A recent case prompted me to share the information with community.
DHCP server pool is 192.168.100.200~192.168.100.250, subnet mask 26,
DHCP relay agent IP 192.168.100.1/24, not included in DHCP server pool subnet
DHCP relay agent will translate broadcasted DHCP from DHCP client to unicast packet.
In this case, source IP = 192.168.100.1, destination IP = DHCP server IP.
Problem is DHCP Client cannot get IP address from DHCP server, although DHCP packet has been relayed by DHCP relay agent.
Root cause is DHCP server has not included DHCP relay agent IP in its pool subnet, and cannot know which pool to use.
As a result, DHCP server simply dropped the DHCP packet relayed by DHCP relay agent, and DHCP client never got IP address assigned.
To fix, need to change DHCP relay agent IP address to be within 192.168.100.192/26, 192.168.100.254 for example.
Please check article from Microsoft for more detail on DHCP relay.
http://technet.microsoft.com/en-us/library/cc779610(v=ws.10).aspx
