Cisco Cisco Prime Network Registrar 8.0 White Paper
© 2013 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information.
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Two Servers - Split Address Space
One technique for redundancy in DHCPv6 is for the two servers to split the address space. For example, if devices
are being assigned addresses from a /64 prefix, one server could be given the 64-bitprefix0:0:0:0/65 from which to
allocate addresses and the other server the 64-bitprefix8000:0:0:0/65 from which to allocate addresses.
When a client gets a lease, it will get it from one of the servers and will usually be able to renew the address from
that same server. Only if that server is down during the renewal period (50 percent to 85 percent of the lease time;
see the “Longer Lease Times” section below), will the client likely end up having to obtain a new address.
But, if the client gets a lease from a different server, it will transition to that new address gracefully per the IPv6
rules. (Note: Some client types, such as cable modems, do not necessarily follow the IPv6 address transition rules
and may switch to the new address immediately.)
There are two possibilities for configuring Cisco Prime Network Registrar DHCPv6 servers for split address
spaces:
●
Configure /64 prefixes with a /65 range (recommended).
●
Configure /65 prefixes (not recommended).
Configuring the /64 prefixes with a /65 range is the recommended approach. The reason for this is that if a
DHCPv6 client issues a Confirm message, and if the /64 prefixes are configured, either server will respond with
success to the Confirm if the client has an address in that /64 prefix. If instead the /65 prefixes are configured, one
of the servers will respond with "NOT ON LINK", which is not desirable (as this will cause the client to solicit a new
address).
An example of prefixes configured for address assignment is shown below (note that many unset/default attributes
have been excluded from this discussion for clarity):
On Server 1:
nrcmd> prefix server1
100 Ok
server1:
address = 2001:db8::/64
dhcp-type = [default=dhcp]
range = 2001:db8::/65
On Server 2:
nrcmd> prefix server2
100 Ok
server2:
address = 2001:db8::/64
dhcp-type = [default=dhcp]
range = 2001:db8:0:0:8000::/65
Note:
The above applies to prefix delegation as well. For example, if you wanted to delegate prefixes from a /48
prefix, you would configure a prefix delegation prefix on both servers with the /48 prefix but one server with the
range of 48-bitprefix0:0:0:0:0/49 and the other the range of 48-bitprefix8000:0:0:0:0/49.