Router Advertisement (RA) Messages

The RA message option 1, SLAAC is the default option for the router. The router interface can be configured for three options:

  1. SLAAC – Which says I’m all you need (Prefix, Prefix-length, Default Gateway)”
  2. SLAAC and DHCPv6 stateless–My information are here but you also need to get other information like DNS addresses from a DHCPv6 server.
  3. DHCPv6 Only – I can’t give you any information. Send a request to DHCPv6 server for all your required information.

RA Option 1- SLAAC

The ICMPv6 RA message is a suggestion to a device on how to get an IPv6 global unicast address. The device operating system is final authority to get an IPv6 address.This message suggests that the receiving device use the information in the RA message to create its own IPv6 global unicast address. The DHCPv6 services are not required for SLAAC.

Basically, SLAAC is no central server required to allocate global unicast addresses. The SLAAC is not keeping a list of devices and their addresses. The client device uses the information in the RA message to create its own global unicast address. The host device sends a request for addressing information to the local router. The local router advertises its addressing information (Prefix, Prefix length and Default Gateway) through RA message towards host computer. The two parts of the address are created as follows:

  • Prefix– Received in the RA message
  • Interface ID– Uses the EUI-64 process or by generating a random 64-bit number

RA Option 2 – SLAAC and DHCPv6 stateless

The router’s interface can be configured to send a router advertisement using SLAAC and stateless DHCPv6. A stateless DHCPv6 server distributes DNS server addresses and domain names only. It does not allocate global unicast addresses.The RA Option 2 (SLAAC and Stateless DHCPv6) functions are here:

  • SLAAC to create its own IPv6 global unicast address, router’s link-local address and the RA’s source IPv6 address for the default gateway address.
  • A stateless DHCPv6 server to obtain other information like DNS server address and a domain name.

RA Option 3 – Stateful DHCPv6

Stateful DHCPv6 is work just like DHCP for IPv4 addresses. A device can get its addressing information including a global unicast address, prefix length, and the addresses of DNS servers automatically using the services of a stateful DHCPv6 server. This option suggests devices:

  • The link-local address of the router, the RA’s source IPv6 address for the default gateway address.
  • A stateful DHCPv6 server to obtain a global unicast address, DNS server address, domain name and all other information.

A stateful DHCPv6 server allocates and maintains a list of devices which receive IPv6 address.The default gateway address can only be obtained from the RA message. The stateless or stateful DHCPv6 server does not afford the default gateway address.