This article describes why "preserving and restoring IPv6 prefixes assigned using DHCPv6 Prefix Delegation" feature is not working with PPPoE.

If "preserving and restoring IPv6 prefixes assigned using DHCPv6 Prefix Delegation" feature is configured, e.g. the following configuration is present:

set access address-preservation address-types delegated-prefix
set access address-preservation aging-time <>

The expectation is that the subscriber's delegated prefix will be preserved when this subscriber logs out, regardless of whether the subscriber was using plain DHCPv6 or DHCPv6 over PPPoE. However, after logout the output of "show network-access address-assignment preserved" command is empty and the same DHCPv6 over PPPoE subscriber gets allocated another delegated prefix after login.

user@device> show network-access address-assignment preserved
user@device>

"Subscriber delegated prefix preservation" feature (which is called "preserving and restoring IPv6 prefixes assigned using DHCPv6 Prefix Delegation" in the Feature Explorer) has the following functionality:

It supports preserving and restoring a subscriber’s delegated prefix through
multiple logins.

Receiving the same delegated prefix from the MX prevents an IA-PD change, which
triggers renegotiation for all hosts on the LAN behind the CPE.

To utilise this functionality, a configuration command is executed. This
functionality defaults to disabled. It is enabled, via the access
address-preservation configuration command.

A subscriber is identified by the subscriber’s Agent-Circuit-Id. Each
participating subscriber must have an Agent-Circuit-Id. The Agent-Circuit-Id
must be present within upstream control traffic through standard means (option 18 for DHCPv6).

If the access address-preservation prefix configuration is set and a subscriber
has an Agent-Circuit-Id, the subscribers prefix will be preserved through
multiple logins. The Agent-Circuit-Id is the key that will be used to store the
subscriber’s prefix when the subscriber logs out. When the subscriber, with the
same Agent-Circuit-Id, logs in again the prefix is retrieved and presented to
the subscriber.

Implementation of this feature relies on presence of Agent-Circuit-Id in the control traffic. This information is usually inserted by an access node into PPPoE discovery packets and DHCPv4/DHCPv6 packets. However, insertion of Agent-Circuit-Id into DHCPv6 packets encapsulated in PPPoE is usually not supported by access nodes and DHCPv6 over PPPoE packets arrive on MX without Agent-Circuit-Id information. Which means that ACI is usually present in PPPoE discovery packets, plain DHCPv4/DHCPv6 packets, but not in DHCPv6oPPPoE packets.

MX is not able to correlate Agent-Circuit-Id present in PPPoE discovery packets with IPv6 prefixes assigned to this CPE using DHCPv6 prefix delegation. Without Agent-Circuit-Id present in DHCPv6oPPPoE traffic MX is not able to preserve IPv6 prefixes assigned using DHCPv6 Prefix Delegation, and this feature doesn't work. However, if access node is able to insert option 18 into DHCPv6oPPPoE packets, this feature will work.