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Troubleshooting DHCP.

There are a number of reasons that can cause DHCP problems, for example;

  Errors in configuration.
*  Software defects in operating systems
*  Corrupt or deleted NIC drivers or DHCP/BOOTP relay agents.

 To try and resolve these problem areas in DHCP, apply the following methods

 1: Check for IP Address Conflicts

DHCP clients connect to the network using a leased IP address. This is supposed to be renewed after an expiration date, if the client does not renew the lease, the DHCP server can reassign that IP address to another client on the network. When the client decides to log on to the network, it requests an IP address. If the DHCP server does not respond promptly, the client uses the last IP address. This can cause an IP address conflict because two clients on the network are using the same IP.

Use the following commands to verify IP address conflicts:

Show ip dhcp conflict: This displays all conflicting addresses recorded by the DHCP server. DHCP server uses the ping command to detect conflicts. If an address conflict is detected, the address is removed from the pool and will not be assigned until an administrator resolves the conflict.

This example displays the detection method and time;

Router1# show ip dhcp conflict
IP address Detection Method Detection time Ping July 15 2009 6:15 PM Gratuitous ARP July 26 2009 8:12 AM
2: Check Physical Connectivity

Use the show interface{interface} command to verify if the router interface acting as the default gateway for the client is operational. If the state of the interface is anything other than up, that shows inactivity on the port, including requests made by DHCP client.

3: Test connections with other Client using a Static IP Address.

This is common method use to troubleshoot DHCP issues or other wireless problems, start by configuring a static IP address on a client workstation, test for network connection. If the client is unable to access the network with a static IP address, then the root cause of the problem is not DHCP. At this point, it’s best to start considering troubleshooting networks connectivity.

4: Confirm Switch Port Configuration.

If the DHCP client is unable to obtain an IP address from the DHCP server on startup, attempt to obtain an IP address from the DHCP server by manually forcing the client to send a DHCP request.

If you are dealing with a switched network – switch between the client and the DHCP server,


*   That the switchport has STP PortFast enabled and trunking/channeling disabled (The default configuration is PortFast disabled and trunking/channeling auto) if applicable. These configuration changes resolve the most common DHCP client issues that occur with an initial installation of a Catalyst switch.


5: Verify the source of Clients IP Address.

It is important to distinguish whether DHCP is functioning correctly when the client is on the same subnet or VLAN as the DHCP server. If the DHCP is working correctly, the problem may be the DHCP/BOOTP relay agent. If the problem persists even with testing DHCP on the same subnet or VLAN as the DHCP server, the problem may actually be with the DHCP server

Tips on troubleshooting  DHCP

*   Check Physical connection

*   Check for IP address conflict

*   Test connectivity with other clients using static IP addresses

*   Confirm switch port configuration

*   Verify the source of clients address

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