Chapter Overview

1
Chapter Overview

• Understanding DHCP
• Configuring a DHCP Server
• Troubleshooting DHCP

2
Understanding DHCP

• Dynamic Host Configuration Protocol (DHCP) is a
  service and a protocol that work together to
  automatically assign Internet Protocol (IP)
  addresses and other Transmission Control
  Protocol/Internet Protocol (TCP/IP) configuration
  settings to computers on a network.
• DHCP overcomes the limitations of manually
  configuring TCP/IP clients and tracking IP
  addresses.

3
DHCP Overview

• DHCP dynamically assigns IP addresses to clients
  from a pool.
• Each time a DHCP client starts, it requests IP
  addressing information from a DHCP server.
• When a DHCP server receives the request, it
  selects IP addressing information from a pool of
  addresses (called a scope) and offers it to the
  DHCP client.
• If the client accepts the offer, the IP
  addressing information is leased to the client
  for a specified time.

4
DHCP Overview (Cont.)

• As the lease interval progresses, the client
  computer renews the address assignment each time
  the computer is started.
• If the lease expires without a renewal, the IP
  address is returned to the pool for reassignment.
• DHCP is based on open standards published by the
  Internet Engineering Task Force (IETF).

5
DHCP Client/Server Interaction
6
Manual vs. Automatic Configuration

• Configuring TCP/IP manually on client computers
  has many limitations.
• Errors in assigning or typing IP addresses can
  lead to difficult-to-trace network problems.
• Manual configuration requires a large amount of
  administrative maintenance.
• Using DHCP to automatically configure IP
  addressing information overcomes all of these
  limitations.

7
DHCP Address Allocation Types

• The core function of DHCP is to assign IP
  addresses.
• There are three types of IP address allocation
  manual, automatic, and dynamic.
• Manual allocation an administrator assigns a
  specific IP address for the DHCP server to
  provide to a specific DHCP client.
• This method is necessary for some computers that
  require permanently assigned IP addresses.

8
DHCP Address Allocation Types (Cont.)

• Automatic allocation the DHCP server supplies
  clients with IP addresses taken from a common
  pool, and the clients permanently retain the
  assigned addresses.
• This method works well when computers are rarely
  moved.
• Dynamic allocation the DHCP server supplies IP
  addresses to clients from a pool on a leased
  basis.
• The client must periodically renew the lease or
  the address expires and is returned to the pool.
• Most DHCP installations use dynamic allocation.

9
How DHCP Works

• A typical transaction between a DHCP client and
  server has four phases
• DHCPDISCOVER the DHCP client requests
  configuration parameters from a DHCP server.
• DHCPOFFER the DHCP server offers IP addressing
  information to the requesting client.
• DHCPREQUEST the client accepts or renews an IP
  address assignment.
• DHCPACK the DHCP server acknowledges the
  clients acceptance of an offered IP address.

10
DHCP Client/Server Communications
11
DHCP Communications

• When a DHCP client computer is booted, it
  initiates communication with a DHCP server by
  generating a series of DHCPDISCOVER broadcast
  messages.
• At this point, the client has no IP address and
  is said to be in the init state.
• The clients broadcasts are limited to the local
  subnet unless a DHCP relay agent is used, in
  which case the broadcasts are relayed to DHCP
  servers on other subnets.

12
DHCP Communications (Cont.)

• When a DHCP server receives a DHCPDISCOVER
  message from a client, it generates a DHCPOFFER
  message containing IP addressing information and
  other optional TCP/IP parameters.
• The client might receive DHCPOFFER messages from
  multiple DHCP servers.
• The client accepts one of the offered IP
  addresses by generating a DHCPREQUEST broadcast
  message that contains
• The address of the server that offered the
  address
• The offered IP address

13
DHCP Communications (Cont.)

• When the server receives the DHCPREQUEST, it
• Commits the offered IP addressing information to
  its database
• Sends a DHCPACK message to the client,
  acknowledging the completion of the process
• If the server cannot complete the address
  assignment, it transmits a DHCPNAK message to the
  client and the process begins again.
• Finally, the client performs an Address
  Resolution Protocol (ARP) test to ensure that no
  other system on the network is using the assigned
  IP address.

