Stateless Address Autoconfiguration (SLAAC)

Stateless Address Autoconfiguration (SLAAC) is a method that allows a device to obtain its prefix, prefix length, and default gateway address information from an IPv6 router without the use of a DHCPv6 server. Using SLAAC, devices rely on the local router’s ICMPv6 Router Advertisement (RA) messages to obtain the necessary information.

IPv6 routers periodically send out ICMPv6 Router Advertisement (RA) messages to all IPv6-enabled devices on the network. By default, Cisco routers send out RA messages every 200 seconds to the IPv6 all-nodes multicast group address. An IPv6 device on the network does not have to wait for these periodic RA messages. A device can send a Router Solicitation (RS) message to the router, using the IPv6 all-routers multicast group address. When an IPv6 router receives an RS message it will immediately respond with a router advertisement.

Even though an interface on a Cisco router can be configured with an IPv6 address, this does not make it an “IPv6 router”. An IPv6 router is a router that:

IPv6 routing is not enabled by default. To enable a router as an IPv6 router, the ipv6 unicast-routing global configuration command must be used.

Note: Cisco routers are enabled as IPv4 routers by default.

The ICMPv6 RA message contains the prefix, prefix length, and other information for the IPv6 device. The RA message also informs the IPv6 device how to obtain its addressing information. The RA message can contain one of the following three options, as shown in the figure:

Routers send ICMPv6 RA messages using the link-local address as the source IPv6 address. Devices using SLAAC use the router’s link-local address as their default gateway address.