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Troubleshooting DHCP

DHCP Troubleshooting

It maybe famous last words, but DHCP does not give many problems.  However if you are suffering from an APIPA address or a mis-configuration, then check out these symptoms and their associated cures.

IPCONFIG will be your number one troubleshooting tool.  Take the time to learn all its switches.  For example IPCONFIG /all, /release /renew.

Topics for DHCP Troubleshooting

• APIPA (Automatic Private IP Address)
• DHCP Server has stopped
• DHCP Database is corrupted
• Trap - Orientation, finding a Scope Option
• Trap - Subnet mask
• Trap - Static address
• Trap - DHCP Relay Agent, Interface
• DHCP Bottlenecks
• Summary

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APIPA (Automatic Private IP Address)

When you run IPCONFIG, if you see address beginning 169.254.x.y, this is known as APIPA.  There may be nothing wrong, on a small network this could be 'By design'.  However on a business network, more than likely it means that the DHCP server is down, or the Relay Agent is not doing its job.

DHCP Server has stopped

If the DHCP server is newly installed then check that it has been Authorized in Active Directory by an Enterprise Admin.  If it is Authorized (Green down arrow on server Icon), then check that the scope is activated.

DHCP Database is corrupted

When you suspect that the DHCP database is corrupt, firstly check through the system and application event logs.  If your worst fears are confirmed, then you have two choices, either just ruthlessly delete the affected scopes and start again.  Alternatively, attempt to restore the database from backup.  See here for more information.

Trap - Orientation, finding a Scope Option

Are you looking for a property of an individual scope, or a property of the DHCP server option?  For example, the clients have a default gateway address of 10.10.56.200, but you do not know where this IP address is coming from. As a rule of thumb if you cannot find what you are looking for on the server, try the scope, and vica versa!  See more about configuring DHCP scope

Trap - Subnet mask

You cannot change a subnet once you have configured a scope.  All that you can do is delete that scope and the start again.

Trap - Static address

It goes without saying that the very DHCP server itself, must have a fixed IP address.  The DHCP server cannot be its own client.

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Trap - DHCP Relay Agent, Interface

Make sure that you add the interface to the Relay Agent.  The Relay Agent is found under the Routing and RAS server icon.  Whilst you add the interface itself, by right-clicking the Relay Agent object, select New Interface from the short cut menu.  See more on DHCP Relay Agent

DHCP Bottlenecks

If you suspect that there is more DHCP activity than necessary, then setup a performance log and monitor the key counters, for example Requests /sec.  See more about Performance Monitor here.

Summary

DHCP servers are normally well behaved, however here are a selection of tips and traps for when you are stuck.

Related DHCP Server topics

• Benefits of DHCP - Saves Work
• DHCP Strategies - Client and Server
• DHCP - Install and Configure Server Options
• DHCP - Relay Agents
• APIPA - When all else fails
• DHCP - User Classes
• DHCP - Database, Backup, Reconcile and Audit
• DHCP - Troubleshooting

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