IPv6 Global Unicast Address & Host Link-Local Address

IPv6 Global Unicast Address (GUA) Assignment

• IPv6 GUA is manually configured on a router using the ipv6 address ipv6-address/prefix-length interface configuration command.
• Windows hosts can be manually configured or dynamically acquire IPv6 GUA configurations.
• Manual configuration can be time-consuming and error-prone.
• Most Windows hosts dynamically acquire IPv6 GUA configurations.

IPv6 Host Link-Local Address

• Automatic IPv6 addressing uses Internet Control Message Protocol version 6 (ICMPv6) Router Advertisement (RA) messages to autoconfigure.
• IPv6 link-local addresses are automatically created when the host boots and the Ethernet interface is active.
• The "%" and number at the end of the link-local address is also known as a Zone ID or Scope ID, used by the OS to associate the LLA with a specific interface.
• DHCPv6 is defined in RFC 3315.

IPv6 GUA dynamic assignment

• IPv6-enabled routers periodically send ICMPv6 RAs to simplify dynamic IPv6 configuration.
• Hosts can dynamically be assigned a GUA using stateless and stateful services.
• All stateless and stateful methods use ICMPv6 RA messages to suggest how hosts create or acquire their IPv6 configuration.
• The actual decision is ultimately up to the host, despite following the router's RA suggestion.

RA Message Flags

• How a client obtains an IPv6 GUA depends on settings in the RA message.
• The ICMPv6 RA message includes three flags:
  • A flag (Address Autoconfiguration): Use Stateless Address Autoconfiguration (SLAAC) to create an IPv6 GUA.
  • O flag (Other Configuration): Additional information is available from a stateless DHCPv6 server.
  • M flag (Managed Address Configuration): Use a stateful DHCPv6 server to obtain an IPv6 GUA.

SLAAC (Stateless Address Autoconfiguration) Overview

• SLAAC allows hosts to create unique IPv6 global unicast addresses without a DHCPv6 server.
• SLAAC is a stateless service without a server maintaining network address information.
• SLAAC sends periodic ICMPv6 RA messages (every 200 seconds).
• RA provides addressing and configuration information.
• A host can send a Router Solicitation (RS) message requesting an RA.
• SLAAC can be deployed as SLAAC only, or SLAAC with DHCPv6.

SLAAC Enabling

• Configured Router (R1) G0/0/1 IPv6 addresses include:
  • Link-local IPv6 address fe80::1
  • GUA/subnet 2001:db8:acad:1::1, 2001:db8:acad:1::/64
  • IPv6 all-nodes group ff02::1
• R1 joins the all IPv6 multicast group and sends RA messages with address configuration to hosts using SLAAC.
• The IPv6 all-routers group responds to the IPv6 multicast address ff02::2.
• The show ipv6 interface command verifies R1 has joined the IPv6 all-routers group (ff02::2).
• R1 sends RA messages every 200 seconds to IPv6 all-nodes multicast address ff02::1.

SLAAC Only Method Flags

• RA messages have the following flags set:
• A=1: Informs the client to use the IPv6 GUA prefix in the RA and dynamically create its own Interface ID.
• O=0 and M=0: Informs the client to use the additional information in the RA message (DNS server, MTU, and default gateway information).
• A Windows command ipconfig confirms that PC1 (client) has generated an IPv6 GUA using the R1 RA.
• The default gateway address is the LLA of the R1 G0/0/1 interface.

ICMPv6 RS (Router Solicitation) Messages

• A router sends RA messages every 200 seconds or when it receives an RS message from a host.
• IPv6-enabled hosts send an RS message to the IPv6 all-routers multicast address of ff02::2.
• A host initiates as such:
  • PC1 has booted andsends an RS message to the IPv6 all-routers multicast address of ff02::2 requesting an RA.
  • R1 generates an RA and sends it to the IPv6 all-nodes multicast address of ff02::1; PC1 uses this information to create a unique IPv6 GUA.

