When designing a DHCP infrastructure, it is important to take into account the topology of the network being serviced. By determining the topology, the designer will be able to anticipate where the load on the DHCP servers may be high and identify single points of failure that may cause DHCP services to be disrupted.
There are two different areas that need to be examined:
The physical layout of the network
The number of users in each physical location
By determining the physical layout of the network, the designer will be able to create a list of subnets that need to be serviced by DHCP. This information will be needed when scopes are created later.
Another important factor is the placement of DHCP relay agents. The physical layout of the network establishes which routers and subnets will need to be serviced by relay agents.
The number of users in each location helps determine the placement of DHCP servers. If there are a small number of users located in a single location, the DHCP server may be placed in a remote subnet with a DHCP Relay Agent set up on the router to listen for DHCP requests. This eliminates the need to place a server physically on the LAN where the users reside. If the WAN link goes down, the number of users disrupted is minimized.
If some of your DHCP clients at remote sites are Windows 2000 or Windows 98 systems and the WAN link goes down, they will not be able to contact a DHCP server. As mentioned earlier in this chapter, when these operating systems fail to contact a DHCP server, they resort to using APIPA to obtain an IP address. As a result, once the WAN link is restored, they will not be able to achieve network connectivity with the rest of the production network until the APIPA address is released. Connectivity is restored once a new address lease is obtained from the DHCP server. See Chapter 7, for information about disabling APIPA in Windows 2000 and Windows 98.
In situations where the number of users is high, the DHCP server should be placed locally. In this case the loss of the WAN link will not disrupt DHCP service.