There are two mechanisms to assign a DNSv6 name server to an IPv6 SLAAC hosts:
Stateless DHCPv6
The client starts a stateless DHCPv6 transaction by sending an Information Request message.
PPPoE session
An Information Request message is always authenticated:
When no DNSv6 name servers are received during authentication, then DHCPv6 relay is performed irrespective of whether the DNSv6 name servers are associated with the PPP session or not. The DNSv6 name servers in the Reply message from the DHCP server (or defaults if not available from DHCP) are sent to the client. These DNSv6 name servers are not associated with the PPP session.
When DNSv6 name servers are received during authentication, DHCPv6 proxy is performed and the DNSv6 name servers are included in a DNS Recursive Name Server Option (23) of the Reply message sent on behalf of a DHCPv6 server. The DNSv6 name servers are not associated with the PPP session.
Because the Information Request for PPP SLAAC hosts are always authenticated, a mid-session change of DNSv6 name servers using CoA is not supported. Instead, the DNSv6 name servers can be included in the RADIUS Access Accept message.
IPoE session
An Information Request message is authenticated based on the ipoe-session min-auth-interval value. When IPoE sessions are disabled, the authentication is based on the re-authentication command in the RADIUS authentication policy.
Unauthenticated Information Request
When DNSv6 name servers different from defaults are associated with the IPoE session, DHCPv6 proxy is performed and the DNSv6 name servers are included in a DNS Recursive Name Server Option (23) of the Information Reply message sent on behalf of a DHCPv6 server.
Extended DNSv6 name servers saved in the IPoE session are not included in the Reply message to the client.
When default or no DNSv6 name servers are associated with the IPoE session, DHCPv6 relay is performed. The DNSv6 name servers in the Reply message from the DHCP server (or defaults if not available from DHCP) are sent to the client. These DNSv6 name servers are now associated with the IPoE session.
Mid-session change of DNSv6 name servers using CoA is supported: the DNSv6 name servers in the CoA are associated with the IPoE session and included in the reply to the next unauthenticated Information Request (proxy-server).
Authenticated Information Request
When no DNSv6 name servers are received during authentication, then DHCPv6 relay is performed irrespective of whether DNSv6 name servers are associated with the IPoE session or not. The DNSv6 name servers in the Reply message from the DHCP server (or defaults if not available from DHCP) are sent to the client. These DNSv6 name servers are now associated with the IPoE session.
When DNSv6 name servers are received during authentication, then DHCPv6 proxy is performed and the DNSv6 name servers are included in a DNS Recursive Name Server Option (23) of the Reply message sent on behalf of a DHCPv6 server. These DNSv6 name servers are now associated with the IPoE session.
Mid-session change of DNSv6 name servers using CoA is not supported for authenticated Information Requests. Instead, the DNSv6 name servers can be included in the RADIUS Access Accept message
Router Advertisements
A Recursive DNS Server (RDNSS) option as defined in RFC 6106, IPv6 Router Advertisement Options for DNS Configuration, is included in the Router Advertisement sent to the IPv6 SLAAC host.
The following CLI command includes the RDNSS option in IPv6 Router Advertisements for SLAAC hosts and sets the RDNSS lifetime:
config>service>ies>sub-if>grp-if>ipv6>rtr-adv
config>service>vprn>sub-if>grp-if>ipv6>rtr-adv
config>subscr-mgmt>rtr-adv-plcy
dns-options
[no] include-dns - Set/reset inclusion of the RDNSS server
option 25 on this group-interface
[no] rdnss-lifetime - Maximum time the RDNSS address is valid
in this group-interface
The configuration at the group interface level is common to all subscriber sessions active on the interface. The configuration in a router advertisement policy overrides the group interface configuration for the sessions associated with the policy.