Using multiple DHCP servers decreases the probability of having a DHCP-related
network access failure, but it does not guarantee against it. If a DHCP server for a
particular subnet fails, the other DHCP server might be unable to service all of the
requests from new clients. This can use every available address in the server’s
limited pool.
Youcan bias which DHCP server uses all of its addresses first. DHCP clients tend
to select the DHCP server that is offering more options. Tobias service toward the
DHCP server with 70% of the available addresses, offer fewer DHCP options from
the server that holds 30% of the available addresses for the subnet.
Do you Already have BOOTP Servers in your Network?
If you already have BOOTP clients and servers in your network, consider replacing
your BOOTP servers with DHCP servers. DHCP servers can optionally serve
BOOTP clients the same IP configuration information as current BOOTP servers.
If you cannot replace your BOOTP servers with DHCP servers or want both to
serve your network, perform the following steps:
1. Turn off BOOTP support in your DHCPserver.
2. Make sure that your BOOTP servers and DHCP servers do not give out the
same IP addresses.
3. Configure any BOOTP/DHCP Relay Agents to forward the BOOTP/DHCP
broadcasts to both the appropriate BOOTP and DHCP servers.
Youalso need to consider that BOOTP and DHCP servers cannot both run on any
individual system. Tocontinue running your BOOTP servers in addition to your
DHCP servers, you need to run them on separate systems.
A DHCP server allocates a permanent IPaddress to a BOOTP client. If subnets are
renumbered in such a way that a BOOTP-assigned address is unusable, you must
restart the BOOTP client and obtain a new IP address.
Do you have Hosts with Special Requirements?
Youmight have hosts with individual or special administrative needs, such as the
following:
Permanent lease
Assign permanent leases to designated hosts by specifying an infinite lease
time. Additionally,the DHCP server allocates a permanent lease to BOOTP
clients that explicitly request it, as long as support for BOOTP clients is
enabled. The DHCP server also allocates a permanent lease to DHCP hosts
that explicitly request it.
Specific IP address
Reserve a specific address and configuration parameters for a specific DHCP
(or BOOTP) client host on a particular subnet.
Specific configuration parameters
Allocate specific configuration information to a client regardless of its subnet.
Manually defined workstations
Explicitly exclude addresses from DHCP subnets for existing hosts that do not
use DHCP or BOOTP for configuring their IP network access.
DHCP servers and clients automatically check to see if an IP address is in use
before allocating or using it. However, they are unable to detect addresses of
Chapter17. DHCP Server 407