Question

A three-tier campus network uses a Layer 3 distribution switch as a DHCP relay agent. Access switches insert DHCP Option 82 (relay information) as packets pass through them. When clients on VLAN 30 request addresses, the central IOS DHCP server logs show the requests arriving, but no DHCPOFFER reaches the clients. Debugging on the relay shows: 'DHCPD: relay information option exists, but giaddr is zero' followed by the packet being dropped. The relay agent has 'ip helper-address' configured correctly on the VLAN 30 SVI. What is the most likely cause?

Accepted Answer

The relay agent needs 'ip dhcp relay information trust-all' or per-interface 'ip dhcp relay information trusted' to accept the pre-inserted Option 82 with giaddr 0

Answer

The relay agent's default 'ip dhcp relay information policy replace' behavior is discarding packets that already contain Option 82 from an untrusted downstream device with a zero giaddr

Answer

The DHCP server pool is missing the 'network' statement for the VLAN 30 subnet

Answer

The access switch is not configured as a DHCP snooping trusted port toward the relay