59-2
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
•
Port 67 (the DHCP server port) is closed in the Cisco DHCP/BOOTP default configuration. There
are two logical parts to the
service dhcp
command: service-enabled and service running. The DHCP
service is enabled by default, but port 67 does not open until the DHCP service is running. If the
DHCP service is running, the
show ip sockets details
or the
show sockets detail
command displays
port 67 as open.
•
The Cisco DHCP relay agent is enabled on an interface only when you configure the
ip
helper-address
command. This command enables a DHCP broadcast to be forwarded to the
configured DHCP server.
Information About
Cisco IOS DHCP Server
•
Overview of the DHCP Server, page 59-2
•
DHCP Attribute Inheritance, page 59-2
•
DHCP Server Address Allocation Using Option 82, page 59-3
•
Disabling Conflict Logging, page 59-4
•
•
•
DHCP Static Mapping, page 59-7
•
DHCP Server Operation, page 59-8
•
Static Route with the Next-Hop Dynamically Obtained Through DHCP, page 59-9
Overview of the DHCP Server
The Cisco DHCP server accepts address assignment requests and renewals from the client and assigns
the addresses from predefined groups of addresses within DHCP address pools. These address pools can
also be configured to supply additional information to the requesting client such as the IP address of the
Domain Name System (DNS) server, the default device, and other configuration parameters. The Cisco
DHCP server can accept broadcasts from locally attached LAN segments or from DHCP requests that
have been forwarded by other DHCP relay agents within the network.
DHCP Attribute Inheritance
The DHCP server database is organized as a tree. The root of the tree is the address pool for natural
networks, branches are subnetwork address pools, and leaves are manual bindings to clients.
Subnetworks inherit network parameters and clients inherit subnetwork parameters. Therefore, common
parameters (for example, the domain name) should be configured at the highest (network or subnetwork)
level of the tree.
Inherited parameters can be overridden. For example, if a parameter is defined in both the natural
network and a subnetwork, the definition of the subnetwork is used.
Address leases are not inherited. If a lease is not specified for an IP address, by default, the DHCP server
assigns a one-day lease for the address.
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