Design Option 2: BOOTP/DHCP Forwarding on Routers
Updated: November 10, 2008
Applies To: Windows Server 2008
In TCP/IP networking, routers are used to interconnect hardware and software on different subnets and forward IP packets between them. To support and use the DHCP service across multiple subnets, the routers connecting each subnet should comply with DHCP/BOOTP relay agent capabilities described in RFC 1542.
To be compliant with RFC 1542 and provide relay agent support, each router must be able to recognize BOOTP and DHCP protocol messages and process (relay) them appropriately. Because these messages are sent through the same UDP port (port 67) and have a similar message structure, a router with BOOTP-relay agent capability typically relays both DHCP and BOOTP packets. Most routers support DHCP/BOOTP relay; if your routers do not, contact your router manufacturer or supplier to find out if a software or firmware upgrade is available to support this feature. Most routers require the address of the DHCP server to be configured; this is often referred to as the helper address. For more information about DHCP/BOOTP forwarding, see the DHCP Technical Reference and Managing BOOTP Clients in the DHCP Server Operations Guide.
If you choose to deploy more than one DHCP server, place them on different subnets to achieve fault tolerance. The servers should not have common IP addresses in their scopes; each server should have a unique pool of addresses. If the DHCP server on the local subnet shuts down and the routers support DHCP/BOOTP relay, requests can be relayed to a remote subnet. The DHCP server at that location can respond to requests as long as it maintains a scope of IP addresses for the requesting subnet. If the remote server has no scope defined for the requesting subnet, it cannot provide IP addresses even if it has available addresses for other scopes. If each DHCP server has a pool of addresses for each subnet, it can provide IP addresses for remote clients whose own DHCP server is offline.
A relay agent implementation that is recommended for the best routed DHCP network performance uses two DHCP servers that are attached to two different subnets. The relay agents operating on each of the routers that connect the subnets have varied delay intervals; one is set to four seconds and the other uses no delay, which eliminates the risk of an undesirable flood of DHCP packets through randomly selected network paths.
The advantages of BOOTP/DHCP forwarding on routers include:
Single DHCP server can support many subnets: The use of RFC 1542-compatible routers allows a large routed network to be supported by a single DHCP server, although additional servers are typically deployed for fault tolerance.
Low management overhead: Because a single, or a few, DHCP servers can support a large routed network, its administration is simpler and the system is more efficient to manage.
The disadvantages of BOOTP/DHCP forwarding on routers include:
All routers must be compatible: This design requires that all routers in the network are compatible with RFC 1542. If there are points in the network where the routers cannot relay BOOTP/DHCP messages, then DHCP will not be available to subnets behind these routers.
Additional scope complexity: Although this design can be run on a single DHCP server, the scope configuration of that server can become complex. Each supported subnet will require its own DHCP scope so that the correct IP addresses and associated options are leased for clients on each subnet. Incorrectly configured DHCP scopes will, therefore, have a critical impact on network availability.