2-3The DHCP server assigns an IP address to the client in the following order from an interface addresspool or a global address pool:3) If there is an address pool where an IP address is statically bound to the MAC address or ID of theclient, the DHCP server will select this address pool and assign the statically bound IP address tothe client.4) Otherwise, the DHCP server observes the following principles to select a dynamic address pool.z If the client and the server reside in the same network segment, the smallest address pool thatcontains the IP address of the receiving interface will be selected.z If the client and the server do not reside in the same network segment (that is, a DHCP relay agentis in-between), the smallest address pool that contains the IP address specified in the giaddr fieldof the client’s request will be selected.z If no assignable IP address is available in the selected address pool, the DHCP server will notassign any IP address to the client because it cannot assign an IP address from the parent addresspool to the client.DHCP IP Address PreferencesA DHCP server assigns IP addresses in interface address pools or global address pools to DHCPclients in the following sequence:1) IP addresses that are statically bound to the MAC addresses of DHCP clients or client IDs.2) The IP address that was ever assigned to the client3) The IP address designated by the Option 50 field in a DHCP-DISCOVER message4) The first assignable IP address found in a proper DHCP address pool5) If no IP address is available, the DHCP server queries lease-expired and conflicted IP addresses. Ifthe DHCP server finds such IP addresses, it assigns them; otherwise the DHCP server does notassign an IP address.IRF SupportIn an IRF (intelligent resilient framework) system, DHCP servers operate in a centralized way to fit theIRF environment.z DHCP servers run (as tasks) on all the units (including the master unit and the slave units) in aFabric system. But only the one running on the master unit receives/sends packets and carries outall functions of a DHCP server. Those running on the slave units only operate as the backup tasksof the one running on the master unit.z When a slave unit receives a DHCP-REQUEST packet, it redirects the packet to the DHCP serveron the master unit, which returns a DHCP-ACK or DHCP-NAK packet to the DHCP client and at thesame time backs up the related information to the slave units. In this way, when the current masterunit fails, one of the slaves can change to the master and operates as the DHCP serverimmediately.z DHCP is an UDP-based protocol operating at the application layer. When a DHCP server in afabric system runs on a Layer 2 network device, DHCP packets are directly forwarded by hardwareinstead of being delivered to the DHCP server, or being redirected to the master unit by UDPHELPER. This idles the DHCP server. DHCP packets can be redirected to the DHCP server on themaster unit by UDP HELPER only when the Layer 2 device is upgraded to a Layer 3 device.
