In IPv4, devices have two ways to get IPv4 addressing information, which includes an IPv4 address, subnet mask, default gateway address, domain name, and Domain Name Service (DNS) server address:
• Static or manual configuration
• Dynamically from a DHCPv4 server
As with IPv4, IPv6 addresses can be statically assigned. However, when it comes to dynamic addressing, IPv6 has a different approach. IPv6 uses the ICMPv6 Router Advertisement message to suggest to devices how to obtain their IPv6 addressing information. An IPv6 router sends a Router Advertisement (RA) message periodically (every 200 seconds in Cisco IOS) or when it receives a Router Solicitation request from a device.
The RA message is typically sent to the all-IPv6 devices multicast address (ff02::1), so every IPv6 device on the link (network) receives it. (It can also be sent as a unicast message.) Other routers do not forward RA messages.
The Router Advertisement message includes addressing information for IPv6 devices that includes the following:
• The network prefix and prefix length, along with other information about the link (subnet).
• The address of the default gateway. (This is a link-local address of the router's egress interface, the source IPv6 address of the RA message.)
• Three flags that are used to suggest to a device how to obtain its IPv6 addressing information. These flags are the Autonomous Address Configuration Flag (A flag), the Other Configuration Flag (O flag), and the Managed Address Configuration Flag (M flag).
• Optional information such as a domain name and a list of DNS server addresses.
Unlike an IPv4 device, an IPv6 device can determine all of its addressing dynamically without the services of a DHCP server.
As shown in Figure 2-7, the Router Advertisement message can be one of three methods:
• Method 1=Stateless Address Autoconfiguration (SLAAC): The device uses the information in the RA message for all of its addressing needs, including using the prefix in the RA to create an IPv6 global unicast address. The device will use the source IPv6 address of the RA as its default gateway. Method 1 is the default in Cisco IOS.
• Method 2=SLAAC and stateless DHCPv6 server: Similar to Method 1, the device uses SLAAC to create a global unicast address and uses the source IPv6 address of the RA as the address for the default gateway. However, this method also suggests to the device that it needs to contact a stateless DHCPv6 server for additional information that is not contained in the RA message. This information may be a list of DNS server addresses. It's important to note that a stateless DHCPv6 server does not provide or maintain any IPv6 global unicast addressing information. A stateless server only provides network information common to all devices on the network.
Note The RA message doesn't specify what information is available from the stateless DHCPv4 server. A list of DNS addresses should be included in the RA message only if both the router and the receiving device support this capability (see RFC 6106, IPv6 Router Advertisement Options for DNS Configuration).
• Method 3=Stateful DHCPv6 server: This method is similar to DHCP for IPv4. The RA message suggests to the device that it use a DHCPv6 server for all its IPv6 addressing needs, including a global unicast address. However, the device must dynamically obtain its default gateway address from the RA message, as in the previous two methods.
Note Cryptographically Generated Addresses (CGA) is a third option in which to generate the Interface ID. This is done by computing a cryptographic one-way hash function from a public key and involves the Secure Neighbor Discovery Protocol (SEND). CGA and SEND are beyond the scope of this book. For more information on CGA and SEND, I recommend the book, IPv6 Security, by Eric Vyncke.
In either case, the device is able to create its own global unicast address without the services of DHCPv6.
Neither a stateless nor stateful DHCPv6 server provides the default gateway address. It can only be dynamically obtained from the RA message. This means that even if a stateful DHCPv6 is used, the RA messages must be sent so that hosts can dynamically receive their default gateway information.
When the RA suggests the use of stateful DHCPv6, some host operating systems will use both stateful DHCPv6 and SLAAC for creating two different addresses. In most cases, network administrators that implement stateful DHCPv6 don't usually want the devices to create another address using SLAAC. Therefore, the information in the RA message must be configured so that the host uses the RA message for the default gateway address but does not assign a second address using SLAAC.
About the Author
Rick Graziani has been an instructor of computer networking and computer science courses at Cabrillo College in Aptos, California since 1994. Rick also teaches networking courses in the Computer Engineering department at the University of California, Santa Cruz and is on the Curriculum Engineering team for Cisco Networking Academy.
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