If you already know a prefix which is delegated from a server, you don't want to use the prefix delegation, and you can configure your network using the prefix statically. It may be useful in many cases. However, you may want to configure your network dynamically.
To use the prefix delegation, use -P option. Here, you need to specify an interface which an IPv6 address is to be configured, SLA-id, host-id and prefix length, separated by slant. For example, to configure an IPv6 address for fxp0 with SLA-id is 1 and host-id is 0:0:0:1, you can specify as following:
-P 'fxp0/1/0:0:0:1/64'
Here, you can omit prefix length as the default is 64. Usually, /64 is used for Ethenet. However, /128 should be used for a point-to-point link such as an interface which is used for an IPv6 over IPv4 tunnel itself.
When host-id is omitted, an address based on EUI-64 is assumed.
You can configure more than one interface. In this case, you can specify it separated with comma like follows:
-P 'fxp0/1,fxp1/2,gif0/f//128'
If prefix length is not /128, the dtcps set to announce RA to the interface automatically. If you don't want this behavior, make sure to specify -a option.