Router Advertisement (RA) Messages
The ICMPv6 Router Advertisement (RA) message is a suggestion to a device on how to get an IPv6 global unicast address. The device operating system is also the final authority to get the IPv6 address. Furthermore, the ICMPv6 RA message consists of the following:
- Network prefix along with the prefix length
- Default gateway address
- DNS addresses along with the domain name
There are three options for Router Advertisement (RA) messages which used to get an IPv6 address automatically. The RA message option 1, SLAAC is the default option for the router. We can configure the router interface for the other option manually:
SLAAC
There are two types of IPv6 address auto-configuration. One is the old type to automatically configure IP address from IPv4 DHCP. The other type is to make the auto-configuration in IPv6 which empower the hosts to make the auto-configuration by themselves without the need to communicate to anybody else on the network. IPv6 makes the life of the network administrators easier, especially when it comes to dealing with huge address space provided by IPv6. IPv6 address number is very larger than IPv4.
SLAAC is the default RA option which says I’m all you need (Prefix, Prefix-length, Default Gateway)” As a result, an IPv6 host can configure for itself complete address settings automatically. Using SLAAC a router interface is assigned a 64-bit prefix, and then the last 64 bits of its address is derived by the host or router with help of EUI-64 process. The figure below illustrates the SLAAC Process.
The host computer sends Router Solicitation (RS) message to the Router and the router reply with RA message including the IPv6 Prefix, Prefix length and all other related information.
SLAAC and DHCPv6 stateless
We can configure the router interface to send a router advertisement (RA) message using SLAAC and stateless DHCPv6. A stateless DHCPv6 server distributes DNS server addresses and domain names only. It does not allocate global unicast addresses. SLAAC and Stateless DHCPv6 is RA option 2 which says my information is here but you also need to get other information like DNS addresses from a DHCPv6 server.
SLAAC create its own IPv6 global unicast address, router’s link-local address and the RA’s source IPv6 address for the default gateway address and a stateless DHCPv6 server obtain other information like DNS server address and a domain name. The figure below illustrates the SLAAC and DHCPv6 process.
- The clients send RS message to the router for IPv6 address prefix, prefix length and other related information.
- The Router reply with Router Advertisement (RA) message including prefix, prefix length and about DHCPv6 server.
- The client starts the DHCPv6 process with a DHCPv6 server.
Stateful DHCPv6
It is work like DHCP for IPv4 addresses. A device can get its addressing plan and information including a global unicast address, prefix length, and the addresses of DNS servers automatically using the services of a stateful DHCPv6 server. The RA message in this option says that I can’t give you any information you needed. Send a request to DHCPv6 server for all your required information. 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. The figure below illustrates the DHCPv6 process.
- The host sends a request for IPv6 address assignment including other related information.
- The server reply with assigned IPv6 address including with other related information like lease time, default gateway and DNS server address.
