Chapter 11
Configure and Verify Layer 2 Discovery Protocols
This chapter ensures that you are ready for questions related to two simple but important topics from the Network Access section of the CCNA 200-301 exam blueprint from Cisco Systems. This chapter presents just a portion of the Network Access area. The other chapters in Part II also review VLANs, trunks, Rapid PVST+, and wireless technologies.
This chapter covers the following essential terms and components:
▸ Layer 2 protocols
▸ Cisco Discovery Protocol (CDP)
▸ Link Layer Discovery Protocol (LLDP)
Topic: Configure and verify Cisco Discovery Protocol (CDP)
CramSaver
If you can correctly answer these CramSaver questions, save time by skimming the ExamAlerts in this section and then completing the CramQuiz at the end of this section and the Review Questions at the end of the chapter. If you are in doubt at all, read everything in this chapter!
1. What is the Cisco Layer 2 protocol for discovering neighbors?
_________
2. What is the default status on a switch or router for Cisco’s proprietary Layer 2 neighbor discovery protocol?
_________
Cisco Discovery Protocol is a Layer 2 protocol that allows Cisco devices to communicate information about each other to their directly connected neighbors. This can be useful when you are unsure of the topology. Cisco IP phones also use it to communicate their capabilities and VLAN information to their local switch. CDP messages from a Cisco router are not forwarded by a directly connected Cisco switch. Two routers connected to the same switch would not see each other’s CDP messages. On Ethernet, CDP messages are encapsulated within standard Ethernet frames, and the destination MAC address is a multicast MAC address. This actually means that non-Cisco standards-based switches flood CDP frames out all their interfaces. So, Cisco switches are not standards compliant when it comes to CDP frames, but Cisco devices connected by a non-Cisco switch will successfully exchange CDP messages.
CDP is enabled by default on all interfaces of Cisco routers and switches. To ensure that it has not been disabled globally on a device or to ensure that it has not been “trimmed” off an interface, you can use the commands shown in Example 11.1.
Example 11.1 Ensuring That CDP Is Running on a Device and on an Interface
Switch# configure terminal Enter configuration commands, one per line. End with CNTL/Z. Switch(config)# cdp run Switch(config)# interface gi1/0 Switch(config-if)# cdp enable Switch(config-if)# end Switch# %SYS-5-CONFIG_I: Configured from console by console Switch#
To verify that CDP is running globally, use the show cdp command. To verify that CDP is indeed enabled on an interface, use the show cdp interface command. To view the information collected by CDP about neighboring devices, use the show cdp neighbors detail command.
Administrators might disable CDP globally or on certain interfaces if they are concerned about a device sharing information with an unauthorized neighbor. This concern is legitimate and often leads to disabling CDP on specific public-facing interfaces.
ExamAlert
Remember that CDP is its own Layer 2 protocol. It does not rely on other protocols. To quote Cisco: “Cisco Discovery Protocol is a Layer 2, media-independent, and network-independent protocol that networking applications use to learn about nearby, directly connected devices.”
CramQuiz
1. What technology does CDP rely on in its operation?
A. TCP
B. UDP
C. ICMP
D. Layer 2
2. What modern technology heavily relies on CDP?
A. Cisco VoIP phones
B. WebEx
C. Catalyst switches
D. DTP
CramQuiz Answers
1. D is correct. CDP operates at Layer 2 and does not use TCP, UDP, or ICMP.
2. A is correct. Cisco VoIP phones rely on CDP for neighbor discovery and capabilities exchange.
Topic: Configure and verify Link Layer Discovery Protocol (LLDP)
CramSaver
1. What is the open standard Layer 2 protocol for discovering neighbors?
_________
2. What is the global configuration command to ensure that a Cisco device speaks LLDP?
_________
CDP is obviously a Cisco-specific solution. The open standard protocol for discovering network neighbors is Link Layer Discovery Protocol (LLDP). Note that CDP and LLDP basically serve the same purpose: They provide a method for network devices to communicate information about themselves.
Unlike CDP, LLDP is not enabled globally by default on IOS. When enabled globally, however, it is also by default enabled to both transmit and process incoming LLDP frames on all Ethernet interfaces. Example 11.2 shows the commands that may be used to configure LLDP.
Example 11.2 Ensuring That LLDP Is Running Globally and on an Interface
Switch# configure terminal Enter configuration commands, one per line. End with CNTL/Z. Switch(config)# lldp run Switch(config)# interface gi1/0 Switch(config-if)# lldp transmit Switch(config-if)# lldp receive Switch(config-if)# end Switch# %SYS-5-CONFIG_I: Configured from console by console Switch#
To verify that LLDP is running globally, use the show lldp command. To verify that LLDP is indeed enabled on an interface, use the show lldp interface command. To view the information collected by LLDP about neighboring devices, use the show lldp neighbors detail command.
Because LLDP is defined as part of the Ethernet standard, it is available only on Ethernet interfaces. If you need to discover neighbors over non-Ethernet interfaces, your only option is to run CDP.
CramQuiz
1. What interface-level command ensures that an interface will process incoming LLDP frames?
A. lldp receive
B. lldp enable
C. lldp run
D. lldp all
2. Which statement regarding LLDP is false in relation to Cisco devices?
A. LLDP is not enabled by default on IOS devices.
B. Cisco invented LLDP.
C. LLDP is compatible with CDP.
D. After being enabled globally, LLDP must be enabled on specific interfaces.
CramQuiz Answers
1. A is correct. You use the lldp receive command on an interface in order to ensure that the device processes incoming LLDP frames on that interface.
2. B is correct. LLDP is an open standard protocol that is specified in the IEEE 802.3 standard; it was not invented by Cisco Systems.
Review Questions
1. What command disables CDP globally on a device?
A. no cdp run
B. no cdp enable
C. cdp disable
D. cdp stop
2. What command ensures that CDP is running on a specific interface?
A. cdp enable
B. cdp on
C. cdp run
D. cdp accept
3. What is most often meant by the term “trimming” CDP?
A. Using version 1 of CDP to make it more efficient
B. Forwarding CDP messages to a different subnet by using a relay
C. Not running CDP on all interfaces
D. Globally disabling CDP
4. Which statement about CDP is correct?
A. CDP relies on TCP.
B. CDP is disabled by default.
C. CDP is disabled on all serial interfaces by default.
D. CDP is a Layer 2 protocol.
5. On a Cisco device, what command would you use on an interface to ensure that the interface is sending LLDP frames?
A. lldp run
B. lldp forward
C. lldp transmit
D. lldp send
Answers to Review Questions
1. A is correct. You use the no cdp run command to globally disable CDP on a device. The cdp disable and cdp stop commands do not exist.
2. A is correct. You use the cdp enable command on an interface to run CDP on that specific interface.
3. C is correct. Trimming CDP refers to disabling it selectively on certain interfaces.
4. D is correct. CDP is a media- and protocol-independent Layer 2 protocol.
5. C is correct. On an interface on a Cisco device, you can use the lldp transmit command to ensure that the interface sends LLDP frames.
Additional Resources
Cisco Discovery Protocol (CDP)
https://www.ajsnetworking.com/2-1-b-i-cdp-lldp
Link Layer Discovery Protocol (LLDP)