Chapter 13. Dynamic Host Control Protocol

This chapter covers the following topics:

  • Understanding DHCP—Describes the process of allocating IP addresses with DHCP.

  • DHCP Roles—Defines the server, relay, and client used by DHCP.

  • Troubleshooting DHCP—Lays out a seven-step process for troubleshooting DHCP.

This chapter is about using IPv4 DHCP (the Dynamic Host Control Protocol) with Cisco IOS. Cisco IOS devices may act as DHCP servers, relays, or clients, and this chapter deals with each case. This chapter also provides the configuration syntax with working examples.

The purpose of the "Do I Know This Already?" quiz is to help you decide what parts of this chapter to use. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.

The 11-question quiz, derived from the major sections in the "Foundation Topics" portion of the chapter, helps you determine how to spend your limited study time.

Table 13-1 outlines the major topics discussed in this chapter and the "Do I Know This Already?" quiz questions that correspond to those topics.

Table 13-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping
Foundation Topics SectionQuestions Covered in This SectionScore
Understanding DHCP1–2 
DHCP Roles3–10 
Troubleshooting DHCP11 
Total Score  


The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong for purposes of the self-assessment. Giving yourself credit for an answer you correctly guess skews your self-assessment results and might provide you with a false sense of security.

1.List the following actions in order: A-Acknowledgement, B-Discover, C-Offer, and D-Request.
  1. ABCD

  2. BADC

  3. CABD

  4. BCDA

2.DHCP is a descendant of what protocol?
  1. BootP

  2. RARP

  3. GARP

  4. SDLC

3.Which of the following are roles a device can assume in DHCP?
  1. Server

  2. Designee

  3. Relay Agent

  4. Client

  5. Communicating Peer

4.Given the following configuration, users are not able to communicate through the router. What should be changed to fix the configuration?
interface f0/1
     ip address
interface f0/0
     ip address
ip dhcp pool exp_pool

  1. Illegal IP address

  2. Pool name is not valid

  3. Pool is not applied

  4. No default gateway in DHCP

  5. Illegal/inappropriate mask

5.What is the default lease time for an IOS DHCP server?
  1. 1 day

  2. 2 days

  3. 3 days

  4. 4 days

6.How do you configure an IOS device as a DHCP client?
  1. service dhcp

  2. ip address dhcp

  3. Match IP address to network

  4. ipconfig/renew

7.Which command is used to configure DHCP relay agents?
  1. ip helper-address

  2. ip address dhcp

  3. ip dhcp relay-agent

  4. service dhcp relay-agent

8.Which of the following ports are forwarded by UDP by default?
  1. UDP 51

  2. UDP 666

  3. UDP 500

  4. UDP 67

9.Which command is used to add a UDP port that should have broadcasts relayed through DHCP?
  1. ip helper-address

  2. ip address dhcp

  3. ip dhcp relay-agent

  4. ip forward-protocol udp

10.Which UDP broadcasts are automatically forwarded by an IOS DHCP relay?
  1. NTP (port 37)

  2. DNS (port 53)

  3. TFTP (port 69)

  4. TACACS (port 49)

  5. NetBIOS name service (port 137)

  6. NetBIOS datagram service (port 138)

11.What command generates the following output?
IP address   Hardware address    Lease expiration        Type  00a0.9802.32de      Feb 01 1998 12:00 AM    Automatic

  1. show ip dhcp conflict

  2. show ip dhcp pool

  3. show ip dhcp binding

  4. dhow ip dhcp database

The answers to this quiz are found in Appendix A, "Answers to Chapter 'Do I Know This Already?' Quizzes and Q&A Sections." The suggested choices for your next step are as follows:

  • 7 or less overall score—Read the entire chapter. This includes the "Foundation Topics," "Foundation Summary," and "Q&A" sections.

  • 8–9 overall score—Begin with the "Foundation Summary" section, and then go to the "Q&A" section at the end of the chapter. If you have trouble with these exercises, read the appropriate sections in "Foundation Topics."

  • 10–11 overall score—If you want more review on these topics, skip to the "Foundation Summary" section, and then go to the "Q&A" section. Otherwise, move to the next chapter.