IP Addressing: DHCP Configuration Guide, Cisco IOS XE Release 3S (Cisco ASR 1000)
DHCP Relay Agent Support for Unnumbered Interfaces
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DHCP Relay Agent Support for Unnumbered Interfaces

DHCP Relay Agent Support for Unnumbered Interfaces

Last Updated: October 25, 2012

Relay agents are used to forward requests and replies between clients and servers when they are not on the same physical subnet. Relay agent forwarding is distinct from the normal forwarding of a device, where IP datagrams are switched between networks somewhat transparently. Relay Agents receive Dynamic Host Configuration Protocol (DHCP) messages and then generate a new DHCP message to send out on another interface.

The Cisco DHCP relay agent supports unnumbered interfaces. The DHCP relay agent automatically adds a static host route specifying the unnumbered interface as the outbound interface.

This concept module describes the how the Cisco DHCP Relay Agent supports Unnumbered Interfaces and includes the following sections:

Prerequisites for DHCP Relay Agent Support for Unnumbered Interfaces

  • The interface must be configured to be unnumbered with a helper address.
  • The interface must be configured with an IP helper address pointing to the destination Dynamic Host Configuration Protocol (DHCP) server.

Information About DHCP Relay Agent Support for Unnumbered Interfaces

Overview of DHCP Relay Agent Support for Unnumbered Interfaces

The Cisco Dynamic Host Configuration Protocol (DHCP) relay agent now supports unnumbered interfaces. The DHCP relay agent automatically adds a static host route specifying the unnumbered interface as the outbound interface.

When the interfaces are configured as unnumbered with IP helper addresses, the DHCP Relay Agent support for those interfaces gets automatically enabled.

Benefits of DHCP Relay Agent Support for Unnumbered Interfaces

The Cisco Dynamic Host Configuration Protocol (DHCP) Relay Agent Support for Unnumbered Interfaces feature reduces configuration tasks and costs. In most cases, whenever an unnumbered interface is configured, a static route for any host beyond the unnumbered interface must be manually configured by a network administrator. For DHCP relay, this static route is automatically maintained.

The ability to support unnumbered interfaces provides the following benefits:

  • Allows DHCP clients across multiple unnumbered interfaces to share IP addresses.
  • Eliminates wastage of IP addresses configured to subnets for numbered interfaces.
  • Removes the requirement for static host route information, which now is handled dynamically by DHCP relay.

Additional References for DHCP Relay Agent Support for Unnumbered Interfaces

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