Chapter 36 - Implementing DHCP

Question 1

What is DHCP?

Answer 1

• Dynamic Host Configuration Protocol
• A protocol that a host uses to dynamically learn its IP address, subnet mask, DNS server, and default gateway

Question 2

Benefits of DHCP over static assignment

Answer 2

• Less room for user error
• Mobility

Question 3

What are the four different types of DHCP messages?

Answer 3

• Discover - Sent by a DHCP Client to find a DHCP Server (Broadcast).
• Offer- Sent by a DHCP Server to offer a specific IP address (along with other information) as a lease to a DHCP Client. If the DHCP Client allows receipt of unicast packets before having IP configuration established it will be sent as Unicast. Otherwise it will be sent as Broadcast.
• Request - Sent by a DHCP Client to ask the DHCP Server to lease the IP address listed in the previous Offer message. Sent as Broadcast.
• Acknowledgement - Sent by a DHCP server to assign the address and list the other necessary network information. Can be sent as Broadcast or Unicast.

Question 4

What addresses will a DHCP Client use as its src and dst addresses if it does not already have an IP when sending a DHCP Discover?

Answer 4

• If sent via broadcast:
  src: 0.0.0.0
  dst: 255.255.255.255

Question 5

What addresses will a DHCP Server use as its src and dst addresses when responding to a DHCP Discover message?

Answer 5

• If sent via broadcast:
  src: <DHCP>
  dst: 255.255.255.255</DHCP>

Question 6

How does a host know whether a DHCP Offer is meant for it or not when sent via broadcast?

Answer 6

• As the Offer is sent via broadcast, all hosts on the link will receive it
• In the original Discover message, a client ID (chaddr) is included which includes the sending host’s MAC address
• The responding server includes the client ID in its Offer so that only the host with the same client ID will know that the Offer is meant for it
• The other hosts will ignore the Offer

Question 7

What does the ‘ip helper-address <server-ip>' command do?</server-ip>

Answer 7

Tells an interface that receives messages from DHCP clients to:
- Watch for incoming DHCP messages with a destination IP of 255.255.255.255
- Change the packet’s source IP to the router’s incoming interface IP address
- Change the packet’s destination IP to that of the DHCP server (listed in the command)
- Route the packet to the DHCP server

Question 8

What does a DHCP relay do?

Answer 8

• A DHCP relay is used when a subnet does not contain its own DHCP server, but there is a DHCP server somewhere else on the network that DHCP client messages should be forwarded to.

Question 9

What information does a DHCP server need to know in order to serve a host?

Answer 9

• Subnet ID and mask - The DHCP server will use this information to know all addresses in a subnet and to not hand out the network and broadcast addresses
• Reserved (excluded) addresses - Addresses that cannot be leased, for example, addresses that have been statically assigned
• Default router(s) - The IP address of the routers on the subnet
• DNS server IP addresses - Self explanatory

Question 10

What are the 3 methods of allocation that a DHCP server can use?

Answer 10

• Dynamic Allocation - The default
• Automatic Allocation - Sets the DHCP lease time to infinite.
• Static Allocation - Preconfigures an address for a specific client at the server based on the client’s MAC address.

Question 11

True or False. A DHCP server can also be used to provide the IP address of a TFTP server on the network that devices should use.

Answer 11

True.

Question 12

What are the two rules for configuring a DHCP relay on a network?

Answer 12

• Does the network contain DHCP clients?
• Does the network not contain a DHCP server?
  If both are true then a DHCP relay is required on the interface connecting to that subnet from the router.

Question 13

What command(s) can you use to see if an interface is being used as a DHCP Relay?

Answer 13

• ‘show ip int <int>'
  - If it is then it will show "Helper address is <DHCP>"</DHCP></int>
• ‘show running-config’

Question 14

What command can you use to cause a switch or router interface to get its IP via DHCP?

Answer 14

‘ip address dhcp’ in interface config mode

Question 15

How can you find out if a switch/router interface has had its IP configured via DHCP?

Answer 15

Use the ‘show dhcp lease’ command

Question 16

True or False. A router or switch using DHCP to obtain its IP address will only show the IP if DHCP has been successful.

Answer 16

True.

Question 17

When looking at a Cisco routing table, how can you tell if a route has been created from information learned via DHCP?

Answer 17

• The administrative distance by default will be 254
• IOS oddly lists them in the legend as ‘S’ meaning static

Question 17

What information does a host need to learn from a DHCP server?

Answer 17

• DNS server IP
• Its own IP
• Its own subnet mask
• The gateway for its subnet

Question 18

What extra information does the Windows command ‘ipconfig /all’ show above just ‘ipconfig’?

Answer 18

• ‘ipconfig /all’ shows all IP information for the host including the DNS server(s)
• ‘ipconfig’ only shows the host’s IP, subnet mask, and default gateway

Question 19

What command can you use on a Windows device to see its routing table?

Answer 19

‘netstat -rn’

Question 20

What commands does MacOS use that is similar to Windows’ ‘ipconfig’ command?

Answer 20

• ‘ifconfig’ - Shows the host’s IP address and subnet mask
• ‘networksetup -getinfo’ - Shows the host’s IP address, subnet mask, and default gateway
• ‘networksetup -getdnsservers’ - Shows the host’s DNS servers

Question 21

What command can you use on MacOS to see a device’s routing table?

Answer 21

‘netstat -rn’

Question 22

True or False. ‘ifconfig’ only works for MacOS.

Answer 22

False. It also works for older Linux distributions (or ones where it has been enabled) and does the same as it does on MacOS (shows the host’s IP address and subnet mask)

It has since been replaced by the ‘ip address’ command.

Question 23

True or False. ‘netstat -rn’ only works for MacOS.

Answer 23

False. It also works for older Linux distributions (or ones where it has been enabled) and does the same as it does on MacOS (shows the routing table)

It has since been replaced by the ‘ip route’ command.

Question 24

When using the 'ipconfig /all' command, what does 'Preferred' mean next to the device's IP address?

Answer 24

It means that this device has been given that IP address before so if its lease is renewed it will ask to receive that address again.

Question 25

What is a DHCP Release message?

Answer 25

A message sent from a client to a DHCP server to advise that its address is no longer required and therefore can be put back into the pool of available addresses. Sent as unicast.

Question 26

What does the Bootp Flags field in a DHCP packet do?

Answer 26

- Tells the receiver of the packet whether to respond with Broadcast or Unicast. - For example if a PC sends a DHCP Discover with Bootp Unicast, the server will respond with unicast (e.g. with the client's MAC and IP rather than 255.255.255.255).

Question 27

What commands would you use to configure a Cisco router as a DHCP server?

Answer 27

- To specify a range of addresses that won't be given to clients 'ip dhcp excluded-address ' - To create a DHCP pool 'ip dhcp pool ' - To assign an address range to be handed out by the pool (from dhcp config mode) 'network

' - To assign a DNS server address to the pool (from dhcp config mode) 'dns-server

' - To specify the domain name of the network (from dhcp config mode) 'domain-name ' - To specify the default gateway (from dhcp config mode) 'default-router ' - To specify the lease time (from dhcp config mode) 'lease ' OR 'lease infinite'

Question 28

What command can you use to see all dhcp clients that have received addresses from a router acting as a DHCP server?

Answer 28

'show ip dhcp binding'

Question 29

What is a DHCP Decline message?

Answer 29

A message that a client uses to decline the lease of an IP address from a server during the DORA process.