Dhcp Address Pools - Cisco Catalyst 4500 Series Software Configuration Manual

Chapter 59 Configuring the Cisco IOS DHCP Server

DHCP Address Pools

You can configure a DHCP address pool with a name that is a string (such as "engineering") or an integer
(such as 0). Configuring a DHCP address pool also puts the device into DHCP pool configuration
mode—identified by the (dhcp-config)# prompt—from which you can configure pool parameters (for
example, the IP subnet number and default device list).
DHCP defines a process by which the DHCP server knows the IP subnet in which the DHCP client
resides, and it can assign an IP address from a pool of valid IP addresses in that subnet. The process by
which the DHCP server identifies the DHCP address pool to use for a client request is described in the
Configuring Manual Bindings section.
The DHCP server identifies and uses DHCP address pools for a client request, in the following manner:
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Cisco DHCP server software supports advanced capabilities for IP address allocation. See the
Configuring DHCP Address Allocation Using Option 82 section for more information.
DHCP Address Pool with Secondary Subnets
Each subnet is a range of IP addresses that the device uses to allocate an IP address to a DHCP client.
The DHCP server multiple subnet functionality enables a Cisco DHCP server address pool to manage
additional IP addresses by adding the addresses to a secondary subnet of an existing DHCP address pool
(instead of using a separate address pool).
Configuring a secondary DHCP subnetwork places the device in DHCP pool secondary subnet
configuration mode—identified by the (config-dhcp-subnet-secondary)# prompt—where you can
configure a default address list that is specific to the secondary subnet. You can also specify the
utilization rate of the secondary subnet, which allows pools of IP addresses to dynamically increase or
reduce in size depending on the address utilization level. This setting overrides the global utilization rate.
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If the client is not directly connected to the DHCP server (the giaddr field of the DHCPDISCOVER
broadcast message is nonzero), the server matches the DHCPDISCOVER with the DHCP pool that
has the subnet that contains the IP address in the giaddr field.
If the client is directly connected to the DHCP server (the giaddr field is zero), the DHCP server
matches the DHCPDISCOVER with DHCP pools that contain the subnets configured on the
receiving interface. If the interface has secondary IP addresses, subnets associated with the
secondary IP addresses are examined for possible allocation only after the subnet associated with
the primary IP address (on the interface) is exhausted.
If the DHCP server selects an address pool that contains multiple subnets, the DHCP server allocates
an IP address from the subnets as follows:
When the DHCP server receives an address assignment request, it looks for an available IP address
in the primary subnet.
When the primary subnet is exhausted, the DHCP server automatically looks for an available IP
address in any of the secondary subnets maintained by the DHCP server (even though the giaddr
does not necessarily match the secondary subnet). The server inspects the subnets for address
availability in the order of subnets that were added to the pool.
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Information About Cisco IOS DHCP Server
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