Cisco Catalyst 4500 Series, Руководство по настройке программного обеспечения, Страница: 1554 / 2086

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 59 Configuring the Cisco IOS DHCP Server
Information About Cisco IOS DHCP Server
Note If the giaddr matches a secondary subnet in the pool, the DHCP server allocates an IP address
from that particular secondary subnet (even if IP addresses are available in the primary subnet
and irrespective of the order of secondary subnets that were added).
The secondary subnet in the pool is supported only for directly connected clients. To avoid
multiple IP address allocation from multiple subnets, you should configure secondary IP address
on the interface connected to clients. Note that the secondary subnets should not be used in pools
that are used for servicing requests from DHCP relay.
Manual Bindings
An address binding is a mapping between the IP address and MAC address of a client. The IP address
of a client can be assigned manually by an administrator or assigned automatically from a pool by a
DHCP server.
Manual bindings are IP addresses that are manually mapped to MAC addresses of hosts that are found
in the DHCP database. Manual bindings are stored in the NVRAM of the DHCP server. Manual bindings
are just special address pools. There is no limit to the number of manual bindings, but you can configure
only one manual binding per host pool.
Automatic bindings are IP addresses that have been automatically mapped to MAC addresses of hosts
that are found in the DHCP database. Because the bindings are stored in the volatile memory of the
DHCP server, binding information is lost in the event of power failures or on device reloads. To prevent
the loss of automatic binding information, a copy of the automatic binding information is stored on a
remote host called the DHCP database agent. The bindings are periodically written to the database agent.
When the device reloads, the bindings are read from the database agent to the DHCP database in the
DHCP server.
Note
We strongly recommend that you use database agents. However, Cisco DHCP server can function even
without database agents.
Some DHCP clients send a client identifier (DHCP option 61) in the DHCP packet. To configure manual
bindings for such clients, you must enter the client-identifier command with the hexadecimal values that
identify the DHCP client. To configure manual bindings for clients that do not send a client identifier
option, you must enter the hardware-address DHCP pool configuration command with the hexadecimal
hardware address of the client.
Depending on your release, the DHCP server sends infinite lease time to the clients for which manual
bindings are configured.
Depending on your release, the DHCP server sends lease time that is configured using the lease
command to clients for which manual bindings are configured.
Note
You cannot configure manual bindings within the same pool that is configured with the network
command in DHCP pool configuration mode. See the Configuring DHCP Address Pools section for
information about DHCP address pools and the network command.