IPv6 anycast,hmm?

EdTheLadEdTheLad Member Posts: 2,111 ■■■■□□□□□□
Can somebody who understands ipv6 anycast addresses please give me an explanation as how these can achieve something different to what i can achieve with a unicast address.

Anycast address is taken from the same range as a unicast address.
Anycast address disables DAD (who cares since the duplicate addresses are on different layer 3 segments)
Anycast address cannot be used in the source address of a packet( not important for me, the anycast address will be assigned to the router interface and i wont be sourcing traffic from this interface.

Lets say i have 10 servers dispersed throughout my network, all servers share the same ipv6 unicast address. These servers are connected to various router interfaces spread through my network, and the routers advertise the interface subnet to the core.The path to the destination server is dependent on where the source is in relation to the destination subnet. So the source will pick the lowest cost path to a server i.e. the closest server.What is anycast offering me over this solution? as far as i can see its the same thing with a fancy new name.
Networking, sometimes i love it, mostly i hate it.Its all about the $$$$


  • dtlokeedtlokee Member Posts: 2,378 ■■■■□□□□□□
    So fundamentally there is such a thing as IPv4 anycast althought there is no formal definition for what makes it anycast. Anycast addressing is typically used for content distribution networks and server load balancing based on geographic location, redundancy... The only real difference I see when it comes to IPv6 anycast are 2 things which you hit on one:

    1. It won't detect it as a duplicate IP address if on the same segment but that is not likely to be why you are using anycast.
    2. Looking at the rule that an IPv6 anycast address can't be used as the source IP address there is a bit of a problem in the typical implementation of IPv4 anycast. In IPv4 you were really using any given IP address on the network and letting the routers figure out the "closest" destination. What happens to a long lived TCP session when the routers change their routing tables and now the "closest" anycast destination is no longer the server you have a TCP session with? It breaks. In IPv6 anycast the return traffic will be sourced from the actual IPv6 address of that node so if the routing tables shift you are still talking to the same server and the session remains. This could have implications on security of course, and things like NAT.. but with IPv6 who needs NAT.
    The only easy day was yesterday!
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