Saturday, February 13, 2010

Ipv6 Anycast Addresses

I spent some time over the past few days learning more about the many types of IPv6 addresses out there. A new address type made specifically for IPv6 is called the Anycast Address. IPv6 Anycast addresses are global addresses, theses addressed can be assigned to more than one interface unlike a Ipv6 unicast address. Anycast is designed to send a packet to the nearest interface that is apart of that anycast group.

The sender creates an anycast packet and fowards the packet to the anycast address as the destination address which goes to the nearest router. The nearest router or interface is found by using the metric of a routing protocol. However in a LAN setting the nearest interface is found depending on the order the neighbors were learned. The anycast packet in a LAN setting forwards the packet to the neighbor it learned about first. Anycast was first proposed in 1993 but even to this data there isn't much usage as of yet. There are actually only a few anycast addresses currently assigned!

The source sending the anycast path can use the address to control the paths that traffic flows. For example, when a customer has multiple connections to multiple IP's using BGP. The customer can create a different anycast address for each ISP, and then configure the same anycast address on the closest router to that specific ISP. Therefore the routers along the source's path to the ISP can determine the shortest route based on the IPv6 anycast address. Which then forwards the packet based on the routers closest anycast address link. Another example would be on a LAN link. All the routers on the same LAN can have the same IPv6 address so that distant devices only need to identify the anycast address.


  1. Sorry, IPv4 has been using anycast addressing for years. DNS servers use it religiously.

  2. Really, thanks for that tidbit Jeremy I had no clue that it's been around since IPv4. I'll do a little bit of research on my end to learn more about it! I'm guessing that's why the book mentioned that it has been around for so long already.