DHCP Troubleshooting

Show ip dhcp snooping statistics details ---------> To check the dhcp snooping packet drops.

To enable the DHCP relay on switch in a vlan by following below command.


service dhcp

ip helper-address 1.1.1.1 ------- To enable  DHCP relay on 6500 switch under SVI. Define DHCP server ip address.



Below are the flowchart of DHCP server /client packets during troubleshooting.



DHCP Discover (TX123456) ----------->PC sends DHCP discover to broadcast for the DHCP ip address
       |
      V
DHCP offer  (TX123456)----------------->Reply from DHCP server with client ip address
      |
     V
DHCP request  (TX123456) ----------->  DHCP client send again request with allocated ip address
      |
     V
 DHCP Acknowledgement  TX123456)   ----------->Acknowdgement received from DHCP server.
      |
     V
DHCP inform   -----------> DHCP informa is used if PC/client needs additional information like option 66 and option 67.
      |
     V
DHCP Acknowledgment.  -------> DHCP server acknowledge to inform packet by using DHCP ack to client with option 66 and option 67.


Debugging command to use when DHCP relay agent is in use

debug ip dhcp server packet

Not working logs:  

Aug 16 14:24:03.919 EDTT DHCPD: setting giaddr to 10.1.1.1.
Aug 16 14:24:03.919 EDTT DHCPD: BOOTREQUEST from 0178.acca.b2d3.f2 forwarded to 10.10.10.10.
Aug 16 14:24:03.923 EDTT DHCPD: BOOTREQUEST from 0178.acca.b2d3.f2 forwarded to 10.10.10.20.
Aug 16 14:24:04.731 EDTT DHCPD: setting giaddr to 10.1.1.1.
Aug 16 14:24:04.731 EDTT DHCPD: BOOTREQUEST from 01d4.8561.b441.4d forwarded to 10.10.10.10.
Aug 16 14:24:04.735 EDTT DHCPD: BOOTREQUEST from 01d4.8561.b441.4d forwarded to 10.10.10.20
Aug 16 14:24:05.263 EDTT DHCPD: setting giaddr to 10.1.1.1.

 
Working logs:

Aug 17 01:23:17.899 EDTT DHCPD: setting giaddr to 1.1.1.1.
Aug 17 01:23:17.899 EDTT DHCPD: BOOTREQUEST from 0111.1114.1b16.c1 forwarded to 10.10.10.10.
Aug 17 01:23:17.899 EDTT DHCPD: BOOTREQUEST from 0111.1114.1b16.c1 forwarded to 10.10.10.20.
Aug 17 01:23:18.163 EDTT DHCPD: forwarding BOOTREPLY to client 0022.2233.1223.
Aug 17 01:23:18.163 EDTT DHCPD: Forwarding reply on numbered intf
Aug 17 01:23:18.163 EDTT DHCPD: creating ARP entry (10.11.11.11, 0022.2233.1223).
Aug 17 01:23:18.163 EDTT DHCPD: unicasting BOOTREPLY to client 0022.2233.1223 (10.11.11.11).
Aug 17 01:23:18.167 EDTT DHCPD: setting giaddr to 10.1.1.1.

debug ip dhcp snooping packet mac <client mac address>



 Not working logs:
 DHCP_SNOOPING: process new DHCP packet, message type: DHCPDISCOVER, input interface:

Working logs: 

 DHCP_SNOOPING: process new DHCP packet, message type: DHCPDISCOVER, input interface:

DHCP_SNOOPING: process new DHCP packet, message type: DHCPOFFER,