Stateful and Stateless DHCPv6

Three ways hosts get IPv6 configuration

IPv4 campus designs often funnel almost everything through DHCP: address, mask, gateway, DNS. IPv6 splits responsibilities more cleanly among Router Advertisements, SLAAC, and DHCPv6. HCIA-Datacom expects you to distinguish three patterns:

The exam loves one crisp contrast: stateful DHCPv6 assigns addresses; stateless DHCPv6 does not—it only provides other parameters while SLAAC handles addressing.

SLAAC: Stateless Address Autoconfiguration

SLAAC allows a host to configure one or more IPv6 addresses without a DHCPv6 server handing out the host identifier.

High-level steps:

1. Interface enables IPv6 and obtains a link-local address (with DAD).
2. Host sends RS (optional but common) and/or receives periodic RA.
3. RA carries a prefix (typically /64) with the A (autonomous) flag set, meaning “you may autoconfigure from this prefix.”
4. Host builds an address: prefix + interface identifier (EUI-64, privacy extension, stable private, or OS policy).
5. Host runs DAD on the new address.
6. Host installs an on-link route for the prefix and often a default route via the advertising router (from RA router lifetime).

Why /64 matters

SLAAC as commonly taught assumes a 64-bit interface identifier space. Advertising odd prefix lengths for SLAAC LANs is a design smell and a troubleshooting clue. HCIA static labs may still configure manual /64s without SLAAC, but when the question says “autoconfiguration from RA,” think /64 + A flag.

SLAAC strengths and limits

Stateful DHCPv6

Stateful DHCPv6 is the model closest to classic DHCPv4 address assignment:

• The DHCPv6 server allocates an IPv6 address (or prefix, in prefix-delegation stories beyond pure HCIA host focus) to the client.
• The server maintains state (bindings/leases).
• The client uses DHCPv6 message exchanges (Solicit/Advertise/Request/Reply family—know the idea, not necessarily every timer).
• The server can also supply DNS servers, domain search lists, and other options.

Hosts are steered toward managed configuration when RAs set the M (Managed address configuration) flag. Conceptual reading of flags:

Real networks combine flags carefully. For HCIA multiple-choice, map:

• Need server-assigned address + tracking → stateful DHCPv6 (M-managed model).
• Need DNS but addresses from RA/SLAAC → stateless DHCPv6 (O-other model).

Default gateway remains an ND/RA concern in many designs even when DHCPv6 is present—do not assume every DHCPv6 server must hand out a gateway the IPv4 way unless the scenario says so.

Stateless DHCPv6

Stateless DHCPv6 means the DHCPv6 server does not assign addresses and does not keep per-address lease state for those hosts. Instead:

1. Hosts form addresses with SLAAC from RA prefixes.
2. Hosts still contact DHCPv6 to request other configuration parameters.
3. The most exam-famous “other” parameter is DNS server addresses (and related name-service options).

If an item says: “Hosts autoconfigure addresses but still need DNS from a server,” the answer is stateless DHCPv6 (SLAAC + DHCPv6 for other info), not “stateful only,” and not “ARP.”

Side-by-side decision table

Interaction with ND (do not isolate the protocols)

Even with perfect DHCPv6, hosts still need ND:

• Link-local formation and DAD.
• NS/NA to reach the default router and on-link peers.
• RS/RA for prefixes, flags, and router discovery.

DHCPv6 does not replace Neighbor Discovery. Saying “DHCPv6 replaces ARP” is still wrong—ND replaces ARP; DHCPv6 replaces or supplements parts of IPv4 DHCP’s configuration role.

Huawei / campus perspective for HCIA

At associate level, Huawei materials emphasize the concepts and how they fit dual-stack campus design more than a full DHCPv6 server CLI lab for every platform feature. Still, connect to VRP skills you already have:

• Interfaces need ipv6 enable and addressing (manual, or host SLAAC toward a router sending RAs).
• Routers participating as default gateways must run IPv6 on the client-facing interface and advertise appropriately (RA behavior is router-side).
• Verification remains display ipv6 interface brief, neighbor tables, and connectivity tests (ping ipv6).
• For infrastructure devices in labs, static ipv6 address is common even when clients use SLAAC/DHCPv6.

When troubleshooting “host has no IPv6 DNS” versus “host has no IPv6 address,” branch correctly:

Worked scenario A — SLAAC only

RA advertises 2001:db8:10::/64 with A=1, M=0, O=0, router lifetime > 0.

• Host builds 2001:db8:10::<iid>, DAD succeeds.
• Default route points to the router’s link-local.
• DNS might be missing unless another mechanism exists—exam may call that out as a limitation.

Worked scenario B — Stateless DHCPv6

RA advertises the same prefix with A=1, M=0, O=1.

• Host SLAAC-builds its address as before.
• Host queries DHCPv6 for other config and learns DNS 2001:db8:dns::53.
• Server does not hand out the host’s GUA.

Worked scenario C — Stateful DHCPv6

RA has M=1 (and design may clear A or combine per policy).

• Host obtains address from DHCPv6 pool, for example 2001:db8:10::100.
• Server records the binding.
• DNS can be included in the same DHCPv6 answers.
• ND still resolves the gateway MAC when sending off-link.

Comparison to IPv4 DHCP (exam language)

Exam traps for DHCPv6 and SLAAC

• Defining stateless DHCPv6 as “DHCPv6 without a server.” Stateless still uses a server for options; it is stateless regarding address leases.
• Claiming stateful DHCPv6 is the only way to get a default gateway.
• Saying SLAAC requires DHCPv6.
• Mixing M and O flag meanings (M → managed addresses; O → other config).
• “DHCPv6 replaces Neighbor Discovery.” False.
• Assuming every host with a GUA automatically has DNS.
• Forgetting DAD still runs on SLAAC and DHCPv6-assigned addresses.

Mastery checklist

• Draw the three-column table: SLAAC / stateful / stateless.
• Explain one sentence each for M and O flags.
• Answer “who assigns the address?” and “who provides DNS?” for any scenario stem.
• Keep ND in the picture for gateway and L2 resolution.

With address assignment models clear, the final section applies VRP configuration to IPv6 static routes—the control-plane tool for reachability beyond connected prefixes when dynamic IPv6 routing is not yet in play.