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22-4
Cisco CNS Network Registrar User’s Guide
OL-6240-02
Chapter 22 Advanced DHCP Server Properties
Defining Advanced Server Parameters
Table 22-1 DHCP Advanced Parameters
Advanced Parameter Action Description
max-dhcp-requests set/
unset
Controls the number of buffers that the DHCP server allocates for
receiving packets from DHCP clients and failover partners. If this
setting is too large, a burst of DHCP activity can clog the server with
requests that become stale before being processed. This results in an
increasing processing load that can severely degrade performance as
clients try to obtain a new lease, and affects the ability to handle
bursts. A low buffer setting throttles requests and could affect server
throughput. If the server runs out of buffers, packets are dropped.
A good rule or thumb is to increase the buffers if you expect a high
load (in a steady state or when experiencing frequent stress times)
or you have a fast multiprocessor system.
In a nonfailover deployment, the default setting (500) is sufficient.
In a failover deployment, you can increase it to 1000 if the DHCP
logs indicate a consistently high number of request buffers. You
should then also modify the number of DHCP responses (see the
max-dhcp-responses parameter) to four times the request buffers.
When using LDAP client lookups, buffers should not exceed the
LDAP lookup queue size defined by the total number of LDAP
connections and the maximum number of requests allowed for each
connection. Set the LDAP queue size to match the LDAP server’s
capacity to service client lookups.
Some log messages that might trigger a change in this value are:
4493 DHCP ERROR "DHCP has used xx of its yy request
buffers: the server is dropping a request."
4494 DHCP WARNING "DHCP has used xx of yy request packets.
Requests will be ignored if no packet buffers are
available."
5270 DHCP WARNING "DHCP has used xx of its yy request
buffers: the server is congested -- will not keep the
client last-transaction-time to within value but will keep
it to within value seconds."
Required. The default is 500.