Chapter 8: DHCP

Chapter 8: DHCP

1. The DHCPv4 server dynamically allocates, or leases, an IPv4 address from a set of addresses for a limited period of time chosen by the server or until the client no longer needs the address.

2. Operation of DHCPv4

   • Origin of the lease
     • DHCP detection (DHCPDISCOVER)
     • DHCP offer (DHCPOFFER)
     • DHCP request (DHCPREQUEST)
     • DHCP acknowledgment (DHCPACK)
   • Renovation of the lease
     • DHCP request (DHCPREQUEST)
     • DHCP acknowledgment (DHCPACK)

3. DHCPv4 message format

   • Operation code (OP)
     • 1 indicates a request message and the value 2 is a response message.
   • Type of hardware
     • 1 is Ethernet, 15 is Frame Relay and 20 is a serial line
   • Hardware address length
     • the length of the address.
   • Jumps
     • controls the forwarding of messages
   • Transaction identifier
     • the client uses it to match the request with responses received from the servers
   • Seconds
     • prioritize responses when there are several pending client requests
   • Indicators
     • The value 1 in this field tells the DHCPv4 server that the response should be sent as a broadcast.
   • Client's IP address
     • The client places its own IPv4 address in this field only if it has a valid IPv4 address while it is in the linked state
   • Your IP address
     • it is used by the server to assign an IPv4 address to the client.
   • IP address of the server
     • is used by the server to identify the server address that the client should use
   • IP address of the gateway
     • Facilitates communications of DHCPv4 requests and responses between the client and a server
   • Customer hardware address
     • specifies the physical layer of the client.
   • Server name
     • text nickname or a DNS domain name
   • Boot file name
     • it is used by a client in an optional way to request a certain type of boot file
   • DHCP options
     • contains the DHCP options = basic operation

4. Discover (Discovery) and Offer (Offer) messages from DHCPv4

   • The client transmits a DHCPDISCOVER message on its local network when it starts or detects an active network connection, DHCPDISCOVER is an IPv4 broadcast. DHCPOFFER.
   • This message includes initial configuration information for the client.

5. Configuring a basic DHCPv4 server

   • Step 1: Exclude IPv4 addresses
     • use the ip dhcp excluded-address command
   • Step 2: Configure a DHCPv4 pool
     • set of addresses that must be assigned. Create a set with the specified name and place the router in configuration mode.
   • Step 3: Configure specific tasks
     • the tasks to finish the configuration of the DHCPv4 pool are indicated
   • Disabling DHCPv4
     • To disable the service, use the global configuration mode no service dhcp command.

6. Verification of DHCPv4

   • No specific DHCP router interface configuration is required. If a computer is connected to a network segment that has a pool of DHCPv4 available, the computer can obtain an IPv4 address from the appropriate pool automatically.

7. Relay of DHCPv4

   • To locate servers and receive services, clients often use broadcast messages.

8. Configuring a router as a DHCPv4 client

   • The Ethernet interface is used to connect to a cable modem or a DSL modem. Use the dhcp ip address interface configuration mode command.

9. Configuring a wireless router as a DHCPv4 client

   • they connect to an ISP using a cable modem or DSL.

10. Problem solving tasks

    • show ip dhcp conflict muestra todos los conflictos de direcciones que registran el servidor de DHCPv4

11. Verification of router DHCPv4 configuration

    • Step 1
      • Verify that the ip helper-address command is configured in the correct interface.
    • Step 2
      • Verify that the global configuration command no service dhcp has not been configured.

12. Debugging DHCPv4

    • The extended ACL is used with the debug ip packet command to display only DHCPv4 messages.

13. Automatic state-independent address configuration (SLAAC)

    • Automatic stateless address configuration (SLAAC).
      • Router request message (RS)
      • Router announcement message (RA)
    • Dynamic host configuration protocol for IPv6 (DHCPv6 with status)

14. Operation of SLAAC

    • EUI-64: mediante el proceso EUI-64, la PC1 crea una IID utilizando su dirección MAC de 48 bits.
    • Random generation: the 64-bit IID can be a random number generated by the client operating system.

15. SLAAC and DHCPv6

    • SLAAC (router advertisement only)
    • DHCPv6 without status (router advertisement and DHCPv6)
    • DHCPv6 with status (DHCPv6 only)

16. SLAAC option

    • This option tells the client to use the information that is included in the RA message exclusively.

17. DHCPv6 option without status

    • stateless informs the client to use the RA message information for addressing, but that there are more configuration parameters available from a DHCPv6 server.

18. DHCPv6 operations

    • DHCPv6 client without status
      • the client sends a DHCPv6 INFORMATION-REQUEST message to the DHCPv6 server where it requests only configuration parameters, such as the DNS server address.
    • DHCPv6 client with status
      • the client sends a DHCPv6 REQUEST message to the server to obtain an IPv6 address and all other server configuration parameters.

19. Configuring a router as a stateless DHCPv6 server

    • Step 1: Enable IPv6 routing = ipv6 unicast-routing
    • Step 2: Configure a DHCPv6 pool = ipv6 dhcp pool pool-name
    • Step 3: Configure the pool parameters
    • Step 4: Configure the DHCPv6 interface = ipv6 dhcp server package-name

20. Configuring a router as a DHCPv6 client without status

    • The ipv6 address autoconfig command enables automatic configuration of IPv6 addressing through SLAAC.

21. Verification of DHCPv6 without status

    • The debug output shows all DHCPv6 messages sent between the client and the server, including the DNS server and domain name options that were configured on the server.

22. Configuring a router as a DHCPv6 client with status

    • The ipv6 address dhcp interface configuration mode command enables the router to function as a DHCPv6 client on this interface.

23. Verification of DHCPv6 with status

    • the show ipv6 dhcp pool command verifies the pool name of DHCPv6 and its parameters

24. Configuring a router as a DHCPv6 relay agent

    • The configuration of a DHCPv6 relay agent is similar to the configuration of an IPv4 router as a DHCPv4 relay.

25. Problem solving tasks

    • The problem resolution process of DHCPv6 is similar to that of DHCPv4.

26. Verification of router DHCPv6 configuration

    • the ipv6 interface configuration mode command nd managed-config-flag is used.
    • the interface configuration mode command ipv6 nd other-config-flag is used

27. Debugging DHCPv6

    • The debug ipv6 dhcp detail command is useful for verifying the reception and transmission of DHCPV6 messages.