IP Addresses and Services Configuration Guide for Cisco 8000 Series Routers, IOS XR Releases

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IP Addresses and Services Configuration Guide for Cisco 8000 Series Routers, IOS XR Releases

DHCPv6 relay source address

Updated: August 12, 2026

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This topic describes the DHCPv6 relay source address feature on the Cisco 8000 Series Router.

The DHCPv6 relay source address is a configuration feature that

  • allows the selection of a specific IPv6 address from the relay source-interface,

  • ensures consistent source address selection for DHCPv6 server communication, and

  • eliminates the need to modify firewall rules when new IPv6 addresses are added to an interface.

Table 1. Feature History Table

Feature Name

Release Information

Feature Description

Configure DHCPv6 relay source address

Release 25.1.1

Introduced in this release on: Fixed Systems (8200 [ASIC: Q200, P100], 8700 [ASIC: P100, K100]); Centralized Systems (8600 [ASIC:Q200]); Modular Systems (8800 [LC ASIC: Q100, Q200, P100])

You can now select an IPv6 address from the configured relay source-interface to be used as the source address for forwarding packets to a server. By selecting a fixed source address, the need to frequently update firewall rules when new, lower-value IPv6 addresses are added is minimized.

Previously, the router automatically used the lowest numbered IPv6 address configured on that interface as the source address.

Benefits of DHCPv6 relay source address

The DHCPv6 relay source address feature provides these benefits:

  • Flexibility — You can now choose a specific IPv6 address from the relay source-interface, allowing more control over which address is used for packet forwarding.

  • Reduced firewall updates — By selecting a fixed source address, the need to frequently update firewall rules when new, lower-value IPv6 addresses are added is minimized.

  • Enhanced security — With a stable relay source address, security policies can be more consistently applied, reducing the risk of misconfigurations.

  • Improved troubleshooting — Having a predictable relay source address makes it easier to track and troubleshoot network traffic issues.

Configuration guidelines and restrictions for DHCPv6 relay source address

These configuration guidelines and restrictions apply to the DHCPv6 relay source address feature:

  • Removing the configured DHCPv6 relay source-interface IP disables the feature and retains the old behavior of automatically using the lowest numbered IPv6 address configured on that interface as the source address.

  • You can configure the DHCPv6 relay source address under helper-address and in the source-interface as well.

Configure DHCPv6 relay source address

Procedure

  1. Enter the global configuration mode and then enter the DHCP IPv6 configuration mode.

    Example:

    Router# configure terminal
Router(config)# dhcp ipv6

  2. Create a DHCPv6 relay profile.

    Example:

    Router(config-dhcpv6)# profile test relay

  3. Configure the helper address, specifying the interface and source address to use for packets forwarded to that server.

    Example:

    Router(config-dhcpv6-relay-profile)# helper-address vrf default 2011::3 TenGigE0/0/0/0 1001::10
Router(config-dhcpv6-relay-profile)# helper-address vrf default 2011::4

  4. Configure the source interface and source address for the relay profile.

    Example:

    Router(config-dhcpv6-relay-profile)# source-interface TenGigE0/1/0/0 1001::1

  5. Attach the relay profile to an interface and commit the configuration.

    Note

    To disable this feature, use the no form of the command.

    Example:

    Router(config-dhcpv6-relay-profile)# interface Bundle-Ether1 relay profile test
Router(config-dhcpv6-relay-profile)# commit

  6. Verify the DHCPv6 relay source address configuration.

    Example:

    Router# show running-config interface TenGigE0/0/0/0
dhcp ipv6
 profile test relay
  helper-address vrf default 2011::3 TenGigE0/0/0/0 1001::10
  helper-address vrf default 2011::4
  source-interface TenGigE0/1/0/0 1001::1
!
interface Bundle-Ether1 relay profile test

    In this example, when a packet is received on the profile test relay :

    • The address 1001::10, specified as the DHCPv6 relay source address on interface TenGigE0/0/0/0, is used to send packets to server 2011::3.

    • The address 1001::1, specified as the DHCPv6 relay source address on interface TenGigE0/1/0/0, is used to send packets to server 2011::4.