Group-7-DHCPv4.pptx

Switching, Routing and Wireless
Essentials v7.0 (SRWE)
Module 7: DHCPv4

2
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential
Module Objectives
Module Title: DHCPv4
Module Objective: Implement DHCPv4 to operate across multiple LANs
Topic Title Topic Objective
DHCP4 Concepts
Explain how DHCPv4 operates in a small- to medium-
sized business network.
Configure a Cisco IOS DHCP4 Server Configure a router as a DHCPv4 server.
Configure a DHCP4 Client Configure a router as a DHCPv4 client.

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7.1 DHCPv4 Concepts

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DHCPv4 Concepts
DHCPv4 Server and Client
• Dynamic Host Configuration Protocol v4 (DHCPv4) assigns IPv4 addresses and other
network configuration information dynamically. Because desktop clients typically make up the
bulk of network nodes, DHCPv4 is an extremely useful and timesaving tool for network
administrators.
• A dedicated DHCPv4 server is scalable and relatively easy to manage. However, in a small
branch or SOHO location, a Cisco router can be configured to provide DHCPv4 services
without the need for a dedicated server. Cisco IOS software supports an optional, full-
featured DHCPv4 server.
• The DHCPv4 server dynamically assigns, or leases, an IPv4 address from a pool of
addresses for a limited period of time chosen by the server, or until the client no longer
needs the address.
• Clients lease the information from the server for an administratively defined period.
Administrators configure DHCPv4 servers to set the leases to time out at different intervals.
The lease is typically anywhere from 24 hours to a week or more. When the lease expires,
the client must ask for another address, although the client is typically reassigned the same
address.

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DHCPv4 Concepts
DHCPv4 Operation
DHCPv4 works in a client/server mode. When a client communicates with a DHCPv4
server, the server assigns or leases an IPv4 address to that client.
• The client connects to the network with that leased IPv4 address until the lease
expires. The client must contact the DHCP server periodically to extend the lease.
• This lease mechanism ensures that clients that move or power off do not keep
addresses that they no longer need.
• When a lease expires, the DHCP server returns the address to the pool where it can
be reallocated as necessary.

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DHCPv4 Concepts
Steps to Obtain a Lease
When the client boots (or
otherwise wants to join a
network), it begins a four-step
process to obtain a lease:
1. DHCP Discover
(DHCPDISCOVER)
2. DHCP Offer (DHCPOFFER)
3. DHCP Request
(DHCPREQUEST)
4. DHCP Acknowledgment
(DHCPACK)

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DHCPv4 Concepts
Steps to Renew a Lease
Prior to lease expiration, the client begins a two-
step process to renew the lease with the DHCPv4
server, as shown in the figure:
1. DHCP Request (DHCPREQUEST)
Before the lease expires, the client sends a
DHCPREQUEST message directly to the
DHCPv4 server that originally offered the IPv4
address. If a DHCPACK is not received within a
specified amount of time, the client broadcasts
another DHCPREQUEST so that one of the other
DHCPv4 servers can extend the lease.
2. DHCP Acknowledgment (DHCPACK)
On receiving the DHCPREQUEST message, the
server verifies the lease information by returning a
DHCPACK.
Note: These messages (primarily the DHCPOFFER and
DHCPACK) can be sent as unicast or broadcast according
to IETF RFC 2131.

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7.2 Configure a Cisco IOS
DHCPv4 Server

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Configure a Cisco IOS DHCPv4 Server
Cisco IOS DHCPv4 Server
Now you have a basic understanding of how DHCPv4 works and how it can make your
job a bit easier. A Cisco router running Cisco IOS software can be configured to act as a
DHCPv4 server. The Cisco IOS DHCPv4 server assigns and manages IPv4 addresses
from specified address pools within the router to DHCPv4 clients.

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Steps to Configure a Cisco IOS DHCPv4 Server
Use the following steps to configure a Cisco IOS DHCPv4 server:
• Step 1. Exclude IPv4 addresses. A single address or a range of addresses can be
excluded by specifying the low-address and high-address of the range. Excluded
addresses should be those addresses that are assigned to routers, servers, printers,
and other devices that have been, or will be, manually configured. You can also enter
the command multiple times. The command is ip dhcp excluded-address low-
address [high-address]
• Step 2. Define a DHCPv4 pool name. The ip dhcp pool pool-name command creates
a pool with the specified name and puts the router in DHCPv4 configuration mode,
which is identified by the prompt Router(dhcp-config)#.

