by default. It shares hostname, ip address, device type, etc.

Question 2

What is LLDP ?

Answer 2

CDP is an industry standard protocol (802.1ab) for Layer 2 discovery. It shares hostname, ip address, device type, etc.

Question 3

How are CDP messages sent ?

Answer 3

Multicast MAC address 0100.0ccc.ccc

The message is processed by the recieving deviced then discarded, not forwarded so devices can only become neighbors with devices directly connected. They are sent every 60 seconds. By default the CDP holdtime is 180 seconds so if another message isn’t recieved in that time the neighbor is removed from the CDP table. Today only CDPv2 is used.

Question 4

How to disable CDP ?

Answer 4

globally:
R1(config)# no cdp run

on an interface:
R1(config-if)# no cdp enable

Question 5

How to set CDP/LLDP timer and holdtime ?

Answer 5

R1(config)# [cdp | lldp] timer [seconds]

R1(config)# [cdp | lldp] holdtime [seconds]

Question 6

Can you run CDP and LLDP at trhe same time ?

Answer 6

Yes

Question 7

How are LLDP messages sent?

Answer 7

LLDP is disabled by default so it must be enabled globally then on each interface.

R1(config)# lldp run
R1(config-if)# lldp transmit
R1(config-if)# lldp recieve

They are sent to multicast MAC address 0180.c200.000e

The message is processed by the recieving deviced then discarded, not forwarded so devices can only become neighbors with devices directly connected. They are sent every 30 seconds. By default the LLDP holdtime is 120 seconds so if another message isn’t recieved in that time the neighbor is removed from the LLDP table.

Question 8

CDP & LDP commands to view information

Answer 8

R1# show [cdp | lldp]
Shows basic info (timers, version, etc)

R1# show [cdp | lldp] traffic
Shows how many messages have been sent/recieved

R1# show [cdp | lldp] interface
Displays which interfaces its enabled on

R1# show [cdp | lldp] neighbors
Lists neighbors and basic ifo about neighbors

R1# show [cdp | lldp] neighbors detail
Lists more detailed info about neighbors

R1# show [cdp | lldp] entry [name]
Displays specific info for a specific neighbor