On routers configured as DHCPv4 servers, the DHCPv4 process fails if the router is not receiving requests from the client. As a troubleshooting task, verify that the router is receiving the DHCPv4 request from the client. This troubleshooting step involves configuring an ACL for debugging output.
The figure shows an extended ACL permitting only packets with UDP destination ports of 67 or 68. These are the typical ports used by DHCPv4 clients and servers when sending DHCPv4 messages. The extended ACL is used with the debug ip packet command to display only DHCPv4 messages.
The output in the figure shows that the router is receiving DHCP requests from the client. The source IP address is 0.0.0.0 because the client does not yet have an IP address. The destination is 255.255.255.255 because the DHCP discovery message from the client is sent as a broadcast. This output only shows a summary of the packet and not the DHCPv4 message itself. Nevertheless, the router did receive a broadcast packet with the source and destination IP and UDP ports that are correct for DHCPv4. The complete debug output shows all the packets in the DHCPv4 communications between the DHCPv4 server and client.
Another useful command for troubleshooting DHCPv4 operation is the debug ip dhcp server events command. This command reports server events, like address assignments and database updates. It is also used for decoding DHCPv4 receptions and transmissions.