The DHCP for IPv6 Broadband feature
highlights the DHCP enhancements that support IPv6 broadband deployments. This
feature briefly explains the different ways a delegating device selects
prefixes for a requesting device, enabling accounting messages on a device, and
forced release of delegated prefix bindings associated with a PPP virtual
interface when the PPP virtual interface is terminated.
Your software release
may not support all the features documented in this module. For the latest
caveats and feature information, see
Bug Search Tool and the
release notes for your platform and software release. To find information about
the features documented in this module, and to see a list of the releases in
which each feature is supported, see the feature information table.
Use Cisco Feature
Navigator to find information about platform support and Cisco software image
support. To access Cisco Feature Navigator, go to
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An IPv6 prefix
delegating device selects IPv6 prefixes to be assigned to a requesting device
upon receiving a request from the client. The delegating device might select
prefixes for a requesting device in the following ways:
assignment from a pool of available prefixes.
assignment from a pool name obtained from the RADIUS server.
prefix obtained from the RADIUS sever.
Contrary to IPv4
address assignment, an IPv6 user will be assigned a prefix, not a single
address. Typically the Internet service provider (ISP) assigns a 64- or 48-bit
PPP calls a registry
to allow DHCPv6 to append the delegated prefix information to accounting start
and stop messages. When accounting is configured for a DHCPv6 pool, accounting
interim packets are sent to broadband sessions after binding is provided from
Forced Release of a
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.
DHCPv6 invokes a
routine when the virtual interface used by PPP terminates. This routine
automatically releases any delegated prefix bindings associated with the PPP
virtual interface that is being terminated.
When a PPP virtual
interface terminates, the routine runs through the full table of DHCPv6
bindings checking for the matching interface. Because PPP uses a virtual
interface, this subroutine clears any related lease information when the PPP
In IPv6 broadband deployment using DHCPv6, you must enable release of
prefix bindings associated with a PPP virtual interface using the
ipv6 dhcp binding track ppp command. This ensures that DHCPv6
bindings are tracked together with PPP sessions, and in the event of DHCP
REBIND failure, the client initiates DHCPv6 negotiation again.
Example: Enabling the Sending
of Accounting Start and Stop Messages
This example shows
how to enable a device to send accounting start and stop messages.
Device(config)# ipv6 dhcp pool pool1
Device(config-dhcp)# accounting list1
Example: Configuration for
a Prefix Allocated from a Local Pool
DHCPv6 clients are
connected to the DHCPv6 server on Gigabit Ethernet interface 0/0/0. The server
is configured to use parameters from the DHCP pool called dhcp-pool. This pool
provides clients with the IPv6 address of a DNS server and the domain name to
be used. It also specifies that prefixes can be delegated from the prefix pool
called client-prefix-pool1. The prefixes delegated will have valid and
preferred lifetimes of 1800 and 600 seconds respectively. The prefix pool named
client-prefix-pool1 has a prefix of length /40 from which it will delegate
(sub) prefixes of length /48.
ipv6 dhcp pool dhcp-pool
prefix-delegation pool client-prefix-pool1 lifetime 1800 600
description downlink to clients
ipv6 address FEC0:240:104:2001::139/64
ipv6 dhcp server dhcp-pool
ipv6 local pool client-prefix-pool1 2001:DB8:1200::/40 48
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The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
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Table 1 Feature Information for DHCP
for IPv6 Broadband
Enhancements to Support IPv6 Broadband Deployments
Cisco IOS XE
highlights the DHCP enhancements that support IPv6 broadband deployments, such
as, the different ways a delegating device selects prefixes for a requesting
device, enabling accounting messages on a device, and forced release of
delegated prefix bindings associated with a PPP virtual interface when the PPP
virtual interface is terminated.
commands were introduced or modified:
ipv6 dhcp bindings track
ipv6 dhcp pool.
Delegation RADIUS VSA
Cisco IOS XE
When the user
requests a prefix from the prefix delegator, typically the NAS, the prefix is
allocated using DHCPv6.
Cisco IOS XE
Accounting Attribute feature allows DHCPv6 to append delegated prefix
information to accounting start and stop messages.