Juniper JUNOSE 11.1.X - BROADBAND ACCESS CONFIGURATION GUIDE 6-4-2010, Configuration Manual

Page: 145 / 794

■
You cannot delete a pool or a prefix range from which prefixes have been
allocated to requesting routers or DHCPv6 clients. However, you can forcibly
delete such a pool or prefix range by using the
force keyword in the ipv6 local
pool poolName and prefix commands. If a pool is deleted or the prefix range
associated with the pool is deleted, and prefixes have been assigned to DHCPv6
clients or requesting routers, the corresponding DHCPv6 bindings are also deleted.
■
When multiple prefix ranges are configured in a pool, the DHCPv6 prefix
delegation feature allocates prefixes from the configured ranges in the order of
the assigned prefix length. The delegating router or the DHCv6 server attempts
to allocate a prefix from the range with lowest assigned prefix length. If this
attempt fails because the pool has been fully allocated, the server tries to allocate
a prefix from the subsequent prefix ranges. These ranges could have the same
prefix length as the first one or a higher length.
NOTE: Although you can configure an IPv6 local pool with the assigned prefix length
as /128, which implies a full IPv6 address, this assignment is not useful for the
DHCPv6 prefix delegation feature because it assigns a prefix with a length of only
/64 or less. A pool with an assigned prefix length of /128 is useful when complete
IPv6 addresses are assigned to the DHCPv6 clients.
When a PPP user establishes a PPP connection with the E Series router functioning
as a remote access server, the subscriber is first authenticated using the RADIUS
protocol. The Access-Accept message returned from the RADIUS server can contain
different IPv6 attributes, including the Framed-IPv6-Pool attribute, which contains
the name of the IPv6 pool from which a prefix needs to be assigned to the subscriber.
The prefix is assigned to the subscriber using the DHCPv6 prefix delegation feature,
which is covered in the next section.
DHCPv6 Prefix Delegation Example
Consider a scenario in which a number of devices on a home network are connected
to a customer premises equipment, CPE1, which is the requesting router. CPE1 is
connected using a PPP link to the provider edge device, PE1, which is an E Series
router operating as the DHCPv6 server or delegating router. After the IPv6 link is
formed between CPE1 and PE1 and the IPv6 link-local address is created, CPE1
requests and obtains prefixes that are shorter than /64 (usually of length, /48) from
PE1.
CPE1 is connected to the home network. CPE1 divides the single delegated prefix
that it received from PE1 into multiple /64 prefixes and assigns one /64 prefix to
each of the links in the home network. The address allocation mechanism in the
subscriber network can be performed using ICMPv6 neighbor discovery in router
advertisements, DHCPv6, or a combination of these two methods.
When PE1 receives a request for prefix delegation from CPE1, PE1 assigns prefixes
from the list of unallocated prefixes in the IPv6 local pool.
DHCPv6 Local Address Pools for Allocation of IPv6 Prefixes Overview
■ 105
Chapter 1: Configuring Remote Access