3Com 5500-EI PWR Install Manual page 622

Check if an address pool that is on the same network segment with the DHCP clients is configured
on the DHCP server.
Check if a reachable route is configured between the DHCP relay agent and the DHCP server.
Check the DHCP relay agent. Check if the correct DHCP server group is configured on the
interface connecting the network segment where the DHCP client resides. Check if the IP address
of the DHCP server group is correct.
If the address-check enable command is configured on the interface connected to the DHCP
server, verify the DHCP server's IP-to-MAC address binding entry is configured on the DHCP relay
agent; otherwise the DHCP client cannot obtain an IP address.
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