How to Configure Stateful DHCPv6 Server

October 6, 2019

We can configure the stateful DHCPv6 server, similar to a stateless server. The difference between stateless and stateful is that a stateful server also includes IPv6 addressing information similar to a DHCPv4 server and obtains the state of the DHCP.

Similar to stateless DHCPv6, we need to enable unicast routing. The command ipv6 unicast-routing enables IPv6 routing on the router. Unicast routing is also needed for ICMPv6 RA messages.

Configure a DHCPv6 Pool

First, we need the IPv6 addressing pool similar to a stateless DHCPv6 server. 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 the prefix, prefix length, and default gateway information. The client creates its global unicast address, including the default gateway, using the information in the RA message. However, with stateful DHCPv6, the DHCPv6 server must assign all addresses, including other configuration parameters.

We can configure the address prefix and lease time using address prefix <prefix/prefix-length> lifetime <valid-time-in-second | inifinte> command.

Other information is also included in stateful DHCPv6 configuration, usually the DNS server address and the domain name.

Interface Commands

After configuring the DHCPv6 pool, we must bind it to the corresponding interface. The ipv6 dhcp server <pool-name> interface command binds the pool to the interface. The M flag needs and O flag must be 1. This informs the client not to use SLAAC and to obtain all IPv6 addressing, including all configuration parameters, from a stateful DHCPv6 server.

DHCPv6 Stateful Server Example configuration

The figure below illustrates an example of a stateful DHCPv6 server configuration. We are using the same topology used in stateless DHCPv6 configuration. Now, I am going to configure the same topology for stateful DHCPv6. The default gateway is not specified, as the router will automatically send its link-local address as the default gateway.

Configuring a Router as a Stateful DHCPv6 Client

Similar to stateless DHCPv6 client configuration, the ipv6 enable interface configuration mode command is used. This command allows the router to receive a link-local address to send RS messages and participate in the DHCPv6 process.

Next, use the interface mode command ipv6 address dhcp. This command enables the router to behave as a DHCPv6 client on this interface. The figure below illustrates the stateful DHCPv6 client configuration.

Verifying Stateful DHCPv6 Server

We can verify the stateful DHCPv6 server using different commands. The first command we can use for DHCPv6 server verification is the show ipv6 dhcp pool. The figure below illustrates the output of this command.

The command verifies the name of the DHCPv6 pool, including its parameters. Only 1 client is active, which reflects the client DHCP Client receiving its IPv6 global unicast address from this server.

Another command is show ipv6 dhcp binding. The figure below illustrates the output of this command, showing the automatic binding between the interface of the router and the address assigned by the server.

The output illustrates that the DHCPv6 pool name is my pool, and the prefix length is 64. You can also see the DNS server address and the domain name “networkustad-a2bb2f.ingress-alpha.ewp.live. You can see that there is 1 active client. It is a stateful protocol. Therefore, it has a record of the active client.

We can also use the “show running-config” command to verify the stateful DHCPv6 configuration.

Verifying the Stateful DHCPv6 Client

To verify the DHCPv6 client configuration, we can use the show ipv6 interface command. We can also use the show running-config and show ipv6 interface brief commands.

SLAAC and DHCPv6 – Exclusive Details

Asad Ijaz / CCNA /

October 4, 2019

Remember that we already discussed the introduction of the SLAAC and DHCPv6 in Router Advertisement (RA) messages. Now, we will briefly discuss both SLAAC and DHCPv6. Recall that the router sends a Router Advertisement (RA) message. The setting of the Router Advertisement (RA) message decides whether a client can obtain its IPv6 address information automatically using SLAAC, DHCPv6, or a combination of both.

The Router Advertisement (RA) message always contains the Managed Address Configuration flag (M flag) and another configuration flag (O Flag) with a different combination. The combination of M Flag and O Flag tells the host how he can get an IPv6 address. The RA message presents three addressing options for the IPv6-enabled devices using a different combination of M Flag and O Flag.

SLAAC – Use of SLAAC Only
Stateless DHCPv6 – Use of both SLAAC and DHCPv6
Stateful DHCPv6 – Use of DHCPv6 Server Only

The RA message specifies the above three options for the client to obtain an IPv6 address, but the client computer doesn’t need to choose one. The client computer can ignore the RA message and use the services of a DHCPv6 server exclusively.

RFC 4861 recommends detecting Duplicate Addresses (DAD) on any unicast address and address configured with SLAAC and DHCPv6. This ensures that all configured IPv6 addresses are unique on the particular subnet.

SLAAC Option

It is the default option on Cisco routers. The M flag and the O flag value are set to 0 in the RA for this option. It simplifies IPv6 address management by providing plug-and-play IP connectivity without manually configuring hosts. It enables an IPv6 client to create its IPv6 addresses using a combination of locally available information and information advertised by routers through an RA message.

In the SLAAC option, the client uses the information in the RA message exclusively. The information in the RA message includes a prefix, prefix length, DNS server, Maximum Transmission Unit, and default gateway. The client does not use any information from the DHCPv6 server. The IPv6 global unicast address combines the advertised link-local prefix (first 64 bits) with the interface identifier (last 64 bits).

The RA option is configured on an individual router interface. If it is configured with other options, e.g., stateless DHCPv6 or stateful DHCPv6. To re-configure an interface for SLAAC, the M flag and O flags are required to reset to initial values of 0. The configuration command is the following:

Router(config-if)# no ipv6 nd managed-config-flag

Router(config-if)# no ipv6 nd other-config-flag

Stateless DHCPv6 Option

The function of DHCPv6 is similar to DHCPv4.It also runs between a client and a server. Like DHCP for IPv4, DHCPv6 clients and DHCPv6 servers exchange DHCPv6 packets using the UDP. In IPv6, packets cannot be broadcast; therefore, DHCPv6 uses multicast packets. In this case, DHCPv6 clients do not need to be configured with IPv6 addresses of DHCPv6 servers.

In this option, the client used the information in the RA message and a DHCPv6 server. The client gets the addressing information from the RA message, but additional parameters for configuration are available from a DHCPv6 server. The client creates its IPv6 global unicast address using the RA message’s prefix and length, along with the EUI-64 process or a randomly generated ID.

After configuring the IP addressing information, the client communicates with a stateless DHCPv6 server to get supplementary information. This process is called stateless DHCPv6 because the server does not maintain any client state information, such as the state of available and allocated IPv6 addresses. The server only provides configuration parameters for clients, not IPv6 addresses.

The O flag is 1, and the M is 0 for the stateless DHCPv6 option. The O flag value of 1 tells the client that additional configuration information is available from a stateless DHCPv6 server. To configure and modify the RA message, enter into the concern interface mode and use the following command:

Router(config-if)# ipv6 nd other-config-flag

Stateful DHCPv6 Option

It works like DHCP for IPv4 addresses. A device can get its addressing plan and information, including a global unicast address, prefix length, and the addresses of DNS servers, automatically using the services of a stateful DHCPv6 server. The RA message in this option informs the client that I can’t give you any information you need. Send a request to the DHCPv6 server for all your required information. This option is stateful DHCPv6 because the DHCPv6 server keeps all records and states of IPv6 addresses.

The M and O flags must be set to 1 in stateful DHCPv6. The following command is used to change the M flag and O flag from 0 to 1 to apply stateful DHCPv6:

Router(config-if)# ipv6 nd managed-config-flag