SonicOS IPv6 Prefix Delegation (DHCPv6)
Description
DHCPv6 Prefix Delegation, the ISP assigns a block of IPv6 address space (the Delegated Prefix) to the firewall via DHCPv6. The firewall uses this prefix to address downstream LAN-side interfaces and, where required by the ISP, the WAN interface itself.
DHCPv6 Prefix Delegation
Background
When a WAN interface is configured as a DHCPv6 client with prefix delegation enabled, the ISP provides a Delegated Prefix, a block of IPv6 address space from which the firewall can assign subnets to downstream (LAN-side) interfaces. In standard operation, this delegated prefix is used only for LAN-side interface addressing.
Some ISP environments require the customer firewall to also hold an IPv6 address from the delegated prefix on the WAN interface itself, in addition to the standard LAN-side use.
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SonicOS 8: Administrators can assign an IPv6 address derived from the ISP-delegated prefix to the WAN interface itself. This is configured through the Advanced Tab of the WAN interface using the Add Downstream IPv6 Address Delegated from DHCP-PD option. |
Prerequisites
- The WAN interface must be configured with IP Assignment set to DHCPv6.
- Enable DHCPv6 prefix delegation must be toggled on in the General Tab.
- The ISP must have assigned a Delegated Prefix to the interface.
Configuration
The following fields are available when adding a downstream IPv6 address from the delegated prefix on the WAN interface:
- Delegated Prefix Assignment: select the delegated prefix received from the ISP via DHCPv6.
- Preferred IPv6 Address: the specific IPv6 address, derived from the delegated prefix, to assign to the WAN interface.
- Preferred Prefix Length: the prefix length for the assigned address.
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For step-by-step configuration instructions, refer to the SonicOS 8 Administration Guide.
Comparison
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DHCPv6 Prefix Delegation |
RA Prefix Delegation |
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Prefix Source |
DHCPv6 server (ISP assigns prefix via DHCPv6) |
Upstream router (prefix advertised via Router Advertisement) |
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WAN Interface Address from Prefix |
Supported, assign an address from the delegated prefix to the WAN interface itself |
Not applicable, prefix is received on the WAN interface from RA |
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Downstream Prefix Distribution |
Sub-prefixes assigned to LAN-side interfaces |
Sub-prefixes delegated to LAN-side interfaces via route mode |
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Forwarding Mode |
Route mode |
Route mode |
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NDP Proxy Equivalent |
No |
Yes, implements NDP Proxy functions without L2 proxy behavior |
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Configuration Location |
WAN IPv6 Interface -> <Edit> -> General Tab -> (Enable DHCPv6 prefix delegation & send preferred delegated prefix)
WAN IPv6 Interface -> <Edit> -> <Advanced Tab> -> (Delegated Prefix Assignment, Preferred IPv6 Address, Preferred Prefix Length) |
WAN IPv6 Interface -> <Edit> -> Advanced Tab -> Advanced Settings
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Real-World Use Cases
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Use Case |
Scenario |
Outcome |
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ISP Requiring Delegated Prefix Address on WAN Interface |
An ISP delivers a Delegated Prefix to the firewall via DHCPv6 and requires the WAN interface itself to hold an IPv6 address from that prefix, in addition to the LAN-side use of the delegated range. |
DHCPv6 Prefix Delegation is configured on the WAN interface. An IPv6 address is added from the delegated prefix via the Advanced Tab using the Add Downstream IPv6 Address Delegated from DHCP-PD option, satisfying the ISP provisioning requirement. |
Summary
SonicOS 8 supports DHCPv6 Prefix Delegation on WAN interfaces, enabling the firewall to receive a delegated IPv6 prefix from the ISP via DHCPv6. The firewall can also assign an IPv6 address to the WAN interface from the delegated prefix, addressing ISP provisioning requirements that extend beyond standard LAN-side prefix usage.
This capability ensures proper integration with ISP networks that require WAN-side addressing as part of the delegated prefix, while still allowing distribution of sub-prefixes to downstream interfaces.
DHCPv6 Prefix Delegation is configured on the WAN interface under Network > Interfaces > <Edit IPv6 Interface> > Advanced Tab.
