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Chapter 8 DHCP (DHCPv4 (Configuring a Basic DHCPv4 Server (Configure…

- - - - DHCPv4 assigns (leases) IPv4 addresses and other network configuration information dynamically.
        
        In a small branch or SOHO location, a Cisco router can be configured to provide DHCPv4 services without the need for a dedicated 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.
    - - DHCPv4 works in a client/server mode
        
        Lease Origination
        
        When the client boots (or otherwise wants to join a network), it begins a four step process to obtain a lease.
        
        D O R A / B U B U
        
        DHCP Offer (DHCPOFFER): When the DHCPv4 server receives a DHCPDISCOVER message, it reserves an available IPv4 address to lease to the client. the DHCPv4 server sends the binding DHCPOFFER message to the requesting client.
        
        The server also creates an ARP entry consisting of the MAC address of the requesting client and the leased IPv4 address of the client
        
        DHCP Request (DHCPREQUEST): When the client receives the DHCPOFFER from the server, it sends back a DHCPREQUEST broadcast message
        
        This message is used for both lease origination and lease renewal.
        
        the DHCPREQUEST serves as a binding acceptance notice to the selected server for the parameters it has offered and an implicit decline to any other servers that may have provided the client a binding offer.
        
        DHCP Acknowledgment (DHCPACK): the server verifies the lease information with an ICMP ping to that address to ensure it is not being used already,
        
        Creates a new ARP entry for the client lease, and replies with a DHCPACK message
        
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        The DHCPACK message is a duplicate of the DHCPOFFER, except for a change in the message type field.
        
        DHCP Discover (DHCPDISCOVER): a client starts the process with a broadcast DHCPDISCOVER message with its own MAC address to discover available DHCPv4 servers.
        
        The client uses Layer 2 and Layer 3 broadcast addresses to communicate with the server.
  - - - 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.
        
        Step 1. Excluding IPv4 Addresses
        
        some IPv4 addresses in a pool are assigned to network devices that require static address assignments. Therefore, these IPv4 addresses should not be assigned to other devices
        
        A single address or a range of addresses can be excluded by specifying the low-address and high-address of the range
        
        Step 2. Configuring a DHCPv4 Pool
        
        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 this prompt Router(dhcp-config)#.
        
        R1(config)ip dhcp pool myDHCPpool
        
        Step 3. Configuring Specific Tasks
        
        Disabling DHCPv4
        
        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.
    - - The show running-config | section dhcp command output displays the DHCPv4 commands configured on a router.
        
        the operation of DHCPv4 can be verified using the show ip dhcp binding command.
        
        show ip dhcp server statistics, is used to verify that messages are being received or sent by the router.
    - - Enterprise servers are usually located in a server farm. 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.
        
        If a outer router is not configured as a DHCPv4 server, it does not forward broadcast packets. Because the DHCPv4 server is located on a different network, a PC cannot receive an IP address using DHCP.
        
        A solution is to configure a Cisco IOS helper address. This solution enables a router to forward DHCPv4 broadcasts to the DHCPv4 server.
        
        When a router forwards address assignment/parameter requests, it is acting as a DHCPv4 relay agent.
        
        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
        
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    - - A Cisco 2960 switch can function as a DHCPv4 server.
        
        Assings and manages IPv4 addresses from identified address pools that are associated with specific VLANs ans SVIs
        
        The Cisco Switch Database Manager (SDM) provides multiple templates for the Cisco 2960 switch.
        
        Templates can be enabled to support specific roles depending on how the switch is used in the network
        
        Display current SDM preference: S1# show sdm prefer
        
        The default template does not
        support static routing.
        
        If IPv6 addressing has been enabled, the template will be dual-ipv4-and-ipv6 default.
        
        Change the SDM Preference
        
        S1(config)# sdm prefer lanbase-routing
        Changes to the running SDM preferences have been stored, but cannot take effect until the next reload: S1# reload
        
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        The Cisco 2960 switch can also function as a Layer 3 device and route between VLANs and a limited number of static routes.
  - - - Troubleshooting Task 2: Verify Physical Connectivity
        
        First, use the show interfaces interface command to confirm that the router interface acting as the default gateway for the client is operational.
        
        If the state of the interface is anything other than up, the port does not pass traffic, including DHCP client requests.
      - Troubleshooting Task 1: Resolve IPv4 Address Conflicts
        
        An IPv4 address lease can expire on a client still connected to a network.
        
        If the client does not renew the lease, the DHCPv4 server can reassign that IPv4 address to another client
        
        When the client reboots, it requests an IPv4 address. If the DHCPv4 server does not respond quickly, the client uses the last IPv4 address.
        
        The situation then arises where two clients are using the same IPv4 address, creating a conflict.
        
        The server uses the ping command to detect clients. The client uses Address Resolution Protocol (ARP) to detect conflicts.
        
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        The show ip dhcp conflict command displays all address conflicts recorded by the DHCPv4 server
      - Troubleshooting Task 3: Test Connectivity using a Static IP Address
        
        When troubleshooting any DHCPv4 issue, verify network connectivity by configuring static IPv4 address information on a client workstation. If the workstation is unable to reach network resources with a statically configured IPv4 address, the root cause of the problem is not DHCPv4. At this point, network connectivity troubleshooting is required.
      - Troubleshooting Task 4: Verify Switch Port Configuration
        
        If the DHCPv4 client is unable to obtain an IPv4 address from the DHCPv4 server on startup, attempt to obtain an IPv4 address from the DHCPv4 server by manually forcing the client to send a DHCPv4 request.
        
        If there is a switch between the client and the DHCPv4 server, and the client is unable to obtain the DHCP configuration, switch port configuration issues may be the cause.
      - Troubleshooting Task 5: Test DHCPv4 Operation on the Same Subnet or VLAN
        
        If DHCPv4 is working correctly when the client is on the same subnet or VLAN, the problem may be the DHCP relay agent.
        
        If the problem persists even with testing DHCPv4 on the same subnet or VLAN as the DHCPv4 server, the problem may actually be with the DHCPv4 server.
    - - If the IPv4 helper address is not configured properly, client DHCPv4 requests are not forwarded to the DHCPv4 server.
        
        Verify that the global configuration command no service dhcp has not been configured.
        
        Verify that the ip helper-address command is configured on the correct interface
  - - - As a troubleshooting task, verify that the router is receiving the DHCPv4 request from the client. This troubleshooting step involves configuring an ACL for debugging output.
        
        The extended ACL is used with the debug ip packet command to display only DHCPv4 messages.
        
        Another useful command for troubleshooting DHCPv4 operation is the debug ip dhcp server events command. This command reports server events, like address assignments and database updates.