Route 300-101 Anycast?

jpm

On pg 19 of the 300-101 book, there is a note on Anycast - "Anycast is an IPv6 concept and is not found in IPv4 networks."
This is entirely false. IPv4 Anycast is a fundamental routing aspect of the Internet's DNS infrastructure and also within many enterprise DNS designs. Does anyone have insight otherwise?

EdTheLad

This depends on the perspective on how you are looking at anycast. As far as an ipv4 device is concerned there is no such thing as anycast. As a network designer, when you assign the same ipv4 address to many devices you look at that address as being anycast when in fact it's a unicast address assigned to multiple devices. With ipv6, the device is aware of the address being anycast. A device does not perform duplicate address detection for an anycast address.

jpm

Interesting view, but I still don't see the overall difference. In our environment, we have DNS servers that peer with our core via OSPF and inject the Anycast address /32 into the routing table. There are four DNS servers that each inject the same route, so each core router will have multiple routes to the /32. Because of the routing metrics in place, the "nearest" anycasted address destination is used for forwarding. How is the behavior of v6 (which we have dual-stacked in this same environment) different?

networker050184

It's really no different in your scenario. Where it is different is the DAD part EdTheLad pointed out. For example you can have two routers on the same segment using the same IPv6 address. Normally duplicate address detection would not allow this in IPv6 unicast addressing. With the anycast functionality it disables this check.

Now, why would you want to do that? I can't really think of any reasons off the top of my head, but I guess someone thought it was useful.