IPv6 Link-Local vs Unique Local – Key Differences

Its always best to stick to the relevant RFC when possible. In this case, your two options aren't mutually exclusive. SIXXS' generator should actually generate you a prefix based on RFC4193's algorithm.

So you would just generate your prefix using SIXXS' tool (or another one you find), and if you feel like it, register it with SIXXS.

There are two different types of anycast. The most common one is where you have multiple routes for the same prefix going to different locations. The routing protocol will choose the best router and the traffic end up at the closest server. One way to do this is to let the DNS servers participate in the routing protocol (RIP in this case) and give them a separate prefix, for example 2001:db8:dddd::/64. Both DNS servers run their own RIP daemon to advertise that prefix. The DNS service is run on i.e. 2001:db8:dddd::1. If a DNS server falls the route it advertised will disappear and the routing protocol will route the traffic to the other DNS server. This works for both IPv4 and IPv6.

IPv6 has a second type of anycast: anycast addresses. They are normal address but configured in such a way that you can have one address on the same LAN configuredon multiple devices. So you can have two DNS servers connected to one LAN with the same addres.