Let's have a look at the Dynamic Host Configuration Protocol. If DHCP is down or leases are full, the new clients cannot access the network. So this is the typical end-user symptom whereby they plug into the ethernet or they try and connect to the wireless and the device can't get an IP address. So again they think the internet is down and they phone you and complain about it. DHCP hands out the IP address and subnet information. It also hands out the default gateway and which DNS servers to use. There are also many DHCP options which allow server configuration information to be passed. For example, the Voice-Over-IP PBX or the Trivial FTP server. Let's have a look at the design recommendations. Firstly we put the DHCP servers near the core of the network and we configure what's called DHCP relaying on each subnet facing interfaces broadcast dhp messages from clients are then relayed to the DHCP servers in the core to avoid raw DHCP servers. Consider setting up DHCP Snooping. If you recall we discussed DHCP Snooping as part of the requirements for campus devices on our network infrastructure elsewhere in this series. DHCP Snooping blocks DHCP replies from non-authorized DHCP servers. Use DHCP even for devices that need fixed IP addresses. These are what are known as static leases because then it makes renumbering in the future much simpler these times or a few hours are okay but you probably want to reclaim ip addresses faster if clients leave the network without releasing so for example for the likes of wireless network infrastructure having shorter lease times might be a good idea for Iv6 many campus administrators turn off slack altogether and try and use DHCP v6 as much as possible. slack is stateless auto configuration. The router on the subnet tells the client what the subnet and the default gateway are. There's no longer a relationship between the client and the IP address recorded on a central system as for DHCP which does make troubleshooting quite a bit harder. DHCP v6 operates in much the same way as DHCP does for IPv4 as for the software we recommend something well known like isc's DHCP configuration is not very difficult but there are many options as for redundancy you probably want to have a pair of servers but setting up a redundant DHCP service isn't covered here it is simple to set up each server to cover half the subnet range because having full failover and synchronization is actually quite complicated to set up. For monitoring it's important that we keep an eye on the log files. We could use something like tenshi for example and look for warnings about the pool usage. Are the ranges allocated about to be filled up? Network equipment can warn of rogue DHCP servers and we mentioned the DHCP Snooping feature earlier.

© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.

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