14
DHCP Leasing

• When a DHCP server dynamically allocates
  settings, the client leases its IP address for a
  specified time and must renew the lease to
  continue using it.
• An IP address lease is typically measured in
  days.
• The lease renewal process begins when a bound
  client reaches renewal time value (or T1 value)
  of its lease and begins generating DHCPREQUEST
  unicast messages to the server that holds the
  lease.

15
DHCP Leasing (Cont.)

• If the DHCP server is available, it responds with
  one of the following messages
• A DHCPACK message, which renews the lease and
  restarts the lease timer
• A DHCPNAK message, which terminates the lease and
  forces the client to begin again
• If the server does not respond, the client begins
  transmitting DHCPREQUEST broadcast messages,
  soliciting an address assignment from any DHCP
  server on the network.
• If the lease expires
• The clients IP address is released
• All of the clients TCP/IP communication ceases,
  except for DHCPDISCOVER broadcast transmissions

16
The DHCP Lease Renewal Process
17
Releasing an IP Address

• A DHCP client can terminate an IP address lease
  at any time by transmitting a DHCPRELEASE message
  to the DHCP server.
• On a computer running Microsoft Windows 2000, use
  the Ipconfig.exe utility to manually release the
  client computers IP address.
• On a computer running Microsoft Windows Me,
  Microsoft Windows 98, or Microsoft Windows 95,
  use the Winipcfg.exe utility.

18
When No DHCP Servers Are Online

• If there is no response from a DHCP server, a
  client computer running Windows 2000 uses
  Automatic Private IP Addressing to automatically
  assign itself an IP address and subnet mask.
• The IP address is from the range 169.254.0.1
  through 169.254.255.254.
• The subnet mask is 255.255.0.0.
• The client continues to check for a DHCP server
  every five minutes.
• If it contacts a DHCP server, the client abandons
  its autoconfigured IP address.

19
Installing a DHCP Server

• Before installing DHCP on a server, consider
  these questions
• Will all of the computers on the network be DHCP
  clients?
• Will a DHCP server supply IP addresses to
  multiple local area networks (LANs)?
• How many DHCP servers are required?
• What IP addressing options will clients obtain
  from a DHCP server?
• Use Add/Remove Programs to install a DHCP server.
• The computer running the DHCP Server service
  should not be a DHCP client.
• You should manually configure a DHCP server.

20
DHCP Relay Agent

• When a router running a DHCP relay agent receives
  broadcasts from DHCP clients, it relays them to
  DHCP servers on other subnets.
• A DHCP relay agent enables a single DHCP server
  to support clients on multiple subnets.
• You should not configure a server running the
  DHCP Server service to function as a DHCP relay
  agent as well.
• To add the DHCP Relay Agent to a computer running
  Windows 2000 Server that acts as a router, use
  Routing And Remote Access.

21
Lesson Summary

• DHCP is a service and a protocol that work
  together to automatically assign IP addresses and
  other TCP/IP configuration settings to computers
  on a network.
• DHCP can allocate IP addresses in three ways
  manual allocation, automatic allocation, and
  dynamic allocation.
• A typical transaction between a DHCP client and
  server has four phases DHCPDISCOVER, DHCPOFFER,
  DHCPREQUEST, and DHCPACK.
• A DHCP relay agent relays DHCP and Bootstrap
  Protocol (BOOTP) messages between clients and
  servers located on different subnets.

22
Configuring a DHCP Server

• As an administrator, you need to know how to
• Authorize a DHCP server
• Create and manage DHCP scopes
• Configure additional DHCP options
• Activate a scope
• Implement multiple DHCP servers

23
Installing and Configuring a DHCP Server

• After you install the DHCP Server service on a
  computer running Windows 2000 Server, you must
  configure the server before it can assign TCP/IP
  settings to clients.
• To implement DHCP
• 1. Install the Microsoft DHCP Server service.
  2. Authorize the DHCP server.
  3. Create an IP address scope and
  activate it. 4. Configure DHCP options.

24
Authorizing a DHCP Server

• When a misconfigured or unauthorized DHCP server
  (called a rogue server) is introduced on a
  network, it can cause problems.
• To avoid the problem of rogue DHCP servers, DHCP
  servers on a Windows 2000 network must be
  authorized in the Active Directory service before
  they can service clients.
• All Windows 2000 DHCP servers must be either
  domain controllers or member servers to be
  authorized in Active Directory and provide DHCP
  service to clients.
• You can authorize a DHCP server by using the DHCP
  console.

