CNT 150 Chapter 7 DHCPv4

DHCPv4 works in what kind of mode ?

Client/Server

When the client boots (or otherwise wants to join a network), it begins a four-step process to obtain a lease:

DHCP Discover (DHCPDISCOVER) DHCP Offer (DHCPOFFER) DHCP Request (DHCPREQUEST) DHCP Acknowledgment (DHCPACK)

may perform an ARP lookup for the assigned address. If there is no reply to the ARP, the client knows that the IPv4 address is valid and starts using it as its own.

DHCP Request (DHCPREQUEST) DHCP Acknowledgment (DHCPACK)

Which two DHCPv4 messages are sent by the server in the lease obtaining process ? DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPACK

DHCPOFFER DHCPACK

Which two DHCPV4 messages are used in the lease renewal process ? DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPACK

DHCPREQUEST DHCPACK

Which message is sent by a DHCPv4 client to initiate the lease obtaining process ? DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPACK

DHCPSDISCOVER

What is inside a DHCPDISCOVER message ?

It contains the devices source MAC address and IP address, and its destination IP, and MAC are broadcast addresses. The purpose of the DHCPDISCOVER message is to find DHCPv4 servers on the network.

Why does DHCP use the lease mechanism ?

It endures that clients that move or power off do not keep addresses they no longer need.

A DHCPREQUEST message is used for what two things ?

It is use for both lease origination(first time request) and lease renewal.

What is the purpose of the DHCPDISCOVER message ?

It is used to find DHCPv4 servers on the network.

What is the job of DHCPv4 ?

It is used to lease IPv4 address from a available pool and other network info to dynamic configure host devices.

When the client receives the DHCPACK message, it does what with the information ?

It logs the configuration information and may perform an ARP lookup for the assigned address. If there is no reply to the ARP, the client knows that the IPv4 address is valid and starts using it as its own.

When the DHCPv4 server receives a DHCPDISCOVER message , what three things happen next ?

It reserves an available IPv4 address to lease to the client, it also creates an ARP entry consisting of the MAC address of the requesting client and the leased IPv4 address of the client, and finally it sends the binding DHCPOFFER message to the requesting client.

Before the lease expires , the client sends what type of message to renew the leased ipv4 address ?

It sends a DHCPREQUEST message directly to the DHCPv4 server that originally offered the IPv4 address.

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

Because the DHCPv4 server is located on a different network , PC1 cannot receive an IP address using DHCP. What must be done R1 for the DHCPv4 server to receive the DHCPDISCOVER message from PC1?

R1 must be configured to relay DHCPv4 messages to the DHCPv4 server.

The router functioning as the DHCPv4 server assigns all IPv4 addresses in a DHCPv4 address pool unless it is configured to exclude specific addresses. Why would you exclude specific addresses ?

Some IPv4 addresses in a pool are assigned to network devices that required static address assignment. Therefore , these IPv4 addresses should not be assigned to other devices.

Use the following steps to configure a Cisco IOS DHCPv4 server:

Step 1: Exclude IPv4 addresses Step 2: Define a DHCPv4 pool name Step 3: Configure the DHCPv4 pool

When setting up DHCP what two things need to be configured ?

The address pool and default gateway router must be configured.

If a DHCPACK is not received within a specified amount of , time the client will do what to renew the leased IPv4 address ?

The client broadcasts another DHCPREQUEST so that one of the other DHCPv4 servers can extend the lease.

How does a PC start the DHCP process ?

The client start the process using a broadcast DHCPDISCOVER message with its own MAC address to discover available DHCPv4 servers.

If a client broadcast a DHCPDISCOVER message and the router is not a DHCP server what happens to the message ?

The message is dropped there is no server to reply. In this scenario, R1 is not configured as a DHCPv4 server and does not forward the broadcast

What happens when the DHCPV4 server receives an DHCPREQUEST message ?

The server verifies the lease information with an ICMP ping to that address to ensure it is not being used already, it will create a new ARP entry for the client lease , and reply with a DHCPACK message.

When would a router be used as a DHCP client ?

There are scenarios where you might have access to a DHCP server through your ISP.

True or False ? These messages (primarily the DHCPOFFER and DHCPACK) can be sent as unicast or broadcast according to IETF RFC 2131.

True

When the client receives the DHCPOFFER from the server, it sends back what ?

a DHCPREQUEST message

What DHCP mode command is used to define the default gateway router ?

default-router (address) (adddress2)

What DHCP mode command is used to assign a DNS server ?

dns-server (address)

What DHCP mode command is used to define the domain name of the dns server ?

domain-name (domain)

To configure an Router Ethernet interface as a DHCP client, what interface configruation mode command is used ?

ip address dhcp

What global config command is used to exclude address from the DHCP pool ?

ip dhcp excluded-address (low-address) (high-address)

What global config command is used to create a dhcp pool and enter dhcp configuration mode ?

ip dhcp pool (pool-name)

What interface config mode command on a router is used to set the interface to relay DHCPDISCOVER messages to a specific DHCPv4 server ?

ip helper-address (ip address of the server )

What windows command is used to release an IPv4 address ?

ipconfig /release

What windows command is used to renew a ipv4 address ?

ipconfig /renew

What DHCP mode command is used to change the lease time of addresses ?

lease {days [hours [ minutes]] | infinite}

What DHCP mode command is used to define the NetBIOS WINS server ?

netbios-name-server address [ address2...address8] Note: Microsoft recommends not deploying WINS, instead configure DNS for Windows name resolution and decommission WINS.

What DHCP mode command is used to define the range of available addresses ?

network (network-number (mask | / prefix-length))

What global config command is used to disable dhcp service on a Cisco router ?

no service dhcp Note: Clearing the DHCP bindings or stopping and restarting the DHCP service may result in duplicate IP addresses being temporarily assigned on the network.

What global config command is used to enable dhcp service on a Cisco router ?

service dhcp

Displays a list of all IPv4 address to MAC address bindings provided by the DHCPv4 service ?

show ip dhcp binding

Displays count information regarding the number of DHCPv4 messages that have been sent and received.

show ip dhcp server statistics

What show command on a router can be used to verify relay configuration ?

show ip interface (int id)

Displays the DHCPv4 commands configured on the router ?

show running-config | section dhcp

When the DHCPREQUEST message is used for lease origination what happens ?

the DHCPREQUEST servers as a binding acceptance notice to the selected server for the info it was offered and implicit decline to any other servers that may have provided the client a binding offer. NOTE: DHCPREQUEST leas origination is a broadcast message.