SLAAC

SLAAC Overview

Not every network has access to a DHCPv6 server. But every device in an IPv6 network needs a GUA. The SLAAC method enables hosts to create their own unique IPv6 global unicast address without the services of a DHCPv6 server.

SLAAC is a stateless service. This means there is no server that maintains network address information to know which IPv6 addresses are being used and which ones are available.

SLAAC uses ICMPv6 RA messages to provide addressing and other configuration information that would normally be provided by a DHCP server. A host configures its IPv6 address based on the information that is sent in the RA. RA messages are sent by an IPv6 router every 200 seconds.

A host can also send a Router Solicitation (RS) message requesting that an IPv6-enabled router send the host an RA.

SLAAC can be deployed as SLAAC only, or SLAAC with DHCPv6.

Enabling SLAAC

Refer to the following topology to see how SLAAC is enabled to provide stateless dynamic GUA allocation.

Assume R1 GigabitEthernet 0/0/1 has been configured with the indicated IPv6 GUA and link-local addresses. Click each button for an explanation of how R1 is enabled for SLAAC.

R1# show ipv6 interface G0/0/1
GigabitEthernet0/0/1 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::1
  No Virtual link-local address(es):
  Description: Link to LAN
  Global unicast address(es):
    2001:DB8:ACAD:1::1, subnet is 2001:DB8:ACAD:1::/64
  Joined group address(es):
    FF02::1
    FF02::1:FF00:1
(output omitted)
R1#

R1(config)# ipv6 unicast-routing
R1(config)# exit
R1#

R1# show ipv6 interface G0/0/1 | section Joined
  Joined group address(es):
    FF02::1
    FF02::2
    FF02::1:FF00:1
R1#

SLAAC Only Method

The SLAAC only method is enabled by default when the ipv6 unicast-routing command is configured. All enabled Ethernet interfaces with an IPv6 GUA configured will start sending RA messages with the A flag set to 1, and the O and M flags set to 0, as shown in the figure.

The A = 1 flag suggests to the client that it create its own IPv6 GUA using the prefix advertised in the RA. The client can create its own Interface ID using either Extended Unique Identifier method (EUI-64) or have it randomly generated.

The O =0 and M=0 flags instruct the client to use the information in the RA message exclusively. The RA includes the prefix, prefix-length, DNS server, MTU, and default gateway information. There is no further information available from a DHCPv6 server.

In the example, PC1 is enabled to obtain its IPv6 addressing information automatically. Because of the settings of the A, O and M flags, PC1 performs SLAAC only, using the information contained in the RA message sent by R1.

The default gateway address is the source IPv6 address of the RA message, which is the LLA for R1. The default gateway can only be obtained automatically from the RA message. A DHCPv6 server does not provide this information.

C:\PC1> ipconfig
Windows IP Configuration
Ethernet adapter Ethernet0:
   Connection-specific DNS Suffix  . : 
   IPv6 Address. . . . . . . . . . . : 2001:db8:acad:1:1de9:c69:73ee:ca8c
   Link-local IPv6 Address . . . . . : fe80::fb:1d54:839f:f595%21
   IPv4 Address. . . . . . . . . . . : 169.254.202.140
   Subnet Mask . . . . . . . . . . . : 255.255.0.0
   Default Gateway . . . . . . . . . : fe80::1%6
C:\PC1>

ICMPv6 RS Messages

A router sends RA messages every 200 seconds. However, it will also send an RA message if it receives an RS message from a host.

When a client is configured to obtain its addressing information automatically, it sends an RS message to the IPv6 all-routers multicast address of ff02::2.

The figure illustrates how a host initiates the SLAAC method.

1. PC1 has just booted and has not yet received an RA message. Therefore, it sends an RS message to the IPv6 all-routers multicast address of ff02::2 requesting an RA.
2. R1 is part of the IPv6 all-routers group and received the RS message. It generates an RA containing the local network prefix and prefix length (e.g., 2001:db8:acad:1::/64). It then sends the RA message to the IPv6 all-nodes multicast address of ff02::1. PC1 uses this information to create a unique IPv6 GUA.

Host Process to Generate Interface ID

Using SLAAC, a host typically acquires its 64-bit IPv6 subnet information from the router RA. However, it must generate the remainder 64-bit interface identifier (ID) using one of two methods:

• Randomly generated – The 64-bit interface ID is randomly generated by the client operating system. This is the method now used by Windows 10 hosts.
• EUI-64 – The host creates an interface ID using its 48-bit MAC address and inserts the hex value of fffe in the middle of the address. Some operating systems default to the randomly generated interface ID instead of the EUI-64 method, due to privacy concerns. This is because the Ethernet MAC address of the host is used by EUI-64 to create the interface ID.

For instance, in the following ipconfig output, the Windows 10 PC1 host used the IPv6 subnet information contained in the R1 RA and randomly generated a 64-bit interface ID as highlighted in the output.

C:\PC1> ipconfig
Windows IP Configuration
Ethernet adapter Ethernet0:
   Connection-specific DNS Suffix  . : 
   IPv6 Address. . . . . . . . . . . : 2001:db8:acad:1:1de9:c69:73ee:ca8c
   Link-local IPv6 Address . . . . . : fe80::fb:1d54:839f:f595%21
   IPv4 Address. . . . . . . . . . . : 169.254.202.140
   Subnet Mask . . . . . . . . . . . : 255.255.0.0
   Default Gateway . . . . . . . . . : fe80::1%6
C:\PC1>

Duplicate Address Detection

The process enables the host to create an IPv6 address. However, there is no guarantee that the address is unique on the network.

SLAAC is a stateless process; therefore, a host has the option to verify that a newly created IPv6 address is unique before it can be used. The Duplicate Address Detection (DAD) process is used by a host to ensure that the IPv6 GUA is unique.

DAD is implemented using ICMPv6. To perform DAD, the host sends an ICMPv6 Neighbor Solicitation (NS) message with a specially constructed multicast address, called a solicited-node multicast address. This address duplicates the last 24 bits of IPv6 address of the host.

If no other devices respond with a NA message, then the address is virtually guaranteed to be unique and can be used by the host. If an NA is received by the host, then the address is not unique, and the operating system has to determine a new interface ID to use.

The Internet Engineering Task Force (IETF) recommends that DAD is used on all IPv6 unicast addresses regardless of whether it is created using SLAAC only, obtained using stateful DHCPv6, or manually configured. DAD is not mandatory because a 64-bit interface ID provides 18 quintillion possibilities and the chance that there is a duplication is remote. However, most operating systems perform DAD on all IPv6 unicast addresses, regardless of how the address is configured.

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