Summary: This article describes how to troubleshoot common DHCP issues.
DHCP issues can arise from time to time when administrating a network and it’s important to first understand how DHCP works and what to do when it doesn’t.
DHCP is a conversation between the Client and the Server, devices in between such as an AP or Switch will pass the packets/frames between each. We often refer to this as the DHCP DORA transaction, DORA being an acronym for Discover, Offer, Request and Acknowledgement.
DHCP Discover (Client) - This is a broadcast packet/frame that is sent by the client device trying to discover the DHCP server and ask for an IP address.
DHCP Offer (Server) - The server receives the broadcast packet/frame from the client device and sends out an IP address to the client via a unicast packet.
DHCP Request (Client) - The client now knows the identity of the DHCP server and will send out a packet/frame via unicast to request the IP address that was provided by the DHCP server or request its last used IP address.
DHCP Acknowledgement (Server) - The server receives the unicast packet and sends an acknowledgement via unicast to the client device stating that its received the request and confirms the agreed upon IP address.
Now knowing how DHCP works, its important to follow each step along the network. The most effective way is to have a wired packet capture running on the switchport of the DHCP server and the network device that the client is connected to. If its hardwired, mirror the switchport the client is connected to or if the client is connected to an AP, run a packet capture on the AP on its Ethernet port.
- Did the DHCP broadcast reach the DHCP server?
- Was the DHCP Offer returned to the client device?
- Did the client device send out a DHCP Request and was it received by the DHCP server?
- Did the server return a DHCP Ack to the client?
If any of these points were not met, DHCP will fail and the client device will use a self assigned IP address.
- Is the DHCP subnet full?
- If using VLANs, is the client VLAN on each network device on the route to the DHCP server?
- If the packet is traversing L3 Routers, is there a DHCP relay in place and is the IP correct?
- Has the IP address already been statically assigned to another device?