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What is IPv6 Stateless Address Autoconfiguration ?

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  • Difficulty Level : Expert
  • Last Updated : 22 Jun, 2022
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The IPv6 addressing scheme is the successor of the IPv4 addressing. The IPv4 addressing scheme used a 32-bit address which translates to over 4 billion unique IPv4 addresses. However, with the expansion of devices that require internet connectivity, this address pool started running out. 

The IPv6 addressing scheme is based on a 128-bit address which translates into a database of about 340 Undecillion routable IPv6 addresses (1 Undecillion = 1036). 

The IPv6 addressing scheme has two ways in which the hosts acquire an IP address.

  • DHCPv6 (Stateful)
  • SLAAC (Stateless)

The IPv6 Stateless Address Auto-Configuration (SLAAC):

The Stateless Address Auto-Configuration enables hosts to generate a unique routable IPv6 address on their own. The router is configured to follow the IPv6 SLAAC protocol and sends out a Router Advertisement periodically.

 

The host can also send a Router Solicitation in order to trigger the Routing Advertisement by the router. The Router Solicitation is sent on the address FF02::2 which is the IPv6 multicast address for all the routers.

The Router Advertisement contains the Prefix Information (prefix (network address), prefix length (subnet mask), and default gateway). The host uses this information to generate an IPv6 address (global unicast address or GUA) for itself. The host then employs Duplicate Address Detection to ensure that its address is unique.

The IPv6 SLAAC Configurations:

In order for the router to be able to send the Router Advertisement and essentially for IPv6 SLAAC to function, these configurations must be set using the CLI of the concerned router.

S.No. Parameters Values
1 Enter Router EXEC mode Router>enable
2  Enter global configuration mode Router#configure terminal
3 Enable IPv6 routing Router(config)#ipv6 unicast-routing
4 Enter interface configuration mode Router(config)#interface interface
5 Configure an IPv6 address Router(config-if)#ipv6 address ipv6-address/prefix-length
6  Enable the interface Router(config-if)#no shutdown

Drawbacks of SLAAC:

  • The SLAAC can be used to generate an IPv6 global unicast address but the acquisition of additional information like that of a DNS server has not been standardized. 
  • All the additional information may be provided in the Router Advertisement but that is not the accepted standard yet. 
  • For this additional information, SLAAC is used in conjunction with DHCP. The SLAAC is responsible for GUA and DHCP for the rest of the information like DNS and others.

Implementation of SLAAC in Cisco Packet Tracer:

To implement the concept practically, we would now configure a router with IPv6 SLAAC with a single host in the Cosco Packet Tracer.  Once the Cisco Packet Tracer is installed, follow the following steps:

Step 1: Form a simple network with a Single Cisco 2911 router and a PC connected via crossover wire, as shown below:

 Single Cisco 2911 router and a PC connected via crossover wire

Step 2: Double click on the router icon and open the CLI prompt, shown below:

 

 

Step 3: To configure the Cisco 2911 Router for IPv6 SLAAC, enter the following commands as shown below:

  • Enter “no” for the initial configuration dialogue.
  • Enter “enable” to enter the Router EXEC mode. (According to configuration 1 in the configuration table)
  • Type in “configure terminal” and press enter. (According to configuration 2 in the configuration table)
  • Type in “ipv6 unicast-routing” this makes the router routable on the IPv6 address FF02::2. This enables the PC to send out the Router Solicitation. (According to configuration 3 in the configuration table)
  • Type in “interface gig 0/0”. The “0/0” part of the command depends on the interface you have connected the router to the PC. (According to configuration 4 in the configuration table)
  • Type in “ipv6 address XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX/YY” where “XXXX:….:XXXX” is the IPv6 address of the network and “YY” is the network mask. (According to configuration 5 in the configuration table)
  • Type in “no shutdown”. (According to configuration 6 in the configuration table)
  • To preview all the changes, type in “do show ipv6 interface gig 0/0”

The Cisco Router 2911 CLI after successfully executing all commands.

As all the configurations have been met, the router is now ready to send out its Router Advertisements. Now close the Router Configuration window.

Step 4: Double click on the PC icon and open the “Desktop” tab:

The PC configuration window

Step 5: Click on the “IP Configuration” option:

The “Desktop” option of the PC configuration.

Step 6: Change the IPv6 Configuration setting from Static to Automatic:

 

 

You will now find that the PC generates a Global Unicast Ipv6 Address on its own, with the Default Gateway address and the Link-Local Address from the Router Advertisement.

Simulation Results:

Practical execution of IPv6 SLAAC in the Cisco Packet Tracer is shown in the graphic below.

In this gif, it can be observed how the PC sends the Router Solicitation to the Cisco 2911 Router and the Router replies back with the Router Advertisement.

After the PC receives the Router Advertisement, the PC automatically generates its own IPv6 global unicast address.


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