IPv6 Global Unicast Address (GUA) Assignment

The assignment of an IPv6 Global Unicast Address (GUA) is a crucial aspect of network configuration, particularly in managing and authenticating devices on a network. The manual configuration of an IPv6 GUA on a router can be executed using the command ipv6 address ipv6-address/prefix-length interface. This manual process is also applicable to Windows hosts, where an IPv6 GUA can be set up similarly; however, it is essential to recognize that this manual entry can be labor-intensive and susceptible to errors. Consequently, the majority of Windows hosts are preset to dynamically acquire their IPv6 GUA configurations, facilitating a more error-free and efficient process.

IPv6 Host Link-Local Address

The link-local address in IPv6 plays a vital role in automatic addressing. When automatic IPv6 addressing is enabled, a host utilizes an Internet Control Message Protocol version 6 (ICMPv6) Router Advertisement (RA) to facilitate its IPv6 configuration. Upon the activation of the Ethernet interface, the host automatically generates a link-local address, which operates locally within the connectivity scope of the device. It is important to note that a device may fail to create an IPv6 GUA if there is no router present on the network segment to guide the configuration process.

It is critical to understand the significance of the Zone ID (or Scope ID) noted by the “%” symbol followed by a numeral at the end of the link-local address. This ID is utilized by the operating system to associate the link-local address with a specific interface. Additional context is provided by referring to the definition outlined in RFC 3315 regarding DHCPv6, which expands on dynamic address assignment methodologies.

Dynamic Assignment of IPv6 GUAs

In IPv6 networks, routers are programmed to periodically send out ICMPv6 RA messages, simplifying how hosts dynamically establish or acquire their IPv6 configurations. A host can attain a GUA through stateless and stateful services. This process inherently relies on the ICMPv6 RA messages which convey suggestions to hosts regarding the creation and acquisition of their IPv6 configurations. Importantly, while operating systems are designed to follow the guidance provided by the RA messages, the ultimate decision-making lies with the host itself.

Router Advertisement Message Flags

How a client secures its IPv6 GUA is determined by specific flags contained within the RA message. The primary flags included in an ICMPv6 RA message are:

A flag - This is the Address Autoconfiguration flag that indicates the usage of Stateless Address Autoconfiguration (SLAAC) for generating an IPv6 GUA.
O flag - The Other Configuration flag suggests that additional configuration information is accessible from a stateless DHCPv6 server.
M flag - The Managed Address Configuration flag instructs the client to engage a stateful DHCPv6 server to secure an IPv6 GUA.

Various combinations of these flags within RA messages convey to the host the available dynamic options for GUA assignment.

Key Takeaways from the Module

The learning outcomes from this module can be summarized as follows:

The manual command for setting IPv6 GUAs on routers is ipv6 address ipv6-address/prefix-length.
Automatic IPv6 addressing allows hosts to dynamically acquire their IPv6 address information.
Hosts will automatically create a link-local address upon booting up and activating the Ethernet interface.
The process of how a client acquires an IPv6 GUA is influenced by the RA message’s settings.
Hosts utilize SLAAC, which relies on ICMPv6 RA messages, to autonomously create their unique IPv6 GUAs without necessitating a DHCPv6 server.
RA messages are dispatched every 200 seconds by default or in response to a host's Router Solicitation (RS) message.

Address Generation Using SLAAC

Utilizing SLAAC, a host typically acquires its 64-bit IPv6 subnet information directly from the router RA. To complete the process, it must generate the remaining 64-bit interface identifier (ID) either randomly or via EUI-64. Furthermore, Duplicate Address Detection (DAD) is implemented through ICMPv6 ensuring that the IPv6 GUA assigned to the host is unique. DAD is conducted by sending an ICMPv6 Neighbor Solicitation (NS) message to a specially computed multicast address, known as a solicited-node multicast address, which mirrors the last 24 bits of the host's IPv6 address.

DHCPv6 Communication Methods

Upon detecting either a stateless or stateful DHCPv6 configuration in the RA, client-hosts begin the DHCPv6 communication process. Communication between servers and clients utilizes designated UDP ports—port 546 for server-to-client messages and port 547 for client-to-server messages. In stateless DHCPv6 configurations, clients utilize RA messages to inform them of addressing while obtaining supplementary parameters from DHCPv6 servers. Conversely, stateful DHCPv6 denotes a configuration where the RA instructs the client to retrieve all relevant addressing information from a stateful DHCPv6 server, with the DHCPv6 server retaining the IPv6 state information.

Router Configuration for DHCPv6 Services

Cisco IOS routers can be configured to serve as DHCPv6 servers, clients, or relay agents. Notably, a router can act as both a DHCPv6 client while simultaneously offering DHCPv6 server functionalities. The functionality of a DHCPv6 server depends on the router informing the host regarding the necessity of contacting a DHCPv6 server for all comprehensive addressing details. To serve as a DHCPv6 relay agent, the router must execute the ipv6 dhcprelay destination ipv6-address [interface-type interface-number] command to designate the DHCPv6 server address and requisite interface to reach the server. This relay configuration is predominately necessary when the DHCPv6 server exists on a geographically separate network from the client workstation. Operational status can be validated using show ipv6 dhcp interface and show ipv6 dhcp binding commands, ensuring the integrity and functionality of the relay agent setup.

IPv6 Global Unicast Address Assignment Insights

IPv6 Global Unicast Address Assignment Insights