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DHCPv6

DHCPv6 enables IPv6 clients to obtain network configuration through a structured exchange between clients, a Layer 3 switch acting as the default gateway, and a DHCPv6 server. The process supports both stateless and stateful address configuration depending on how router advertisement flags are set.

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About this video

DHCPv6 is the IPv6 counterpart to the familiar DHCP process used in IPv4 networks, adapted to work within IPv6's multicast-based communication model and its Neighbor Discovery Protocol framework. When a client joins the network, it begins by sending a router solicitation to all routers on the segment. The Layer 3 switch, acting as the default gateway and a member of the all-routers multicast group, responds with a router advertisement that carries three configuration flags: the auto-configuration flag, the other flag, and the managed flag. The values of these flags determine which configuration mode the client will use to obtain its IP address and supplemental network information. In stateless address autoconfiguration mode, the auto-configuration and other flags are both set to one while the managed flag is set to zero. This instructs the client to generate its own IPv6 address independently while still contacting a DHCPv6 server to retrieve additional options such as DNS server addresses. The client sends a solicit message to the DHCPv6 server multicast address, the server responds with an advertise message, the client issues a request, and the server concludes the exchange with a reply. In stateful mode, the auto-configuration and other flags are set to zero and the managed flag is set to one, directing the client to obtain a fully assigned IP address from the DHCPv6 server rather than generating one on its own. The four-message exchange follows the same structure in both modes. A key distinction from IPv4 DHCP is that IPv6 does not support broadcast. Only the initial client solicit is sent to the DHCPv6 server multicast group address; all remaining messages in the exchange — the advertise, request, and reply — are transmitted as unicast directly between the client and server. This design reduces unnecessary traffic on the network while preserving the structured, negotiated process that ensures clients receive complete and accurate configuration data.

What you'll learn

What's covered

DHCPv6 Process

Aligned to

Cisco CCNA
4.3 Explain the role of DHCP and DNS within the network
1.8 Configure and verify IPv6 addressing and prefix
1.9 Describe IPv6 address types

Key terms

Dynamic Host Configuration Protocol
DHCP
A network protocol that automatically assigns IP addresses and other configuration parameters to devices on a network.
Router
A network device that forwards data packets between networks based on IP addresses.
Default Gateway
The router that a device uses to send traffic to destinations outside its local network.
Neighbor Discovery Protocol
NDP
An IPv6 protocol that replaces ARP and handles router discovery, address autoconfiguration, and address resolution on local links.
Router Advertisement
RA
An ICMPv6 message periodically broadcast by IPv6-enabled routers to announce their presence, advertise network prefixes, and provide default gateway information for SLAAC. Hosts also request RAs immediately on startup by sending a Router Solicitation.
Stateless Address Autoconfiguration
SLAAC
An IPv6 mechanism that allows devices to automatically generate a globally unique IPv6 address using the network prefix from a Router Advertisement combined with a locally generated interface identifier, without requiring a DHCP server. SLAAC uses the EUI-64 process or a random interface ID to create the host portion of the address.
Multicast
A transmission type in which data is sent from one source to a specific group of devices that have subscribed to a multicast group address, rather than to all devices or just one. This differs from broadcast (all devices) and unicast (one device).

Topics

Dhcpv6 Ipv6 Network Protocols Router Advertisements Ndp Networking

Transcript

The Setup

Our layer 3 switch is going to be our default gateway, and so it's actually what's going to be sending out our router advertisements. In this case right here, we're going to specify that our demo Pi 2 is actually going to be a DHCP version 6 server, and so that is going to be what hands out some additional options, some additional information about this network.

The Stateless Process

So what's going to happen is, no matter what, the process is going to start out with the Neighbor Discovery Protocol. This client connects to the network and it's going to send out a router solicitation. That's going to go to all routers that are on the network. This layer 3 switch now has joined that router multicast group, and so it will respond back with a router advertisement, giving some information to this demo laptop.

Some of the information that it will give is some of the flags: the autoconfiguration flag, the other flag, and the managed flag. Let's say in this case right now we want this client to autoconfigure itself, to come up with its own IP address, so that flag is set to one. We also want it to get some additional information from this DHCP version 6 server right here, and so that's set to one. And since those two are set to one, this managed is going to be set to zero.

So this demo laptop will come up with its own IP address, but it also needs that additional information from the DHCP version 6 server. So what it will do now is it will send out a discovery, and it's going to send out that discovery to the multicast address for DHCP version 6 servers. This demo Pi 2 is part of that multicast group, so it will be receiving that, in which case it will respond with a DHCP advertisement with the information inside of it. So that will be sent back to the demo laptop. And then this demo laptop, similar to the process with IPv4 DHCP, will make the request for it, and the Pi then will return with a reply to it.

Now, since IPv6 doesn't have broadcast — when we take a look at the DHCP process for IPv4, we will have some broadcast in there — there's no broadcast for IPv6. So what happens is this first one is a multicast to all the DHCP servers, but the rest of these are all unicast and sent directly to this demo laptop.

Stateful DHCP Version 6

Now, this is for a stateless setup, where the demo laptop comes up with its own IP address. What would it look like if this were actually to be stateful, where we want this demo Pi 2 to come up with an IP address for this demo laptop?

Well, in that case right there, all that we've changed here is this switch right here would actually send out autoconfigure as being a zero, and the other as being zero, and managed as being one. And then this demo laptop would know, okay, I know now what router I'm supposed to use, but now I need to go directly to this demo Pi to get some additional information. And so then it goes through this whole process — really, it goes through this whole thing. So it's very similar, except that it gets an IP address from the DHCP version 6 server along with the other information. So that's the only difference there.

So that is the DHCP version 6 process, if you were to implement a DHCP version 6 server.

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