Anycast Addresses

An IPv6 anycast address is an address that is assigned to more than one interface (typically belonging to different nodes), where a packet sent to an anycast address is routed to the nearest interface having that address, according to the routing protocol's measure of distance.

Anycast addresses, when used as part of a route sequence, permit a node to select which of several Internet service providers it wants to carry its traffic. This capability is sometimes called source selected policies. You implement this by configuring anycast addresses to identify the set of routers belonging to Internet service providers (for example, one anycast address per Internet service provider). You can use these anycast addresses as intermediate addresses in an IPv6 routing header, to cause a packet to be delivered by a particular provider or sequence of providers. You can also use anycast addresses to identify the set of routers attached to a particular subnet or the set of routers providing entry into a particular routing domain.

You can locate anycast addresses from the unicast address space by using any of the defined unicast address formats. Thus, anycast addresses are syntactically indistinguishable from unicast addresses. When you assign a unicast address to more than one interface, that is, turning it into an anycast address, you must explicitly configure the nodes to which the address is assigned in order to know that it is an anycast address.