DHCPv6 configurations with Stateful and Stateless (SLAAC)

DHCPv6 as a concept is very similar to version 4. There are some key differences that we need to know. DHCPv6 supports two very different methods; Stateful and stateless configurations.

Stateful

Stateful configuration works pretty much the same as DHCPv4. The DHCP server assigns the IP address to the client. When DHCPv6 addresses are being sent from a server to a client it uses UDP destination port 546

Stateless (SLAAC)

Stateless on the other hand uses autoconfiguration. Routers running IPv6 can give the prefix of the network and a gateway address to clients looking for an IPv6 address. IPv6 uses the Neighbour Discovery, and one of the things this protocol offers is Route Solicitation and Router Advertisement messages that help an IPv6 device configure an IPv6 address automatically. The router is configured to send out Router advertisements periodically.

Router Advertisement messages inform hosts what to do - There are three flags in the RA messages that play important role in defining how dynamic addressing works on this segment:

• A-flag - if it is set to 1, this informs hosts that they can auto-generate GUA address using SLAAC. If it is set to 0 means that auto-configuration is not allowed for this segment.

• O-flag - if it is set to 1, this informs hosts that they can obtain a DNS server list and a domain name from a Stateless DHCPv6 server, but not addressing information. Typically it works in conjunction with SLAAC for auto-addressing and both the A-flag and the O-flag are set to 1.

• M-flag - if it is set to 1, this informs hosts that they can obtain a global address as well as DNS and a domain name from a Stateful DHCPv6 server. Typically this means that auto-addressing using SLAAC is not allowed on this segment and both the  A-flag and the O-flag are set to 0.

A host sends out a router solicitation message which is a multicast address for all routers which is FF02::2. 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.

2. On the router use the below configurations from the screenshot;

Configurations

Lets look at how we configure SLAAC;

1. Build a simple network similar to the below

-ipv6 unicast-routing (enables the router for IPv6 routing)

-interface gigabitethernet0/0/0 (this is the particular interface being used for the connection to the switch)

-ipv6 address (is an IPv6 address which in this instance in the address of the router)

3.On each PC change the IPv6 configuration 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.

Ipv6, SLAAC and EUI

The EUI (extended Unique Identifier) uses the MAC address to generate a unique 64 bit interface ID. An Ipv6 address is 128bits and a MAC address is 48 bits so an operation needs to be performed. 

• The hexadecimal value of FFF0(16-bits) is added in the middle of the 48-bit mac address.
• The 7th bit from the start is toggled from 0 to 1.

For example: 

For the MAC address FC:99:47:75:CE:E0  the steps are performed as shown in the below:

Cisco routers are configured to use the EUI-64 ID generation by default.

Here is a video showing the configuration in action;