IPv6 Addressing
Anycast Addresses
Anycast AddressesN o t e
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Network size, traffic loads and the potential for network changes make it desirable to build in redundancy for some network services to provide increased service reliability. Anycast addressing provides this capability for applications where it does not matter which source is actually used to provide a service that is offered on multiple sources. Some applications that can benefit from anycast addressing include:
■DNS (UDP)
■time servers
■multicast rendezvous
■syslog devices
■gateways to a common network area.
Similarly, it is also useful in some cases to economically provide redundant paths to a given entity, such as a specific service provider. With IPv6 this can be done efficiently using the anycast address capability to assign the same address to multiple devices providing access to the desired services. An added benefit of utilizing anycast addresses is to reduce the need to configure clients with the addresses of multiple devices offering the same service.
An anycast address is an identifier for a set of interfaces typically belonging to different nodes. Packets sent to an anycast address are delivered to one of the interfaces identified as the “nearest” address, according to the routing protocol's measure of distance.
An anycast address is formatted the same as a unicast address. For this reason, configuring an anycast address on the switch includes using an anycast keyword as part of the command. The prefix for an anycast address should include all areas of the network in which the address is used. For information on configuring an anycast address on the switches covered by this guide, refer to “Statically Configuring An Anycast Address” on page
Duplicate Address Detection (DAD) does not apply to anycast addresses.