Host Process to Generate Interface ID

• Using SLAAC, an end host acquires the 64-bit IPv6 subnet information from the router RA and must generate the remainder 64-bit interface identifier, using either:
  • Randomly generated: The 64-bit interface ID is randomly generated by the end host operating system; used by Windows 10.
  • EUI-64: The host creates an interface ID using its 48-bit MAC address and inserts the hex value of fffe in the middle of the address. Privacy concerns can lead to the Interface ID being randomly generated instead.
• User configurable on Windows, Linux, and macOS.

Duplicate Address Detection (DAD)

• A SLAAC host will use Duplicate Address Detection (DAD) to ensure the IPv6 GUA is unique.
  • The host sents an ICMPv6 Neighbor Solicitation message with a specifically constructed solicited-node multicast address that contains the last 24 bits of the of the host IPv6 Address.
  • If no other devices respond with a Neighbor Advertisement message, then the address is virtually guaranteed to be unique and used by the host.
  • If an NA is received by the host, then the address is not unique, and the host must generate a new interface ID to use.
• IETF recommends that DAD can be used, due to a 64-bit interface ID providing 18 quintillion possibilities.
• Most operating systems perform DAD on all IPv6 unicast addresses, regardless of how the address is configured.

DHCPv6 Operation Steps

• Stateful DHCPv6 does not require SLAAC, while stateless DHCPv6 still does.
• Whether using SLAAC or Stateful DHCPv6, when a Router Advertisement indicates to with DHCPv6:
  • Host sends an RS message to the Router to ask for any IPv6 settings.
  • Router responds with an RA message containing any IPv6 settings.
  • Host sends a DHCPv6 SOLICIT message to the DHCPv6 server to initiate the conversation.
  • DHCPv6 server responds with an ADVERTISE message to indicate the type settings that it is able to provide the host.
  • The host responds to the DHCPv6 server after analyzing the ADVERTISE.
  • DHCPv6 server sends a REPLY message back to the host with the settings.
• Server to client DHCPv6 messages use UDP destination port 546.
• Client to server DHCPv6 messages use UDP destination port 547.

Stateless DHCPv6 operation

• If an RA indicates the stateless DHCPv6 method, the host uses the information with in the RA message for addressing and then contacts a DHCPv6 server for additional information.
• DHCPv6 only provides configuration parameters for clients and does not maintain a list of IPv6 address bindings.
• For example, PC1 receives a stateless RA message containing:
  • The IPv6 GUA network prefix and prefix length.
  • A flag set to 1 informing the hosts to use SLAAC.
  • O flag set to 1 informing the host to look for additional configuration information from a DHCPv6 server.
  • M flag set to the default value of 0.
• PC1 then sends a DHCPv6 SOLICIT message seeking additional information from a stateless DHCPv6 server.

Enable stateless DHCPv6 on an interface

• Stateless DHCPv6 is enabled using the ipv6 nd other-config-flag interface configuration command setting the O flag to 1.
• The highlighted output confirms the RA will tell receiving hosts to use stateless autoconfigure (A flag = 1) and contact a DHCPv6 server to obtain other configuration information (O flag = 1).
• To reset SLAAC only, use the command no ipv6 nd other-config- flag, this will reset the interface to the default SLAAC only option (O flag = 0).

Stateful DHCPv6 Operation

• If an RA indicates a stateful DHCP method, the host contacts a DHCPv6 server for all configuration information.
• The DHCPv6 server is stateful and must maintain a list of IPv6 address bindings.
• For example, PC1 receives a stateful RA message containing:
  • An IPv6 GUA network prefix and prefix length.
  • A flag set to 0 informing the host to contact a DHCPv6 server.
  • O flag set to 0 informing the host to contact a DHCPv6 server.
  • M flag set to the value 1.
• PC1 sends a DHCPv6 SOLICIT message seeking additional configuration information from a stateful DHCPv6 server.

Enable stateful DHCPv6 on an interface

• Stateful DHCPv6 is enabled using the ipv6 nd managed-config-flag interface configuration command, setting the M flag to 1.
• The router will tell the host to obtain all IPv6 configuration information from a DHCPv6 server (M flag = 1).

