The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) server sends its replies to the source address of relayed messages.
Normally, a DHCPv6 relay uses the address of the server-facing interface used to send messages as the source. However, in
some networks, it may be desirable to configure a more stable address (such as a loopback interface) and have the relay use
that interface as the source address of relayed messages. The DHCPv6 relay source configuration feature provides this capability.
Restrictions for Configuring a DHCPv6 Relay Source
If the configured interface is shut down, or if all of its IPv6 addresses are removed, the relay will revert to its standard
The command line interface (CLI) will report an error if the user attempts to specify an interface that has no IPv6 addresses
The interface configuration takes precedence over the global configuration if both have been configured.
Information About DHCPv6 Relay Source Configuration
DHCPv6 Relay Source Configuration
The DHCPv6 server sends its replies to the source address of relayed messages. Normally, a DHCPv6 relay uses the address
of the server-facing interface used to send messages as the source. However, in some networks, it may be desirable to configure
a more stable address (such as a loopback interface) and have the relay use that interface as the source address of relayed
messages. The DHCPv6 Relay Source Configuration feature provides this capability.
The figure below shows a simple network with a single client, relay, and server. The relay and server communicate over 2001:DB8:1::/64,
and the relay has a client-facing interface on 2001:DB8:2::/64. The relay also has a loopback interface configured with address
When the relay receives a request from the client, the relay includes an address from the client-facing interface (Ethernet
1/0) in the link-address field of a relay-forward message. This address is used by the server to select an address pool. The
relay then sends the relay-forward message toward the server. By default, the address of the server-facing (Ethernet 0/0)
interface is used as the IPv6 source, and the server will send any reply to that address.
If the relay source interface is explicitly configured, the relay will use that interface’s primary IPv6 address as the IPv6
source for messages it forwards. For example, configuring Loopback 0 as the source would cause the relay to use 2001:DB8:3:1/64
as the IPv6 source address for messages relayed toward the server.
How to Configure a DHCPv6 Relay Source
Configuring a DHCPv6 Relay Source
Perform the following tasks to configure a DHCPv6 relay source:
Configuring a DHCPv6 Relay Source on an Interface
Perform this task to configure an interface to use as the source when relaying messages.
Command or Action
Enables privileged EXEC mode.
Enter your password if prompted.
Device# configure terminal
Enters global configuration mode.
Device(config)# interface loopback 0
Specifies an interface type and number, and enters interface configuration mode.
Additional References for DHCPv6 Relay Source Configuration
Standards and RFCs
RFCs for IPv6
The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use
these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products
and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.
Feature Information for DHCPv6 Relay Source Configuration
The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for DHCPv6 Relay Source Configuration
DHCPv6 Relay Source Configuration
Cisco IOS XE Fuji 16.8.1a
In some networks that use DHCPv6, it may be desirable to configure a stable address (such as a loopback interface) and have
the relay use that interface as the source address of relayed messages. The DHCPv6 relay source configuration feature provides