Amer Networks E5Web GUI - Manual (Page 374)

5.5. DHCPv6 Servers
cOS Core provides the ability to set up one or more DHCPv6 servers. Configuring these is almost
identical to configuring an IPv4 DHCP server. However, there are some object properties which
are available with DHCPv6 but not with standard IPv4 DHCP. These are as follows:
• Rapid Commit
By default this is disabled. This option makes sense during server solicitation procedure. If the
client has included a rapid commit option in the solicit message and the rapid commit setting
is enabled then the DHCPv6 server responds to the solicit with a reply. The server commits
the assignment of addresses before sending the reply message. The client can assume it has
been assigned the addresses in the reply message and does not need to send a request
message for those addresses.
If this option is left at the default value of being turned off, the server ignores the rapid
commit option and acts as though no rapid commit option were present in the client's solicit
message.
• Preference Value
A preference value can be either sent or not sent to the client. If sending it is enabled, the
default preference value is zero but this can be manually set to be between 0 and 255.
Setting the preference gives the administrator the ability to prioritize one DHCPv6 server
over another. During the server solicitation procedure the client collects received advertise
messages from available DHCPv6 servers. The client typically will contact the server that sent
the advertise message with the highest server preference value.
A preference value of 255 has the highest priority and once such value is received in an
advertise message, the client will immediately begin a client initiated message exchange
with the DHCPv6 Server originated the message. This value therefore should only be used in
an environment with a single server since other servers will be ignored.
Preferences are often used where the administrator wants one server to be the primary with a
higher preference and assigns a lower preference to other backup servers.
• Send Unicast
By default, in negotiations between client and server, the client uses multicast IPv6 address as
a destination for all messages. This option enables the inclusion of the server unicast option
by a DHCPv6 Server in messages sent to client. Once such an option is received by the client,
it can contact the server directly using the server's IPv6 address (which is carried in the server
unicast option).
This allows reduction of the network load as well as offloading to other DHCPv6 Servers
available on the network.
Tip: Speeding up address allocation
If only one DHCPv6 server is configured then the process of IPv6 address allocation can
be significantly speeded up by enabling rapid commit and setting the preference value
of that server to be 255.
With a preference value of 255, message exchange is triggered as soon as soon as the
client receives the solicit message. Rapid commit allows the client to get committed
addresses in the reply message during the solicit-reply message exchange with the
Chapter 5: DHCP Services
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