Operation Manual – DHCPH3C S5600 Series Ethernet Switches Chapter 2 DHCP Server Configuration2-43) The IP address designated by the Option 50 field in a DHCP-DISCOVERmessage4) The first assignable IP address found in a proper DHCP address pool5) If no IP address is available, the DHCP server queries lease-expired andconflicted IP addresses. If the DHCP server finds such IP addresses, it assignsthem; otherwise the DHCP server does not assign an IP address.2.1.4 IRF SupportIn an IRF (intelligent resilient framework) system, DHCP servers operate in acentralized way to fit the IRF environment.z DHCP servers run (as tasks) on all the units (including the master unit and theslave units) in a Fabric system. But only the one running on the master unitreceives/sends packets and carries out all functions of a DHCP server. Thoserunning on the slave units only operate as the backup tasks of the one running onthe master unit.z When a slave unit receives a DHCP-REQUEST packet, it redirects the packet tothe DHCP server on the master unit, which returns a DHCP-ACK or DHCP-NAKpacket to the DHCP client and at the same time backs up the related information tothe slave units. In this way, when the current master unit fails, one of the slavescan change to the master and operates as the DHCP server immediately.z DHCP is an UDP-based protocol operating at the application layer. When a DHCPserver in a fabric system runs on a Layer 2 network device, DHCP packets aredirectly forwarded by hardware instead of being delivered to the DHCP server, orbeing redirected to the master unit by UDP HELPER. This idles the DHCP server.DHCP packets can be redirected to the DHCP server on the master unit by UDPHELPER only when the Layer 2 device is upgraded to a Layer 3 device.