25
The Manage Authorized Servers Dialog Box
26
Creating a DHCP Scope

• When creating a DHCP scope, consider the
  following
• You must create a least one scope for every DHCP
  server.
• You must exclude any static IP addresses you
  manually assign from the scope.
• You must create a separate scope for each subnet.
• You can create only one scope for each subnet on
  a DHCP server.
• DHCP servers do not share scope information.

27
Creating a DHCP Scope (Cont.)

• Depending on the starting and ending IP addresses
  for your scope, the DHCP console suggests a
  default subnet mask.
• If your network requires a different subnet mask,
  you can modify this value.
• Use the DHCP console to create a DHCP scope.

28
The IP Address Range Page of the New Scope Wizard
29
The Lease Duration Page of the New Scope Wizard
30
Managing DHCP Scopes

• A new scope in the DHCP console has four
  subheadings
• Address Pool lists the IP addresses in the scope
• Address Leases lists the IP addresses that are
  currently leased by clients on the network
• Reservations contains IP address assignments for
  specific computers on the network
• Scope Options contains DHCP options that are
  assigned to clients obtaining IP addresses from
  the scope

31
Configuring Additional Options

• You can configure DHCP options to supply client
  computers with TCP/IP configuration parameters in
  addition to IP addresses and subnet masks.
• Commonly used DHCP options are
• 003 Router
• 006 DNS Servers
• 015 DNS Domain Name
• 044 WINS/NBNS Servers
• 046 WINS/NBT Node Type

32
Configuring Additional Options (Cont.)

• Options can be configured at the server or scope
  level.
• Use the DHCP console to configure additional DHCP
  options.
• To configure options for a server, right-click
  the Server Options heading, and then click
  Configure Options.
• To configure options for a scope, right-click the
  Scope Options heading, and then click Configure
  Options.

33
The DHCP Scope Options Dialog Box
34
Configuring the Router in the Scope Options
Dialog Box
35
Activating a Scope

• You must activate the scope before it can begin
  servicing clients.
• If you have not already done so with the New
  Scope Wizard, you can use the DHCP console to
  activate a scope.
• In the DHCP console, right-click the scope you
  want to activate, and then click Activate.

36
Implementing Multiple DHCP Servers

• If your internetwork requires multiple DHCP
  servers
• Create a unique scope for each subnet on each
  server
• Use a router that functions as a DHCP relay agent
  to connect the networks
• Each DHCP server should have
• A scope containing about 75 percent of the
  available IP addresses for the local subnet
• A scope for each remote subnet containing about
  25 percent of the available IP addresses for a
  subnet

37
Lesson Summary

• On a Windows 2000 network, a DHCP server must be
  authorized in Active Directory before it can
  service clients.
• DHCP must be installed on a Windows 2000 domain
  controller or member server in order to be
  authorized.
• Use the DHCP console to
• Create and activate a scope
• Configure DHCP options, such as routers and
  Domain Name System (DNS) servers, for a scope or
  for a server

38
Troubleshooting DHCP

• The most common DHCP client problem is a failure
  to obtain an IP address from a DHCP server.
• The most common DHCP server problems are
• The inability to start the DHCP Server service on
  the network
• The failure of clients to obtain TCP/IP
  configuration settings from a working server

39
Preventing DHCP Problems

• Many DHCP problems involve incorrect or missing
  configuration details.
• To prevent the most common DHCP problems
• Use the 75/25 design rule for balancing scope
  distribution of addresses when multiple DHCP
  servers are used to service the same scope
• Use superscopes for multiple DHCP servers on each
  subnet in a LAN environment
• Use server-side conflict detection on DHCP
  servers only when it is needed

40
Preventing DHCP Problems (Cont.)

• To prevent the most common DHCP problems (Cont.)
• Create reservations on all DHCP servers that can
  potentially service the reserved client
• For server performance, remember that DHCP is
  disk-intensive and purchase appropriate hardware
• Keep audit logging enabled for troubleshooting
  use
• Integrate DHCP with other services, such as
  Windows Internet Name Service (WINS) and DNS
• Use the appropriate number of DHCP servers for
  the number of DHCP-enabled clients on the network

41
Troubleshooting DHCP Clients

• Most DHCP-related problems present themselves as
  a failed IP configuration attempt by a client, so
  you should start DHCP troubleshooting at the
  client.
• If a DHCP-related problem does not originate at
  the client, check the system event log and DHCP
  server audit logs for clues.

