DHCP Client Configuration Problems

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DHCP Client Configuration Problems

All the TCP/IP parameters mentioned previously can cause communication problems if they are not configured correctly. Using a DHCP server can greatly reduce these configuration problems. If the DHCP scope is set up properly, without any typos or other configuration errors, DHCP clients shouldn’t have any configuration problems. It is impossible to completely eliminate human error, but using DHCP should reduce the points of potential errors to just the DHCP servers rather than every client on the network.

Even when there are no configuration problems with DHCP addresses, DHCP clients can get a duplicate IP address from a DHCP server. If you have multiple DHCP servers in your environment, you should have scopes on each DHCP server for different subnets. Usually you have scopes with a larger number of addresses for the local subnet where the DHCP server is located and smaller scopes for other subnets. Creating multiple scopes on one server provides backup for giving clients IP addresses. If the server on the local scope is busy or down, the client can still receive an address from a remote DHCP server. When the router forwards this DCHP request to another subnet, it includes the address of the subnet it came from so that the remote DHCP server knows from which scope of addresses to lease an address to the remote client. Using this type of redundancy, however, can cause problems if you don’t configure the scopes on all the DHCP servers correctly.

The most important part of the configuration is to make sure you don’t have duplicate addresses in the different scopes. On one server, for example, you could have a scope in the range 131.107.2.100 to 131.107.2.170. On the remote DHCP server, you could have a scope of 131.107.2.171 to 131.107.2.200. By setting up the scopes without overlap, you should not have any problems with clients receiving duplicate IP addresses. DHCP servers do not communicate with each other, so one server does not know anything about the addresses the other server has leased. Therefore, you must ensure the servers never give out duplicate information by making sure the scopes for one subnet on all the different DHCP servers have unique IP addresses.

Another common problem with having multiple scopes on one server is entering the configuration parameters correctly. For example, if you enter the default gateway as 131.107.3.1 (instead of 131.107.2.1) for the scope 131.107.2.100 to 131.107.2.170, the clients receiving these addresses will not be able to communicate beyond the local subnet, because they have the wrong router address. With one scope on a DCHP server, you are usually quite sure of what all the configuration parameters should be.

With multiple scopes on one server, however, it is easy to get confused about which scope you are editing and what the parameters should be for that scope. To avoid this type of problem, check each scope’s parameters very carefully to make sure the parameters match the address of the scope, not the subnet where the DHCP server is located.

Also, if the client doesn’t receive an address because the server is down or doesn’t respond in a timely manner, the client is not able to contact anyone. Without an IP address, the IP stack does not initialize and the client can’t communicate at all with TCP/IP.

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