Refer to the exhibit.
The CPE router acts as a DHCP server for the locally attached LAN. After DHCP snooping is enabled on the switch where the DHCP clients are connected, clients are unable to obtain their configuration from the DHCP server. What is the cause of this issue?
The issue arises because the DHCP server drops DHCP packets carrying Option 82 when the relay agent IP address (giaddr) is empty. When DHCP snooping is enabled on a switch, it inserts Option 82 into DHCP packets to indicate their origin. If the DHCP server isn't set up to handle Option 82 correctly or doesn't expect it, it may drop these packets, leading to clients being unable to obtain their configuration from the DHCP server. The debug messages indicating 'inconsistent relay information' and 'relay information option exists, but giaddr is zero' highlight this specific problem.
With the information available, option C seems to be the most relevant: C. The DHCP server drops DHCP packets carrying Option 82 and an empty relay agent IP address. This is because the DHCP server itself is on the router (as evidenced by the presence of the DHCP pool configuration), and there's no need for an ip helper-address on the local router's interface for DHCP server functionality. The error message indicates an issue with DHCP Option 82, which suggests that the problem lies in how the DHCP server or relay agent is handling the packets, rather than a missing DHCP relay configuration. In practical scenarios, when DHCP snooping is configured on a switch, the switch inserts information into the DHCP packets (Option 82) to indicate where the packets came from. If the DHCP server is not expecting or is not configured to handle Option 82, it may drop the packets, hence the error message about inconsistent relay information. This is consistent with the symptoms described in the debug output provided.
Focus on log DHCPD: relay information option exists, but giaddr is zero giaddr (Gateway IP Address) is a field within DHCP packets used in DHCP relay scenarios. It contains the IP address of the relay agent (gateway) that forwards the DHCP request from a client to the DHCP server. This helps the DHCP server assign appropriate IP addresses and configuration to clients on different subnets. Add giaddr on the interface that perform DHCP. " ip dhcp relay information trusted giaddr_IP " command, you indicate that the relay agent IP address should be included in DHCP packets as they are forwarded to the DHCP server.
C is the only one makes sinse
Can someone please explain?
https://ine.com/blog/2009-07-22-understanding-dhcp-option-82 This link explains it much better than I can, because I still am not sure how this works. Hope this helps.
Seems to be ok A,B,D - incorrect