Anycast IPv6 Addresses

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An anycast address identifies multiple interfaces, and is used for one-to-one-of-many communication, with delivery to a single interface. With the appropriate routing topology, packets addressed to an anycast address are delivered to a single interface. In terms of routing distance, a packet addressed to an anycast address is delivered to the nearest interface identified by the address.

To facilitate delivery to the nearest anycast group member, the routing infrastructure must be aware of the interfaces that are assigned anycast addresses and must know their distances in terms of routing metrics. At present, anycast addresses are used only as destination addresses and are assigned only to routers.

Anycast addresses are assigned from the unicast address space. The scope of an anycast address is the scope of the unicast address type from which the anycast address is assigned.

The subnet-router anycast address is predefined and is required. It is used for communication with one of multiple routers attached to a remote subnet, and is created from the subnet prefix for a given interface.

To construct the subnet-router anycast address, the bits in the subnet prefix are fixed at their appropriate values and the remaining bits are set to 0. All router interfaces attached to a subnet are assigned the subnet-router anycast address for that subnet.

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