Before Configuring stateless DHCPv6 server on the Cisco router, it is important to enabling IPv6 unicast routing. The unicast routing is not necessary for stateless DHCPv6 server but it is required for RA messages due to using of ICMPv6. We can enable the IPv6 unicast routing using “ipv6 unicast-routing” command in global configuration mode. Other steps for configuring stateless DHCPv6 is the following:
Configure a DHCPv6 Pool
Just like the DHCPv4 IP addressing pool we need the IPv6 addressing pool. To configure the IPv6 DHCP pool we used ipv6 dhcp pool <pool-name> command in global configuration mode. Executing this command we also enter into the DHCPv6 configuration mode, which is identified by the Router(config-dhcpv6)#
Configure Pool Parameters
The RA message contains necessary information regarding the prefix, prefix length and default gateway. The client creates its global unicast address from the information in RA message and configures its default gateway address which is the link-local address of the router. The DHCPv6 server can be configured for providing additional information that not included in the RA message, for example, DNS server address and domain name etc
Configure the DHCPv6 Interface
After creating the IPv6 addressing pool we need to bind the pool to the corresponding interface. The interface configuration mode command “ipv6 dhcp server <pool-name> binds the DHCPv6 pool to the interface.
When a router receives a DHCPv6 request on this interface it replies with the information configured in the pool. The O flag must be set to 1 and the M flag to 0 for the stateless DHCPv6. The interface mode command ipv6 nd other-config-flag command is used to configure the O flag to 1. RA messages send on this interface with M flag 0 and O flag 1 indicate that additional information is available from a stateless DHCPv6 server.
Example Command of DHCPv6 Stateless
The example configuration topology for stateless DHCPv6 server on the Cisco router is illustrated in the following figure.
Configure Cisco Router as a Stateless DHCPv6 Client
Previously we have configured Cisco Router as DHCPv4 client but here now we will configure Cisco router is DHCPv6 client. Typically DHCPv6 client is a computer, tablet, mobile device, or webcam not a router.
A router that is required to be configured as a DHCPv6 client needs an IPv6 link-local address on the interface. Without a link-local address, the router cannot send and receive IPv6 messages, for example, RS messages and DHCPv6 messages.
When IPv6 address is enabled on the interface the link-local address is created automatically on the interface. This is possible when a global unicast address is configured on the interface or by using the “ipv6 enable” command. When the router creates a link-local address, it can send RS messages and participate in the DHCPv6 process.
So we will use the ipv6 enable command because the router does not yet have a global unicast address and we want to configure the global unicast address from DHCPv6.
The next command for configuring a DHCPv6 client is “ipv6 address autoconfig”. This command enables the automatic configuration of IPv6 addressing using SLAAC. An RA message is then used to inform the client router to use stateless DHCPv6 for address configuration. The figure below illustrates the configuration of the DHCPv6 client on Cisco router command is the following:
Verifying Stateless DHCPv6
We can verify the DHCPv6 server configuration using several commands. The figure below illustrates the output of the command “show ipv6 dhcp pool”. The output of the command verifies the name of the DHCPv6 pool and its parameters. The number of active clients is 0 because there is no state being maintained by the server.
We can also use the show running-config command to verify all the commands that were previously configured.
Verifying the Stateless DHCPv6 Client
We have configured the stateless DHCPv6 client. We can verify the configuration of the client and server from the client. The command show ipv6 interface is used to verify that the router is configured with ”Stateless address autoconfig enabled“ and has an IPv6 global unicast address. We can also verify the configuration using show running-config command. Another command we can use to verify the configuration is debug ipv6 dhcp detail. The command shows the DHCPv6 messages exchanged between the client and the server.