What are the steps to configure a Cisco router as a stateful DHCPv6 (Dynamic Host Configuration Protocol version 6) server?

Enable IPv6 routing, create a DHCPv6 pool with address prefix and options, bind it to an interface, set the M flag, and disable the A flag.

Step-by-step:

1. Enable IPv6 routing:

R1(config)# ipv6 unicast-routing

2. Create the DHCPv6 pool:

R1(config)# ipv6 dhcp pool IPV6-STATEFUL

3. Configure pool with address prefix and options:

R1(config-dhcpv6)# address prefix 2001:db8:acad:1::/64
R1(config-dhcpv6)# dns-server 2001:4860:4860::8888
R1(config-dhcpv6)# domain-name example.com

4. Bind the pool and set flags:

R1(config)# interface g0/0/1
R1(config-if)# ipv6 dhcp server IPV6-STATEFUL
R1(config-if)# ipv6 nd managed-config-flag
R1(config-if)# ipv6 nd prefix default no-autoconfig

5. Verify on clients with ipconfig /all

Critical extra step vs. stateless:

• ipv6 nd managed-config-flag sets M=1 (use DHCPv6 for addresses)
• ipv6 nd prefix default no-autoconfig sets A=0 (do NOT use SLAAC (Stateless Address Autoconfiguration)) — without this, clients might still try to create their own GUA (Global Unicast Address)!

Go deeper:

• RFC 8415 §6 — IA and address assignment — the address-prefix/binding mechanics behind the stateful pool.