42
Invalid IP Address Configuration

• If a DHCP client does not have an IP address
  configured, or has an IP address of 169.254.x.x,
  the client cannot contact a DHCP server to obtain
  an IP address.
• Ensure that
• The client has a functioning network connection
  check cables, network interface adapters, etc.
• Networking software components are installed
• The Obtain An IP Address Automatically option is
  selected in the Internet Protocol (TCP/IP)
  Properties dialog box

43
Missing Configuration Settings

• If a DHCP client is missing configuration
  settings, the DHCP server might not be configured
  to supply those options, or the client might not
  support the options distributed by the server.
• Verify that the options have been configured at
  either the server or scope level.
• Ensure that the DHCP option selections are
  appropriate for your clients.

44
DHCP Servers Do Not Provide IP Addresses

• If clients can access the network but cannot
  obtain IP addresses from a DHCP server, there are
  several possible causes
• The IP address of the DHCP server might have
  changed.
• If DHCP clients must go through a router to use a
  DHCP server, a DHCP relay agent might not be
  available.
• If multiple DHCP servers are used, they might
  have overlapping scopes.

45
Troubleshooting DHCP Servers

• When a DHCP server fails, clients experience
  TCP/IP problems or errors.
• First, ensure that the DHCP Server service is
  running on the server.
• If the service is not started, try to start it
  manually.
• Notice the services startup type (Manual or
  Automatic).
• Check the logs in Event Viewer to determine if
  the service failed to start or stopped because of
  a problem elsewhere in the computer.

46
DHCP Relay Agent Service Is Installed But Not
Working

• Because both the DHCP Relay Agent and the DHCP
  Server service listen for and respond to BOOTP
  and DHCP messages sent using User Datagram
  Protocol (UDP) ports 67 and 68, neither service
  works reliably if both are installed on the same
  computer.
• Install the DHCP Relay Agent and the DHCP Server
  service on separate computers running Windows
  2000 Server.

47
DHCP Console Incorrectly Reports Lease
Expirations

• When the DHCP console displays the lease
  expiration time for reserved clients for a scope,
  it indicates one of the following
• If the scope lease time is set to an infinite
  lease time, the reserved clients lease is also
  shown as infinite.
• If the scope lease time is set to a finite length
  of time, the reserved clients lease uses this
  same lease time.
• To create reserved clients with unlimited lease
  durations, create a scope with an unlimited lease
  duration and add reservations to that scope.

48
DHCP Server Uses Broadcasts to Respond to All
Client Messages

• By default, a Windows 2000 DHCP server attempts
  to transmit all DHCP responses to the limited
  broadcast address (255.255.255.255).
• You can
• Configure the servers registry so that it will
  recognize the broadcast flag of a DHCP client
• Adjust its transmission behavior to send a
  unicast or broadcast response to the client,
  depending on the flag setting in the client
  request

49
DHCP Server Fails to Issue Address Leases for a
New Scope

• When superscopes are not used, only a single DHCP
  scope can be active on the network at one time,
  even if more than one scope is defined and
  activated.
• If you want clients to receive IP addresses from
  a new scope, do one of the following
• Configure the DHCP server to use a superscope
  that includes the old scope and the new scope.
• Change the primary IP address for the DHCP server
  so that it has the same network identifier as the
  new scope.

50
Monitoring Server Performance

• Monitoring the performance of a Windows 2000 DHCP
  server can help in troubleshooting performance
  problems.
• The Windows 2000 DHCP server performance counters
  can monitor
• All types of DHCP messages sent and received by
  the DHCP Server service
• The average amount of processing time spent by
  the DHCP server per message packet sent and
  received
• The number of message packets dropped because of
  internal delays on the DHCP server

51
Lesson Summary

• Most DHCP problems present themselves as a
  failure of clients to obtain IP addressing
  information from a DHCP server.
• When troubleshooting clients, ensure that
• The network connection is functioning
• The client is configured to obtain its IP address
  by using a DHCP server
• When troubleshooting a DHCP server, make sure
  that the DHCP Server service is running.
• To ensure that both services operate properly,
  install the DHCP Relay Agent service and the DHCP
  Server service on separate computers running
  Windows 2000 Server.