126 CHAPTER 10: DHCP S ERVER CONFIGURATIONIP address lease updateAfter a DHCP server dynamically assigns an IP address to a DHCP client, the IP addresskeeps valid only within a specified lease time and will be reclaimed by the DHCPserver when the lease expires. If the DHCP client wants to use the IP address for alonger time, it must update the IP lease.By default, a DHCP client updates its IP address lease automatically by sending aDHCP-REQUEST packet to the DHCP server when half of the lease time elapses. TheDHCP server, in turn, responds with a DHCP-ACK packet to notify the DHCP client ofa new IP lease if the server can assign the same IP address to the client. Otherwise, theDHCP server responds with a DHCP-NAK packet to notify the DHCP client that the IPaddress will be reclaimed when the lease time expires.XRN supportIn an XRN (expandable resilient networking) system, DHCP servers operate in acentralized way to fit the XRN environment.■ DHCP servers run (as tasks) on all the units (including the master unit and the slaveunits) 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.■ When a slave unit receives a DHCP-REQUEST packet, it redirects the packet to theDHCP server on the master unit, which returns a DHCP-ACK/DHCP-NAK packet tothe DHCP client and at the same time backs up the related information to theslave units. In this way, when the current master unit fails, one of the slaves canchange to the master and operates as the DHCP server immediately.■ 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.When you merge two or more XRN systems into one XRN system, a new master unitis elected, and the new XRN system adopts new configurations accordingly. This mayresult in the existing system configurations (including the address pools configuredfor the DHCP servers) being lost. As the new XRN system cannot inherit the originalDHCP server configurations, you need to perform DHCP server configurations for it.When an XRN system is split into multiple new XRN systems, some of the new XRNsystems may be degraded to Layer 2 devices. For a new XRN system degraded toLayer 2 device, although the original DHCP server still exists in the new system, it runidle for being unable to receive any packets. When the XRN system restores to a Layer3 device due to being merged into a new XRN system, it adopts the configurations onthe new XRN system. And you need to perform DHCP server configurations if thenew XRN system does not have DHCP server-related configurations.In an XRN system, the UDP HELPER function must be enabled on the DHCP serversthat are in fabric state.After DHCP server is enabled on a device, the device processes the DHCP packetreceived from a DHCP client in one of the following three modes depending on yourconfiguration.
