Debugging DHCP

Problem

You want to debug a DHCP problem.

Solution

To debug the server events, use the following EXEC command:

Router1#debug ip dhcp server events

The following command will allow you to monitor the actual DHCP-related packets being transmitted and received by the router:

Router1#debug ip dhcp server packet

 

Discussion

The following debug capture shows a router performing normal housekeeping duties, such as updating its address pools, checking for expired leases, assigning new leases, and revoking expired leases:

Router1#debug ip dhcp server events
Sep 15 00:58:17.218: DHCPD: returned 172.25.1.51 to address pool COOKBOOK
Sep 15 00:58:22.566: DHCPD: assigned IP address 172.25.1.51 to client 0100.0103.85e9.87.
Sep 15 01:01:15.056: DHCPD: writing bindings to ftp://dhcp:bindsave@172.25.1.1/dhcp-leases-rtr1.
Sep 15 01:01:15.132: DHCPD: writing address 172.25.1.51.
Sep 15 01:01:15.148: DHCPD: wrote automatic bindings to ftp://dhcp:bindsave@172.25.1.1/dhcp-leases-rtr1.
Sep 15 01:01:58.816: DHCPD: checking for expired leases.
Sep 15 01:03:58.841: DHCPD: checking for expired leases.
Sep 15 01:05:58.859: DHCPD: checking for expired leases.
Sep 15 01:07:58.874: DHCPD: checking for expired leases.
Sep 15 01:09:58.885: DHCPD: checking for expired leases.
Sep 15 01:09:58.885: DHCPD: the lease for address 172.25.1.51 has expired.
Sep 15 01:09:58.885: DHCPD: returned 172.25.1.51 to address pool COOKBOOK.

The next debug capture shows a typical DHCP client transaction between the client and router. The net result is that the router assigns IP address 172.25.1.51 to MAC address 00.0103.85e9.87:

Router1#debug ip dhcp server packet 
Sep 15 01:19:41.211: DHCPD: DHCPDISCOVER received from client 0100.0103.85e9.87 on interface FastEthernet0/0.1.
Sep 15 01:19:43.212: DHCPD: Sending DHCPOFFER to client 0100.0103.85e9.87 (172.25.1.51).
Sep 15 01:19:43.212: DHCPD: creating ARP entry (172.25.1.51, 0001.0385.e987).
Sep 15 01:19:43.212: DHCPD: unicasting BOOTREPLY to client 0001.0385.e987 (172.25.1.51).
Sep 15 01:19:43.216: DHCPD: DHCPREQUEST received from client 0100.0103.85e9.87.
Sep 15 01:19:43.216: DHCPD: Sending DHCPACK to client 0100.0103.85e9.87 (172.25.1.51).
Sep 15 01:19:43.216: DHCPD: creating ARP entry (172.25.1.51, 0001.0385.e987).
Sep 15 01:19:43.216: DHCPD: unicasting BOOTREPLY to client 0001.0385.e987 (172.25.1.51).
Router1#