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Steps to Configure a Cisco IOS DHCPv4 Server (Cont.)
• Step 3. Configure the DHCPv4 pool. The address pool and default gateway router must be
configured. Use the network statement to define the range of available addresses. Use
the default-router command to define the default gateway router. These commands and
other optional commands are shown in the table.
Task IOS Command
Define the address pool. network network-number [mask | / prefix-length]
Define the default router or gateway. default-router address [ address2….address8]
Define a DNS server. dns-server address [ address2…address8]
Define the domain name. domain-name domain
Define the duration of the DHCP lease. lease {days [hours [ minutes]] | infinite}
Define the NetBIOS WINS server. netbios-name-server address [ address2…address8]

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Configuration Example

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DHCPv4 Verification
Use the commands in the table to verify that the Cisco IOS DHCPv4 server is operational.
Command Description
show running-config | section dhcp Displays the DHCPv4 commands configured on the router.
show ip dhcp binding
Displays a list of all IPv4 address to MAC address bindings provided by the
DHCPv4 service.
show ip dhcp server statistics
Displays count information regarding the number of DHCPv4 messages
that have been sent and received

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Verify DHCPv4 is Operational
Verify the DHCPv4 Configuration: As shown in the example, the show running-config
| section dhcp command output displays the DHCPv4 commands configured on R1.
The | section parameter displays only the commands associated with DHCPv4
configuration.

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Verify DHCPv4 is Operational (Cont.)
Verify DHCPv4 Bindings: As shown in the example, the operation of DHCPv4 can be
verified using the show ip dhcp binding command. This command displays a list of all
IPv4 address to MAC address bindings that have been provided by the DHCPv4 service.

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Verify DHCPv4 Statistics: The output
of the show ip dhcp server
statistics is used to verify that
messages are being received or sent by
the router. This command displays count
information regarding the number of
DHCPv4 messages that have been sent
and received.

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Verify DHCPv4 Client Received IPv4
Addressing: The ipconfig
/all command, when issued on PC1,
displays the TCP/IP parameters, as
shown in the example. Because PC1
was connected to the network
segment 192.168.10.0/24, it
automatically received a DNS suffix,
IPv4 address, subnet mask, default
gateway, and DNS server address
from that pool. No DHCP-specific
router interface configuration is
required. If a PC is connected to a
network segment that has a DHCPv4
pool available, the PC can obtain an
IPv4 address from the appropriate
pool automatically.

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Disable the Cisco IOS DHCPv4 Server
The DHCPv4 service is enabled by
default. To disable the service, use
the no service dhcp global
configuration mode command. Use
the service dhcp global
configuration mode command to re-
enable the DHCPv4 server process,
as shown in the example. Enabling
the service has no effect if the
parameters are not configured.
Note: Clearing the DHCP bindings
or stopping and restarting the DHCP
service may result in duplicate IP
addresses being temporarily
assigned on the network.

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DHCPv4 Relay
• In a complex hierarchical network, enterprise servers are usually located centrally. These servers may
provide DHCP, DNS, TFTP, and FTP services for the network. Network clients are not typically on the
same subnet as those servers. In order to locate the servers and receive services, clients often use
broadcast messages.
• In the figure, PC1 is attempting to acquire an IPv4 address from a DHCPv4 server using a broadcast
message. In this scenario, R1 is not configured as a DHCPv4 server and does not forward the
broadcast. Because the DHCPv4 server is located on a different network, PC1 cannot receive an IP
address using DHCP. R1 must be configured to relay DHCPv4 messages to the DHCPv4 server.

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DHCPv4 Relay (Cont.)
• Configure R1 with the ip helper-address address interface configuration command.
This will cause R1 to relay DHCPv4 broadcasts to the DHCPv4 server. As shown in
the example, the interface on R1 receiving the broadcast from PC1 is configured to
relay DHCPv4 address to the DHCPv4 server at 192.168.11.6.
• When R1 has been configured as a DHCPv4 relay agent, it accepts broadcast
requests for the DHCPv4 service and then forwards those requests as a unicast to the
IPv4 address 192.168.11.6. The network administrator can use the show ip
interface command to verify the configuration.

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Other Service Broadcasts Relayed
DHCPv4 is not the only service that the router can be configured to relay. By default,
the ip helper-address command forwards the following eight UDP services:
• Port 37: Time
• Port 49: TACACS
• Port 53: DNS
• Port 67: DHCP/BOOTP server
• Port 68: DHCP/BOOTP client
• Port 69: TFTP
• Port 137: NetBIOS name service
• Port 138: NetBIOS datagram service

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Packet Tracer – Configure DHCPv4
In this Packet Tracer Activity, you will complete the following objectives:
• Part 1: Configure a Router as a DHCP Server
• Part 2: Configure DHCP Relay
• Part 3: Configure a Router as a DHCP Client
• Part 4: Verify DHCP and Connectivity

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7.3 Configure a DHCPv4
Client

