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New to Windows Server 2008 is the IPv6 node item under the DHCP server in the navigation pane, which allows for IPv6 DHCPv6 functionality The original purpose for DHCP was to give out IP addresses, but IPv6 takes care of this automatically DHCPv6 might still be useful for
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con guring options such as DNS servers and setting an address range to use, which allows subnet-speci c, routable IPv6 address space usage There are two modes for DHCPv6, stateless and stateful In stateless mode, the host uses DHCP just to nd con guration options and the IPv6 address used is assigned elsewhere In stateful mode, the host uses DHCP for both con guration and address assignment The mode of the host is based on a router advertisement the host receives in response to a router solicitation and two ags in the returned advertisement, namely ags M and O If both ags are set to 1, the host uses stateful mode; if only O (options) is set, stateless mode is used although options such as DNS are con gured via the DHCP DHCPv6 does not work like DHCPv4 Instead of a broadcast, the DHCPv6 client sends a multicast Solicit message to locate a DHCPv6 server DHCPv6 servers respond with an advertisement of a lease, the client picks one and makes a request, the server sends a reply, and the lease is complete UDP port 547 is used by DHCPv6 servers Just like with a new IPv4 scope, right-click on IPv6, select New Scope, and enter a name and description Instead of a start and end range, you are asked for a 64-bit pre x and a preference (see Figure 7-26) The preference is between 0 and 255 If a client receives multiple DHCP offers, the offer from the higher preference server is used (255 is the highest)
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FIGURE 7-26 Enter the pre x for the 64-bit scope and a preference value
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Like an IPv4 scope, exclusions can be con gured by specifying the address ranges to exclude (see Figure 7-27) You must enter all bytes for the interface ID, for example 0:0:0:10 and not just 10 Next, the lease time must be con gured for both Identity Association for Non-Temporary Addresses (IANA) and Identity Association for Temporary Addresses (IATA), which are settings on the client informing the server of the type of lease it requires As usual, you can activate the scope immediately
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FIGURE 7-27 Con guring lease times for the IPv6 scope There are leases, exclusions, reservations, scope options, and serverspeci c options DHCPv6 con guration and management is the same as for IPv4 you just have bigger addresses and a slightly more complicated lease time con guration
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So far, so good However, there is a catch For fault tolerance reasons it is common to have multiple DHCP servers with scopes broken up using the 80/20 rule This means that two DHCP servers are con gured to assign addresses for the same subnet This leads to an obvious problem: If you have two DHCP servers on the same subnet, they might both lease the
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same IP address to different machines, causing a con ict on the network To avoid this, separate exclusions are set on each of the DHCP scopes with one of the servers leasing 80% of the available addresses, the other leasing the remaining 20% You get to exclusions in a second Figure 7-28 shows an example of this con guration
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Scope 1921681100 1921681199 Exclusions 1921681180 1921681199 Scope 1921681100 1921681199 Exclusions 1921681100 1921681179
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FIGURE 7-28 Out of the scope of 100 addresses, the rst server has 80 available to lease
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If the primary server (the one with 80%) is not available, the other server is able to service some clients until the other server can be brought back into the environment How do you prevent clients from automatically leasing from the second server Place the primary DHCP server on the subnet with the clients it is leasing to and the spare DHCP server on a separate server The router between the subnets should be con gured to forward BOOTP/DHCP requests This means when a client requests an address, the DHCP server on the local subnet would service it rst To ensure the local DHCP server always responds rst, set a delay on the router that is acting as a DHCP relay agent to a value that ensures the local DHCP server always responds rst The second DHCP server has 80% of the addresses for its local subnet and the primary has 20% of its subnet addresses (thus each server giving the other redundancy) Remember, DHCP gives out a lease to a client for a set amount of time If the DHCP server stops working, clients are not immediately affected They still have their lease that they can continue to use They
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