Primary, secondary, and extended name servers

For redundancy purposes multiple name servers can be associated with a subscriber session:

• for NBNS name servers

  a primary and secondary address

• for IPv4 and IPv6 DNS name servers

  a primary, a secondary, and extended addresses

  The extended DNS name servers are an ordered list of addresses beyond primary and secondary and can be provisioned using an SR OS or third party DHCP server only. Extended addresses are not applicable for PPPoE IPCP subscriber hosts.

The order of preference in which the name servers are sent to the client is:

1. primary

2. secondary

3. extended

A client typically contacts the name servers in order of preference.

Typically, all name servers are obtained from the same authentication origin, for example RADIUS, but this is not enforced in SR OS. For each subscriber session, primary, secondary, and extended name servers are independently determined based on the authentication origin priorities.

For example, DNS name server IP addresses obtained from different authentication origins for an IPoE DHCPv4 host (relay):

• a primary DNS server (10.1.1.1) is configured in the Local User Database (LUDB)

• a primary (10.1.2.1) and secondary (10.1.2.2) DNS server is received in a RADIUS Access-Accept message

• the DHCP Offer or Ack message received from the DHCP server contains a domain name server option that includes four DNS servers (10.1.3.1, 10.1.3.2, 10.1.3.3 and 10.1.3.4)

Using default authentication origin priorities, the following DNS name server IP addresses are associated with the subscriber session and included in a domain name server option in the DHCP Ack message sent to the client:

• Primary DNS = 10.1.1.1 (origin = LUDB, highest priority for primary DNS)

• Secondary DNS = 10.1.2.2 (origin = RADIUS, highest priority for secondary DNS)

• Extended DNS 1 = 10.1.3.3 (origin = DHCP)

• Extended DNS 2 = 10.1.3.4 (origin = DHCP)