Cisco Catalyst 4500 Series, Software Configuration Manual

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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
6.
The DHCP server unicasts the reply to the device if the request is relayed to the server by the device.
The device verifies that it originally inserted the option 82 data by inspecting remote ID and possibly
circuit ID fields. The device removes the option 82 field and forwards the packet to the interface that
connects to the DHCP client that sent the DHCP request.
The Cisco software refers to a pool of IP addresses (giaddr or incoming interface IP address) and
matches the request to a class or classes configured in the pool in the order the classes are specified in
the DHCP pool configuration.
When a DHCP address pool is configured with one or more DHCP classes, the pool becomes a restricted
access pool, which means that no addresses are allocated from the pool unless one or more classes in the
pool matches. This design allows DHCP classes to be used either for access control (no default class is
configured on the pool) or to provide further address range partitions within the subnet of the pool.
Multiple pools can be configured with the same class, eliminating the need to configure the same pattern
in multiple pools.
The following capabilities are supported for DHCP class-based address allocation:
• Specifying the full relay agent information option value as a raw hexadecimal string by using the
relay-information hex command in new relay agent information configuration mode.
•
Support for bit-masking the raw relay information hexadecimal value.
• Support for a wildcard at the end of a hexadecimal string specified by the relay-information hex
command.
If the relay agent inserts option 82 but does not set the giaddr field in the DHCP packet, the DHCP server
interface must be configured as a trusted interface by using the
ip dhcp relay information trusted
command. This configuration prevents the server from dropping the DHCP message.
Disabling Conflict Logging
A DHCP database agent is any host (for example, an FTP, a TFTP, or a remote copy protocol [RCP]
server) or storage media on a DHCP server (for example, disk0) that stores the DHCP bindings database.
You can configure multiple DHCP database agents, and the interval between database updates and
transfers for each agent.
Automatic bindings are IP addresses that are automatically mapped to the MAC addresses of hosts that
are found in the DHCP database. Automatic binding information (such as lease expiration date and time,
interface index, and VPN routing and forwarding [VRF] name) is stored in a database agent. The
bindings are saved as text records for easy maintenance.
An address conflict occurs when two hosts use the same IP address. During address assignment, DHCP
checks for conflicts by using ping and gratuitous Address Resolution Protocol (ARP). If a conflict is
detected, the address is removed from the pool. The address is not assigned until the administrator
resolves the conflict.
Note We strongly recommend using database agents. However, the Cisco DHCP server can run without
database agents. If you choose not to configure a DHCP database agent, disable the recording of DHCP
address conflicts on the DHCP server by using the
no ip dhcp conflict logging command in global
configuration mode. If there is a conflict logging but no database agent is configured, bindings during a
switchover are lost when a device reboots. Possible false conflicts can occur causing the address to be
removed from the address pool.