This article describes why "preserving and restoring IPv6 prefixes assigned using DHCPv6 Prefix Delegation" feature may not work for subscribers in VRF.

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. However, after logout the output of "show network-access address-assignment preserved" command is empty and the same 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. (For
example: option-82 sub-option 1 for DHCP, 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.

Due to a SW defect PR1777967 this feature didn't work for subscribers in VRF. In the fixed releases this feature is working and output of "show network-access address-assignment preserved" command supports an optional routing-instance name as an argument:

user@device> show network-access address-assignment preserved routing-instance ?
Possible completions:
  <routing-instance>   Name of routing instance
  SUBSCRIBERS

user@device> show network-access address-assignment preserved routing-instance SUBSCRIBERS
Key                                                        Delegated Prefix     Reservation End
interfaceId_Port //1/1                                     2001:0db8:aaaa:1::/64   May 22 12:06:19