DHCPv6 Router Roles

• A Cisco IOS router can provide DHCPv6 server services.
• It can be configured to be one of the following roles:
  • DHCPv6 Server - Provides stateless OR stateful DHCPv6 services.
  • DHCPv6 Client - Router interface acquires an IPv6 IP config from a DHCPv6 server it relies on.
  • DHCPv6 Relay Agent - Provides DHCPv6 fowarding services when the client and the server are located on different networks.

Configure a Stateless DHCPv6 Server

• The stateless DHCPv6 server option requires that the router advertise the IPv6 network addressing information in RA messages.
• Five steps to configure and verify a router as a stateless DHCPv6 server:
  • Enable IPv6 routing using ipv6 unicast-routing.
  • Define a DHCPv6 pool name using ipv6 dhcp pool POOL-NAME global config command.
  • Configure DHCPv6 pool with options, which may include dns-server X:X:X:X:X:X:X:X and domain-name name.
  • Bind the interface to the pool using ipv6 dhcp server POOL-NAME interface config command. Setting the O flag by using ipv6 nd other-config-flag interface command.
  • RA messages indicate additional info is available from a stateless DHCPv6 server, telling clients to use SLAAC to create their own GUA.
  • Verify that the hosts have received IPv6 addressing info, using ipconfig /all.

Configure a stateless DHCPv6 client

• A Router can also be a DHCPv6 client and obtain an IPv6 configuration from a DHCPv6 server.
• First, enable IPv6 routing using the ipv6 unicast-routing command.
• Next, configure the client router to create a LLA (link-local address). Cisco IOS uses EUI-64 to create the Interface ID.
• Configure the client router to use SLAAC with the command ipv6 address autoconfig.
• Verify that the client router is assigned a GUA (global unicast address) using the command show ipv6 interface brief.
• Finally, verify that the client router received the necessary DHCPv6 information with the command show ipv6 dhcp interface g0/0/1, and look look for for DHCP option information such as DNS server and domain name.

Configure a Stateful DHCPv6 Server

• The stateful DHCP server relies on the RA telling the host to contact a DHCPv6 server to obtain necessary IPv6 network addressing information.
• This is configured in 5 steps:
  • Enable IPv6 routing using ipv6 unicast-routing.
  • Next, define a DHCPv6 pool name using ipv6 dhcp pool POOL-NAME.
  • Now configure the DHCPv6 pool with options such as address prefix, domain name, DHS server IP address etc.
  • Then you bind the interface to the pool using ipv6 dhcp server POOL-NAME interface command.
    • Manually change M flag from 0 to 1 using ipv6 nd managed-config-flag.
    • Manually change A flag from 1 to 0 using ipv6 nd prefix default no-autoconfig this will inform the client not to use SLAAC.
    • Now verify that that the hosts have received the correct settings using ipconfig /all.

Configure stateful DHCPv6 client

• A router needs to enable the configurations below to begin acting as a DHCP client.
• First, enable ipv6 unicast-routing.
• After that, you must create an LLA (link-local address). Cisco IOS uses EUI-64 to create an Interface ID.
• Now configure the client router to use DHCPv6 by using the command ipv6 address dhcp.
• Then, verify that the client router is assigned a GUA by using the command show ipv6 interface brief.
• To complete the process, verify that client router receives all necessary DHCPv6 information by using show ipv6 dhcp interface g0/0/1.

DHCPv6 Server Verification Commands

• Verify the DHCPv6 pool with the command show ipv6 dhcp pool, this command not only names the pool, but also identifies active clients.
• Use the command show ipv6 dhcp to output the IPv6 link-local address of the client, but also display the GUA assigned from the DHCPv6 server.
  • Note: this information is not maintained by the stateless DHCPv6 server.

Configure DHCPv6 relay agent

• If a DHCP server is situated on a different network than the client, the IPv6 router needs to be configured as a DHCPv6 agent.
• The agent will allow the DHCP to become similar to a DHCPv4 relay.
• This process is configured on the interface facing the DHCP and specifies the DHCPv6 server address and egress interfaceto the server.
• This will only require to configure the hop, when the next-hop can use LLA.
• Verify the DCHPv6 agent by verifying its functionality via commands show ipv6 and show ipv6 dhcp binding.