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Configure a DHCPv4 Client
Cisco Router as a DHCPv4 Client
There are scenarios where you might have access to a DHCP server through your ISP. In these
instances, you can configure a Cisco IOS router as a DHCPv4 client.
• Sometimes, Cisco routers in a small office or home office (SOHO) and branch sites have to be
configured as DHCPv4 clients in a similar manner to client computers. The method used depends
on the ISP. However, in its simplest configuration, the Ethernet interface is used to connect to a
cable or DSL modem.
• To configure an Ethernet interface as a DHCP client, use the ip address dhcp
interface configuration mode command.
• In the figure, assume that an ISP has been configured to provide select customers with IP
addresses from the 209.165.201.0/27 network range after the G0/0/1 interface is configured with
the ip address dhcp command.

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Configuration Example
• To configure an Ethernet interface as a DHCP client, use the ip address dhcp interface configuration
mode command, as shown in the example. This configuration assumes that the ISP has been configured to
provide select customers with IPv4 addressing information.
• The show ip interface g0/1 command confirms that the interface is up and that the address was allocated
by a DHCPv4 server.

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Home Router as a DHCPv4 Client
Home routers are typically already set to receive IPv4 addressing information automatically from the
ISP. This is so that customers can easily set up the router and connect to the internet.
• For example, the figure shows the default WAN setup page for a Packet Tracer wireless router.
Notice that the internet connection type is set to Automatic Configuration - DHCP. This
selection is used when the router is connected to a DSL or cable modem and acts as a DHCPv4
client, requesting an IPv4 address from the ISP.
• Various manufacturers of home routers will have a similar setup.

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7.4 Module Practice and Quiz

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Module Practice and Quiz
Packet Tracer – Implement DHCPv4
In this Packet Tracer, you will complete the following objectives:
• Part 1: Configure a Router as a DCHP Server
• Part 2: Configure DCHP Relay
• Part 3: configure a Router as a DCHP Client

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Lab – Implement DHCPv4
In this lab, you will complete the following objectives:
 Part 1: Build the Network and Configure Basic Device Settings
 Part 2: Configure and Verify Two DHCPv4 Servers on R1
 Part 3: Configure and Verify a DHCP Relay on R2

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What Did I Learn In This Module?
• The DHCPv4 server dynamically assigns, or leases, an IPv4 address to a client from a pool of addresses
for a limited period of time chosen by the server, or until the client no longer needs the address.
• The DHCPv4 lease process begins with the client sending message requesting the services of a DHCP
server. If there is a DHCPv4 server that receives the message it will respond with an IPv4 address and
possible other network configuration information.
• The client must contact the DHCP server periodically to extend the lease. This lease mechanism ensures
that clients that move or power off do not keep addresses that they no longer need.
• When the client boots (or otherwise wants to join a network), it begins a four-step process to obtain a
lease: DHCPDISCOVER, then DHCPOFFER, then DHCPREQUEST, and finally DHCPACK. Prior to lease
expiration, the client begins a two-step process to renew the lease with the DHCPv4 server:
DHCPREQUEST then DHCPACK.
• A Cisco router running Cisco IOS software can be configured to act as a DHCPv4 server.
• Use the following steps to configure a Cisco IOS DHCPv4 server: exclude IPv4 addresses, define a
DHCPv4 pool name, and configure the DHCPv4 pool.
• Verify your configuration using the show running-config | section dhcp, show ip dhcp binding, and
show ip dhcp server statistics commands.
• The DHCPv4 service is enabled, by default. To disable the service, use the no service dhcp global
configuration mode command.

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What Did I Learn In This Module? (Cont.)
• Network clients are not typically on the same subnet as enterprise servers providing DHCP, DNS, TFTP, and
FTP services for the network. In order to locate the servers and receive services, clients often use broadcast
messages. The router must be configured to relay DHCPv4 messages to the DHCPv4 server.
• The network administrator can configure the router with the ip helper-address address interface configuration
command and use the show ip interface command to verify the configuration.
• By default, the ip helper-address command forwards the following eight UDP services:
• Port 37: Time
• Port 49: TACACS
• Port 53: DNS
• Port 67: DHCP/BOOTP server
• Port 68: DHCP/BOOTP client
• Port 69: TFTP
• Port 137: NetBIOS name service
• Port 138: NetBIOS datagram service
• To configure an Ethernet interface as a DHCP client, use the ip address dhcp interface configuration mode
command.
• Home routers are typically already set to receive IPv4 addressing information automatically from the ISP. The
internet connection type is set to Automatic Configuration - DHCP. This selection is used when the router is
connected to a DSL or cable modem and acts as a DHCPv4 client, requesting an IPv4 address from the ISP.

Group-7-DHCPv4.pptx

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Group-7-DHCPv4.pptx