4.15 SLAAC Options

Two SLAAC Options

  • Using SLAAC (Stateless Address Autoconfiguration), there are two configuration options available:

    • Strictly SLAAC: This option allows hosts to generate their own addresses using only the information provided in Router Advertisements (RA).

    • SLAAC combined with Stateless DHCPv6: This option allows hosts to use the prefix from RA while also seeking additional information from a DHCPv6 server.

  • The configuration choice is made at the router level and communicated to hosts via flags in RA messages. These messages serve as suggestions rather than strict instructions.

SLAAC Option

  • Functionality:

    • The SLAAC option provides RA messages that include necessary elements for IP configuration, except for the interface identifier (ID), which the host must generate itself.

    • No additional settings are required from a DHCP server.

  • RA Message Content:

    • Contains prefix, prefix length, default gateway, and DNS server information specific to the router interface.

    • The complete RA message exchange is essential for proper configuration, facilitating host configuration by sharing the aforementioned parameters.

Stateless DHCPv6 Option

  • Functionality:

    • Under this option, the RA message dictates to the host to utilize the prefix and prefix length for address generation but directs it to acquire extra details from a stateless DHCPv6 server.

  • Characteristics of Stateless DHCPv6 Server:

    • The server remains stateless and does not allocate IP addresses but supplies configuration details, such as the DNS server's address.

Host Behavior in SLAAC Options

  • Regardless of the option chosen, the host generates its interface ID independently.

  • When opting for Stateless DHCPv6, the host acts as a DHCPv6 client, initiating communication with the DHCP server using UDP destination port 547:

    • Process:

      • Starts with an RS/RA exchange to secure prefix and prefix length for local IPv6 address generation.

      • The host sends a SOLICIT message to multicast address FF02::1:2 to locate a DHCPv6 server.

      • The server responds with an ADVERTISE message, identifying itself to the host.

      • Next, the host sends an INFO-REQUEST message, signaling that it has a generated address and requires additional configuration details only.

      • Lastly, the server sends a REPLY, providing the requested configuration information.

  • The client may perform Duplicate Address Detection (DAD) prior to finalizing configuration, similar to operations in the SLAAC scenario.

Lesson Recap

  • Key Takeaways:

    • Two SLAAC options exist: SLAAC only and SLAAC with Stateless DHCPv6.

    • Stateless DHCPv6 solely provides miscellaneous configuration data without assigning addresses.

    • The RA messages relay configurations but are not obligatory for hosts, allowing independent action.

    • SLAAC requires only the RS/RA exchange for complete address configuration.

    • Adding Stateless DHCPv6 involves the RS/RA exchange followed by DHCPv6-specific message exchanges (SOLICIT, ADVERTISE, INFO-REQUEST, REPLY).

    • The SOLICIT message is used to find a DHCPv6 server, with steps outlined for subsequent messages leading to configuration completion.