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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Dynamic Host Configuration Protocol (DHCP)                      9  DHCP Client/Server Implementation, Features and Issues

DHCP Autoconfiguration / Automatic Private IP Addressing (APIPA) 1 2 DHCP and BOOTP Interoperability

As we saw in the first topic of this section, one of the primary decisions any TCP/IP administrator using DHCP must make is how many DHCP servers to deploy. A single server has the advantage of simplicity, but provides no redundancy in the event of failure. It also means that whenever the DHCP server is down, clients can't get addresses. For these reasons, most larger networks use two or more servers.

When you have two servers or more—and let's just say two for sake of this discussion—you then have another decision to make: how do you divide the address pool between the servers? As I explored in detail in the discussion of DHCP address pools, there are two options: either giving the servers overlapping addresses, or making them non-overlapping. Unfortunately, in classical DHCP, neither is really a great solution. Overlapping ranges mean both servers might try to assign the same address, since DHCP includes no provision for communication between servers. Non-overlapping ranges avoids this problem, but makes only some of the addresses available to each server.

It's strange that the DHCP standard didn't provide better support for cross-server coordination, even though there clearly was a need for it. However, certain DHCP implementations implement an optional feature to in fact allow two servers to have overlapping scopes without address clashes occurring. Again, this is a feature commonly found on Microsoft DHCP servers, and may also be present in other implementations. It is called DHCP server conflict detection.

DHCP Autoconfiguration / Automatic Private IP Addressing (APIPA) 1 2 DHCP and BOOTP Interoperability