3.1.b [ix] DHCPv6 prefix delegation
DHCPv6 Prefix Delegation (DHCPv6-PD) is an extension to DHCPv6. Classical DHCPv6 is typically focused on parameter assignment from a DHCPv6 server to an IPv6 host running a DHCPv6 protocol stack. A practical example would be the stateful address assignment of “2001: db8:: 1” from a DHCPv6 server to a DHCPv6 client. DHCPv6-PD however is aimed at assigning complete subnets and other network and interface parameters from a DHCPv6-PD server to a DHCPv6-PD client. This means that instead of a single address assignment, DHCPv6-PD will assign a set of IPv6 “subnets”. An example could be the assignment of “2001: db8::/ 60” from a DHCPv6-PD server to a DHCPv6-PD client. This will allow the DHCPv6-PD client (often a CPE device) to segment the received address IPv6 address space, and assign it dynamically to its IPv6 enabled interfaces.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2405-2410). . Kindle Edition.
3.1.b [viii] Stateful, stateless DHCPv6
DHCPv6 enables DHCP servers to pass configuration parameters, such as IPv6 network addresses, to IPv6 nodes. It offers the capability of automatic allocation of reusable network addresses and additional configuration flexibility. This protocol is a stateful counterpart to “IPv6 Stateless Address Autoconfiguration” (RFC 2462), and can be used separately, or in addition to the stateless auto-configuration to obtain configuration parameters.
Stateless DHCPv6 is a combination of “stateless Address Autoconfiguration” and DHCPv6. When using stateless-DHCPv6, a device will use Stateless Address Auto-Configuration (SLAAC) to assign one or more IPv6 addresses to an interface, while it utilizes DHCPv6 to receive “additional parameters” which may not be available through SLAAC. For example, additional parameters could include information such as DNS or NTP server addresses, and are provided in a stateless manner by DHCPv6. Using stateless DHCPv6 means that the DHCPv6 server does not need to keep track of any state of assigned IPv6 addresses, and there is no need for state refreshment as result. On network media supporting a large number of hosts associated to a single DHCPv6 server, this could mean a significant reduction in DHCPv6 messages due to the reduced need for address state refreshments. From Cisco IOS 12.4( 15) T onwards the client can also receive timing information, in addition to the “additional parameters” through DHCPv6. This timing information provides an indication to a host when it should refresh its DHCPv6 configuration data.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2396-2403). . Kindle Edition.
3.1.b [vii] SLAAC/DHCPv6 interaction
Stateless Address Autoconfiguration (SLAAC) is one of the most convenient methods to assign Internet addresses to IPv6 nodes. This method does not require any human intervention at all. If one wants to use IPv6 SLAAC on an IPv6 node, it is important that this IPv6 node is connected to a network with at least one IPv6 router connected. This router is configured by the network administrator and sends out Router Advertisement announcements onto the link. These announcements can allow the on-link connected IPv6 nodes to configure themselves with IPv6 address and routing parameters, as specified in RFC 2462, without further human intervention.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2387-2390). . Kindle Edition.
3.1.b [vi] DHCP protocol operations
DHCP Server Function
The DHCPv6 server can provide those configuration parameters that do not require the server to maintain any dynamic state for individual clients, such as DNS server addresses and domain search list options. The DHCPv6 server may be configured to perform prefix delegation. All the configuration parameters for clients are independently configured into DHCPv6 configuration pools, which are stored in NVRAM . A configuration pool can be associated with a particular DHCPv6 server on an interface when it is started. Prefixes to be delegated to clients may be specified either as a list of pre-assigned prefixes for a particular client or as IPv6 local prefix pools that are also stored in NVRAM. The list of manually configured prefixes or IPv6 local prefix pools can be referenced and used by DHCPv6 configuration pools.
The DHCPv6 server maintains an automatic binding table in memory to track the assignment of some configuration parameters, such as prefixes between the server and its clients. The automatic bindings can be stored permanently in the database agent, which can be, for example, a remote TFTP server or local NVRAM file system.
The DHCPv6 client function can be enabled on individual IPv6-enabled interfaces. The DHCPv6 client can request and accept those configuration parameters that do not require a server to maintain any dynamic state for individual clients, such as DNS server addresses and domain search list options. The DHCPv6 client will configure the local Cisco IOS stack with the received information. The DHCPv6 client can also request the delegation of prefixes . The prefixes acquired from a delegating router will be stored in a local IPv6 general prefix pool. The prefixes in the general prefix pool can then be referred to from other applications; for example, the general prefix pools can be used to number router downstream interfaces.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2377-2382). . Kindle Edition.
3.1.b [v] Global prefix configuration feature
The upper 64 bits of an IPv6 address are composed from a global routing prefix plus a subnet ID. A general prefix (for example, /48 ) holds a short prefix, based on which a number of longer, more-specific prefixes (for example, /64) can be defined. When the general prefix is changed, all of the more -specific prefixes based on it will change, too. This function greatly simplifies network renumbering and allows for automated prefix definition. For example, a general prefix might be 48 bits long (” /48″) and the more specific prefixes generated from it might be 64 bits long (“/ 64”). In the following example, the leftmost 48 bits of all the specific prefixes will be the same, and they are the same as the general prefix itself. The next 16 bits are all different.
2001: DB8: 2222::/ 48 Specific prefix: 2001: DB8: 2222: 0000::/ 64
2001: DB8: 2222: 0001::/ 64 Specific prefix: 2001: DB8: 2222: 4321::/ 64
Specific prefix: 2001: DB8: 2222: 7744::/ 64
General prefixes can be defined in several ways:
● Based on a 6to4 interface
● Dynamically, from a prefix received by a DHCP for IPv6 prefix delegation client
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2338-2344). . Kindle Edition.