DHCP is a broad cast protocol.
Router by dfault stops the broadcast. => u need to connect the DHCP server to L2 interface / on the same subnet.
hmmmm, this is avoided by IP Helper address.
it takes broadcast n turns it to unicast.
http://www.networkstraining.com/images/ip-helper-address-explained.jpg
Configuration on Router A
RouterA# conf tRouterA(config)# interface fastethernet0/0RouterA(config-if)# ip address 192.168.1.1 255.255.255.0RouterA(config-if)# ip helper-address 10.10.10.1
Some other important considerations for ip helper-address
By default, the ip helper-address command forwards also some other broadcast protocols in addition to the DHCP (BOOTP) protocol. It forwards by default the following eight UDP broadcast protocols:
UDP 37 (Time protocol)
UDP 49 (TACACS)
UDP 53 (DNS)
UDP 67 (DHCP Server)
UDP 68 (DHCP Client)
UDP 69 (TFTP)
UDP 137 (NetBios)
UDP 138 (NetBios Datagram service)
If you want to add more broadcast protocols to be forwarded, or even remove some of the default forwarded protocols, you can use the “ip forward-protocol” command under global config mode.
Example: Remove the NetBios protocols (137,138) from being forwarded by default, and add NTP protocol 123 to be forwarded by ip helper-address.
RouterA(config)# no ip forward-protocol udp 137RouterA(config)# no ip forward-protocol udp 138RouterA(config)# ip forward-protocol udp 123
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More details: http://www.networkstraining.com/forwarding-a-dhcp-request-using-cisco-ip-helper-address-command/
