DHCP Relay and IP Helper

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• normally, routers do not forward broadcast traffic
  • means that each broadcast domain must be served by its own DHCP server
  • on a large network with multiple subnets, this means provisioning and configuring many DHCP servers
• DHCP relay agent can be configured to provide forwarding of DHCP traffic between subnets
  • routers that can provide this type of forwarding are described as RFC 1542 compliant
  • process:
    • intercepts broadcast DHCP frames
    • applies a unicast address for the appropriate DHCP server
    • and forwards them over the interface for the subnet containing the server
  • DHCP server can identify the original IP subnets from the packet and offer a lease from the appropriate scope
  • DHCP relay performs the reverse process of directing responses from the server to the appropriate client subnet

IP Helper

IP helper is a command set in a router OS to support DHCP relay and other broadcast forwarding functionality.

• can be configured on routers to allow set types of broadcast traffic (including DHCP) to be forwarded to an interface
• supports the function of the DHCP relay agent
• to enable forwarding of DHCP broadcasts on a DHCP relay agent router:
  • based on diagram above
  • interface eth1
  • ip helper-address 10.1.0.200
  • interface eth2
  • ip helper-address 10.1.0.200
• UDP forwarding is a more general application of the same principle
  • used for:
    • DHCP
    • Network Time Protocol (NTP)
    • other broadcast-based applications