Configuring Network VI

Configuring IPv6


Before we move on in this chapter, we need to spend some time
discussing IPv6 address configuration. There are actually several
configuration options when it comes to IPv6 addressing. The first is
to use static assignment. As with IPv4, static IPv6 address
assignments require you to manually assign the entire 128-bit IPv6
address and prefix to the host. 



This can be done from the shell prompt using a command-line utility such as 



ifconfig 


or



ip



Alternatively, you can manually enter the address and prefix in the appropriate
interface configuration file in 



/etc/sysconfig/network



Another configuration option is to use static partial assignment.
Using this method, the prefix is statically assigned, but the
interface ID portion of the address is automatically generated using
the host’s MAC address.




Another configuration option is to use stateless auto-configuration,
which is affectionately called SLAC. Using SLAC, we allow IPv6 hosts
to automatically generate their own interface ID, but we require them
to obtain a correct prefix and default gateway address from a network
router (usually the default gateway) using the Neighbor Discovery
Protocol (NDP). NDP uses the following messages for autoconfiguration:



• Router solicitation (RS) messages are sent from network hosts
  requesting that any listening router respond.


• Router advertisement (RA) messages are sent by the router in
  response to RS messages received from network hosts. These messages
  inform the network hosts of the correct IPv6 prefix they should use,
  along with the default gateway address.



NDP is also used by network hosts to discover the MAC address of other
interfaces on the network, thus completely replacing the ARP when
using IPv6. However, NDP cannot provide network hosts with the address
of your DNS server. It can only provide the network prefix and the
default gateway address.




The final IPv6 address configuration option is to use DHCP. As with
IPv4, IPv6 address assignments can be made automatically using an
updated version of DHCP called DHCPv6. Because IPv6 has so many more
configuration options than IPv4, DHCPv6 operates in a different
manner. There are two different modes:



• Stateful DHCPv6

Configures the DHCP server to provide each IPv6 DHCP
client with an IP address, the default gateway address, and the DNS
server IP address. The DHCP server tracks the status of each IPv6 DHCP
client, hence the name stateful.


• Stateless DHCPv6

Does not assign IPv6 addressees to IPv6 clients,
nor does not track their status. In this configuration, the DHCPv6
server simply assigns the DNS server IP address to network hosts.
Obviously, stateless DHCPv6 does not provide a complete addressing
solution. Instead, it must be used in conjunction with other IPv6
address assignment schemes, such as SLAC.


When an IPv6 host boots up, it follows the process described next to
configure an address on its network interface:


1. The host automatically generates an IPv6 link-local address using
   the link-local prefix (which usually starts with FE80) combined with
   an interface ID derived from its MAC address.


2. The host sends a neighbor solicitation (NS) message addressed to
   its own link-local address to make sure another host on the network
   isn’t already using it.


3. The host sends out a multicast RS message, which should be received
   by all routers on the same network segment. If no routers respond, the
   host attempts to use stateful DHCPv6 to receive an IPv6 address.


4. If a router receives the RS message, it then sends a multicast RA
   message to all hosts on the network segment. The RA message identifies
   how the IPv6 address is to be configured. Possible options include the
   following:


   • Obtain all IPv6 configuration information from a DHCPv6 server. If
     this is the case, the host sends out a REQUEST message to any
     listening DHCPv6 server.

   • Use stateless autoconfiguration to get the prefix and default
     gateway from the RA message and to generate the interface ID portion
     of its address automatically. The address of the DNS must then be
     obtained from a DHCPv6 server.


5. If a static IPv6 address has been configured or if stateful
   autoconfiguration has been used, the host sends an NS message to make
   sure no one else is using its address.








LX0-104 Exam Objectives (T)