Prefix Delegation
In IPv4, clients are typically assigned private addresses on the LAN, and the router performs NAT when sending traffic out to the broader WAN network.
In IPv6, clients use globally addressable addresses. The work of assigning addresses from a prefix is delegated to the router. For detailed information about how prefix delegation works, see RFC3633. At a high level;
"A delegating router is provided IPv6 prefixes for delegation to requesting routers. A requesting router requests prefixes from the delegating router. The delegating router chooses prefixes, and sends them to the requesting router. The requesting router might assign a subnet from a delegated prefix to one of its interfaces, and begin sending router advertisements for the prefix on that link."
Once the router has the prefix to use, different modes can be used to assign addresses to the clients. See https://www.rfc-editor.org/rfc/rfc8415 for details.
SSR Support for Prefix Delegation
The SSR acts as a DHCPv6 client on the WAN-side. When the router advertisement is received, the SSR uses the M and O flags in the advertisement to determine the WAN-side address, as well as the operating mode of the interface; SLAAC, stateful DHCPv6, and stateless DHCPv6. Alternatively, a static address could be set.
The corresponding LAN-side receives the delegated prefix and uses it in router-advertisements to the clients. DHCPv6 server is not supported for the LAN; only SLAAC mode is used.
WAN Configuration
- Set the WAN interface to use DHCPv6. Alternatively, a static address can be configured.
- Enable prefix-delegation mode:
prefix-delegation true. - Specify the group (
pd-group-1) to associate this WAN with the corresponding LAN:prefix-delegation-group pd-group-1
device-interface wan-device
network-interface wan
dhcp v6
prefix-delegation true
prefix-delegation-group pd-group-1
Additional authorization configuration can be provided.
LAN Configuration
Use the following information to set the LAN configuration.
- Set the interface to use
dhcp v6-pd. - Specify the group corresponding to the WAN:
prefix-delegation-group pd-group-1. - Set the subnet-id to send to the WAN router. This identifies the correct prefix to delegate:
prefix-delegation-subnet-id 1. - Enable router-advertisement:
router-advertisement true. This sends the prefix to the clients on the LAN side. Multiple LAN interfaces can have the same group, but must have unique subnet-ids. Only 8 bit IDs are used - 56 bits of the prefix are used for delegation.
device-interface lan-device
network-interface lan
dhcp v6-pd
prefix-delegation-group pd-group-1
prefix-delegation-subnet-id 1
router-advertisement true
Debugging Commands
The following show commands can be used for debugging configuration issues.
show network-interface: Display network-interface data for network-interface.
admin@test1.Fabric128# show network-interface
Tue 2026-02-24 21:51:34 UTC
✔ Retrieving network interfaces...
=========== ======= ========== =========== ============ ====== ============= ========== ========== ======================================== =================== ========== ======== ======
Router Node Device Name Forwarding VLAN Device Type Type DHCP Address Gateway Hostname Status GIID
=========== ======= ========== =========== ============ ====== ============= ========== ========== ======================================== =================== ========== ======== ======
Fabric128 test1 wan-dev wan-intf true 0 ethernet external disabled 2001:db1:1:1::102/64 2001:db1:1:1::101 -- up 1
Fabric128 test1 lan-dev1 lan-intf1 true 0 ethernet external v6-pd 2001:db1:1:2002:f816:3eff:fed4:9fe2/64 -- -- up 2
Fabric128 test1 lan-dev2 lan-intf2 true 0 ethernet external v6-pd 2001:db1:1:2003:f816:3eff:fe25:53ea/64 -- -- up 3
show dhcp mappings: Show each DHCP mapping from an interface to mapping/IP family/config types.
admin@test1.Fabric128# show dhcp mappings
Tue 2026-02-24 21:54:27 UTC
================= ================== ====== ============== ================ ==========================
Src Device Port Dest Device Port VLAN Mapping Type IP Family Type Config Type
================= ================== ====== ============== ================ ==========================
1 0 0 originating ipv6 client
2 0 0 originating ipv6 prefix_delegation_server
3 0 0 originating ipv6 prefix_delegation_server
show dhcp prefix-delegation: Show the prefix learned for prefix-delegation.
admin@gouda.novigrad# show dhcp prefix-delegation
Wed 2020-04-22 14:47:05 UTC
========== ============ ================ ========== ============== ===============
Router Group Name Interface Name Status Prefix Prefix Length
========== ============ ================ ========== ============== ===============
novigrad pd-group-1 t128tuntap1 resolved 2001:db2:1:: 56
Completed in 0.08 seconds
show dhcp v6: Display dhcp lease info for network-interfaces.
admin@gouda.novigrad# show dhcp v6
Wed 2020-04-22 14:47:05 UTC
========== ======= ================== =================== ============ ================================= =============== =================================
Router Node Device Interface Network Interface Dhcp State Address Prefix Length Gateway
========== ======= ================== =================== ============ ================================= =============== =================================
novigrad gouda wan wan-interface Resolved 2001:db8:85a3:0:0:8a2e:370:7334 96 2001:db8:85a3:0:0:8a2e:370:7330
Completed in 0.20 seconds