Module 8

Module 8: SLAAC and DHCPv6

Module Objectives

Module Title: SLAAC and DHCPv6

Objective: Configure dynamic address allocation in IPv6 networks. This module is aimed at providing a comprehensive understanding of dynamic address assignment techniques in IPv6.

IPv6 Global Unicast Address Assignment

Host Configuration:

• IPv6 GUA (Global Unicast Address): This address can be manually configured on routers using the command ipv6 address ipv6-address/prefix-length. It is crucial for enabling communication over the internet.

• Windows Hosts: While Windows hosts can manually configure IPv6 GUAs, this method is often fraught with errors, which is why dynamic acquisition is preferred for efficiency and reliability in network configurations.

Link-Local Address:

• Automatically created by the host upon boot to facilitate basic network connectivity.

• Utilizes ICMPv6 Router Advertisement (RA) messages for autoconfiguration, ensuring that each host can establish essential communication on the local subnet without manual setup.

• Zone ID or Scope ID: Helps in identifying the specific interface associated with the link-local address, which is essential in multi-interface host configurations.

ICMPv6 Router Advertisements

• Routers regularly send periodic ICMPv6 RAs to simplify dynamic IPv6 configuration, informing hosts of the prefix and other configuration parameters needed to obtain a GUA.

• Hosts can be assigned GUAs via both stateless (SLAAC) and stateful (DHCPv6) services, providing flexibility based on network requirements.

RA Message Flags:

• A Flag: Indicates to hosts that they should use SLAAC to self-configure their GUA.

• O Flag: Signifies that additional configuration information may be available from a stateless DHCPv6 server, which can provide parameters like DNS settings.

• M Flag: Directs hosts to use a stateful DHCPv6 server, which gives a complete configuration including the specific IPv6 address.

SLAAC (Stateless Address Autoconfiguration)

Overview:

• SLAAC allows hosts to automatically generate unique IPv6 GUAs independently, without needing a DHCPv6 server. This stateless service enables hosts to configure themselves by leveraging the configuration information disseminated through periodic ICMPv6 RA messages.

• Hosts may also initiate their address configuration by sending Router Solicitation (RS) messages for RAs, ensuring timely updates to their addressing information.

Enabling SLAAC:

• Example Configuration on a Router:

  • Link-local IPv6 address: fe80::1

  • Global Unicast Address/Subnet: 2001:db8:acad:1::1, 2001:db8:acad:1::/64

• The router joins the all-IPv6 multicast group to send RA messages, making sure the hosts in the network receive necessary configuration information.

SLAAC Process:

• RA messages inform hosts to generate their own GUAs.

  • Set A = 1 to indicate SLAAC is enabled for generating the interface ID dynamically.

  • Setting O = 0 and M = 0 limits the information provided exclusively from RA messages, steering the configuration process solely through SLAAC.

RA Message Response:

• Upon receipt of RA messages, hosts send RS messages to the multicast address ff02::2, requesting RAs from routers to ensure they maintain current and accurate addressing information.

Host Process for Interface ID Generation:

• Interface IDs can be:

  • Random: Many modern systems, like Windows 10, utilize a method of generating random interface IDs, thereby enhancing privacy.

  • EUI-64: A method that generates the interface ID from the MAC address of the host; however, privacy concerns have made random methods more favorable.

Duplicate Address Detection (DAD):

• Ensures that the GUA generated is unique in the network.

• The host sends an ICMPv6 Neighbor Solicitation (NS) message using the multicast address to check for existing addresses.

• If no Neighbor Advertisements (NA) are received, the address is deemed unique and valid; if a NA is received, the host will need to generate a new ID to maintain address uniqueness.

DHCPv6 (Dynamic Host Configuration Protocol for IPv6)

Operation Steps:

• Stateful vs. Stateless: The operational steps for DHCPv6 under both modes highlight their functionalities:

  • Host sends an RS message to seek configuration.

  • The router responds with an RA message.

  • Host sends DHCPv6 SOLICIT to the DHCP server.

  • The DHCPv6 server sends an ADVERTISE message back.

  • The host replies to the server.

  • Finally, the DHCP server sends a REPLY message to complete the address assignment and configuration process.

Stateless Operation:

• The RA provides the address information to the host, while the DHCPv6 server supplies other configuration parameters like DNS options, making it simpler for the host to configure itself without needing an IPv6 address from DHCPv6.

Stateful Operation:

• In this mode, the host must contact the DHCPv6 server to obtain a complete configuration, as the server maintains specific IPv6 address bindings critical for network setup.

Enable DHCPv6:

• Stateless Mode: Use the command ipv6 nd other-config-flag to set the O flag on the router to signal hosts that some config info is available via DHCP.

• Stateful Mode: Use the command ipv6 nd managed-config-flag to set the M flag, indicating hosts should seek an IPv6 GUA from DHCPv6.

DHCPv6 Server Configuration:

• As a DHCPv6 Server, configuring it to offer both stateless and stateful services is crucial for accommodating different client requirements in the network.

• Enable IPv6 Routing: Ensures that the DHCPv6 packets can traverse the router successfully.

• Define DHCPv6 Pool: Specifies the range of IPv6 addresses that can be allocated to clients.

• Configure Options: This includes critical parameters like DNS server addresses and domain names.

• Bind Interface to Pool: Ensures that the DHCPv6 pool is associated with the appropriate interface on the router to serve clients effectively.

• Verify Client Acquisition: Use the command ipconfig /all on client systems to confirm successful address assignment and configuration settings.

Verification Commands:

• Use commands like show ipv6 dhcp pool and show ipv6 dhcp binding for troubleshooting to check the availability of pools and current bindings to ensure proper operation and troubleshooting of DHCPv6 services.

Relay Agent Configuration:

• When operating separate client and server networks, configuring the router as a relay agent ensures that DHCPv6 requests from clients are relayed to the DHCPv6 server efficiently, allowing proper configuration even in split network setups.

Module Summary

Key Learnings:

• Understanding the differences between manual and dynamic GUA configurations.

• The importance of automatic link-local address creation at device bootup.

• The role of RA message flags in determining the method of IPv6 addressing.

• Detailed explanations of SLAAC alongside stateful and stateless DHCPv6 operations.

• Techniques for ensuring unique address assignment and verifying configurations using DAD and various DHCP commands.

New Terms and Commands

Terms to Remember:

• Stateless Address Autoconfiguration (SLAAC): Mechanism through which an IPv6 device configures its own address without the need for a server.

• Global Unicast Address (GUA): A unique address that identifies a single interface on the internet.

• Link Local Address (LLA): A non-routable address used for communication within a single link.

• Router Solicitation (RS) & Advertisement (RA): Messages used by hosts to discover routers and their configurations in IPv6.

• DHCPv6 SOLICIT, ADVERTISE, REPLY: The transaction messages exchanged in the DHCPv6 process for dynamic configuration.

Common Commands:

• ipv6 unicast-routing: Enables IPv6 unicast routing on the router.

• ipv6 dhcp pool pool-name: Creates a DHCPv6 address pool with a specified name.

• show ipv6 dhcp interface: Displays information about the DHCP bindings related to the specified interface.