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Alcatel-Lucent | Implementing DHCP Redundancy to Sustain Critical Operations
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IP administrators who implement Split-Scopes using Alcatel-Lucent DHCP servers should check their
NakUnknownClients policy configuration
When implementing the Split-Scopes configuration on Alcatel-Lucent DHCP servers, IP network
administrators must set this policy to False or turned off. Otherwise, the server not configured
for the address in question will NAK Renewal Requests or Init Reboot Requests—causing the client
to issue a Discover packet and start the DHCP process all over again.
Understanding the Alcatel-Lucent Failover DHCP configuration
Unlike the Split-Scopes configuration, which cannot guarantee the relationship between the
intended frontline and redundant servers, the Alcatel-Lucent Failover DHCP configuration clearly
designates a Primary and a backup Failover server. Both the Primary and Failover servers manage
an identical set of IP addresses and receive all Discover traffic. In this configuration, the Primary
server is responsible for sending Offers and granting leases to clients. Although the Failover server
will prepare an Offer in response to a client Discover request, it will not send it out as long as
the Primary server is operational. After the Primary server successfully sends an ACK to complete
the DHCP process, it will send an update to the Failover server to synchronize the active lease
files on both servers.
In the Alcatel-Lucent Failover DHCP configuration, the Failover server polls the Primary server
periodically to insure that it is up and running. As long as the Primary server responds to the
Failover server’s polls, the Failover server will not provide an Offer to the client. However, if the
Primary server fails to answer a number of its polls, the Failover server will assume the Primary
server is down and will take responsibility for sending Offers and granting leases.
The Alcatel-Lucent Failover DHCP configuration: A More Sophisticated Approach
to DHCP Server Redundancy
The Alcatel-Lucent Failover DHCP configuration corrects many of the weaknesses of the
Split-Scopes configuration.
First, the Failover server is a true hot backup of the Primary server. Unlike the Split-Scopes
configuration, there is no need to maintain distinct IP address reserves on the two servers—
minimizing wasted IP addresses while providing true redundancy.
Second, the Alcatel-Lucent Failover DHCP configuration eliminates the considerations referenced
earlier regarding manual DHCP and automatic DHCP clients.
Third, although the polling between the Primary and Failover servers in this configuration generates
some network traffic, eliminating the duplicate Offer packets for each client Discover request
substantially reduces overall traffic levels.
Finally, implementing the Alcatel-Lucent Failover DHCP configuration using Alcatel-Lucent DHCP
servers and VitalQIP enables IP network administrators to virtually eliminate the duplicate
administrative efforts of a Split-Scopes configuration. Once the IP administrator creates the
Primary/Failover relationship in the VitalQIP GUI, he or she assigns all other configurable
items—from scope assignment to server policies and DHCP options—only to the Primary server.
After generating the file to the Primary server, Alcatel-Lucent VitalQIP software automatically
passes the configuration for these items to the Failover server. In an ideal situation, IP network
administrators will establish a one-to-one relationship between the Primary and Failover servers.
However, a distinct advantage of the Alcatel-Lucent Failover DHCP configuration is the ability to
