Hello David
The ipv6 nd prefix command is used to give you control over the individual parameters per IPv6 prefix including if the prefix itself should be advertised or not. By default, if you don’t use this command at all, then only prefixes configured as addresses on the interface are advertised in RAs. By using the ipv6 nd prefix command, you are adjusting or manipulating what is being advertised in the RAs and how it is being advertised.
The fact that the word “default” is used as one of the parameters is a bit misleading. When it is used, you are simply saying that you want the default prefix to be advertised, the default being the prefix configured on the interface itself. The alternative to using the default keyword is to specify another prefix, either by manually typing it in, or by indicating that you want to receive it via a RADIUS server:
R1(config-if)#ipv6 nd prefix ?
X:X:X:X::X/<0-128> IPv6 prefix x:x::y/<z>
default Specify prefix default parameters
framed-ipv6-prefix Advertise prefix obtained from RADIUS server
R1(config-if)#ipv6 nd prefix
So if you use the default keyword, you’re just saying “advertise the default prefix which is the one configured on the interface.” Now any additional parameters added after that are simply applied to the default prefx.
The parameters that you see after the default keyword allow you to specify for how long that prefix will be advertised (valid for X seconds, or infinitely, or expires at a particular time, or is not advertised at all).
Concerning the valid and preferred lifetimes, these are fields found within the RAs as part of the prefix information option. These values are an important part of how SLAAC works. An address considered preferred can be used for both new and existing connections. A valid address can be used only for existing connections. For a more detailed discussion of this, take a look at this NetworkLessons note on the topic of valid and preferred IPv6 addresses.
For more details on how the ipv6 nd prefix command is used, take a look at this Cisco command reference:
I hope this has been helpful!
Laz