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+.. This work is licensed under a Creative Commons Attribution 4.0 International License.
+.. http://creativecommons.org/licenses/by/4.0
+.. (c) Prakash Ramchandran
+
+===================================
+Docker IPv6 Simple Cluster Topology
+===================================
+
+Using external switches or routers allows you to enable IPv6 communication
+between containers on different hosts. We have two physical hosts: Host1 &
+Host2, and we will study here two scenarios: one with Switch and the other
+one with router on the top of hierarchy connecting those 2 hosts. Both hosts
+host a pair of containers in a cluster. The contents are borrowed from
+article [1]_ below, which can be used on any Linux distro (CentOS, Ubuntu,
+OpenSUSE etc) with latest kernel. A sample testing is pointed in the blog
+article [2]_ as a variation using ESXi & older Ubuntu 14.04.
+
+----------------------------
+Switched Network Environment
+----------------------------
+
+Using routable IPv6 addresses allows you to realize communication between
+containers on different hosts. Let’s have a look at a simple Docker IPv6
+cluster example:
+
+.. figure:: images/docker-ipv6-cluster-example.png
+   :name: docker-ipv6-cluster-figure1
+   :width: 100%
+
+   Figure 1: An Docker IPv6 Cluster Example
+
+The Docker hosts are in the ``2001:db8:0::/64`` subnet. Host1 is configured to
+provide addresses from the ``2001:db8:1::/64`` subnet to its containers. It has
+three routes configured:
+
+* Route all traffic to ``2001:db8:0::/64`` via eth0
+* Route all traffic to ``2001:db8:1::/64`` via docker0
+* Route all traffic to ``2001:db8:2::/64`` via Host2 with IP ``2001:db8:0::2``
+
+Host1 also acts as a router on OSI layer 3. When one of the network clients
+tries to contact a target that is specified in Host1’s routing table, Host1
+will forward the traffic accordingly. It acts as a router for all networks it
+knows: ``2001:db8::/64``, ``2001:db8:1::/64``, and ``2001:db8:2::/64``.
+
+On Host2, we have nearly the same configuration. Host2’s containers will get
+IPv6 addresses from ``2001:db8:2::/64``. Host2 has three routes configured:
+
+* Route all traffic to ``2001:db8:0::/64`` via eth0
+* Route all traffic to ``2001:db8:2::/64`` via docker0
+* Route all traffic to ``2001:db8:1::/64`` via Host1 with IP ``2001:db8:0::1``
+
+The difference to Host1 is that the network ``2001:db8:2::/64`` is directly
+attached to Host2 via its docker0 interface, whereas Host2 reaches
+``2001:db8:1::/64`` via Host1’s IPv6 address ``2001:db8:0::1``.
+
+This way every container can contact every other container. The containers
+Container1-* share the same subnet and contact each other directly. The traffic
+between Container1-* and Container2-* will be routed via Host1 and Host2
+because those containers do not share the same subnet.
+
+In a switched environment every host must know all routes to every subnet.
+You always must update the hosts’ routing tables once you add or remove a host
+to the cluster.
+
+Every configuration in the diagram that is shown below the dashed line across
+hosts is handled by Dockeri, such as the docker0 bridge IP address
+configuration, the route to the Docker subnet on the host, the container IP
+addresses and the routes on the containers. The configuration above the line
+across hosts is up to the user and can be adapted to the individual environment.
+
+--------------------------
+Routed Network Environment
+--------------------------
+
+In a routed network environment, you replace the layer 2 switch with a layer 3
+router. Now the hosts just must know their default gateway (the router) and the
+route to their own containers (managed by Docker). The router holds all routing
+information about the Docker subnets. When you add or remove a host to this
+environment, you just must update the routing table in the router instead of on
+every host.
+
+.. figure:: images/routed-network-environment.png
+   :name: docker-ipv6-cluster-figure2
+   :width: 100%
+
+   Figure 2: A Routed Network Environment
+
+In this scenario, containers of the same host can communicate directly with
+each other. The traffic between containers on different hosts will be routed
+via their hosts and the router. For example, packet from Container1-1 to
+Container2-1 will be routed through Host1, Router, and Host2 until it arrives
+at Container2-1.
+
+To keep the IPv6 addresses short in this example a ``/48`` network is assigned
+to every host. The hosts use a ``/64`` subnet of this for its own services and
+one for Docker. When adding a third host, you would add a route for the subnet
+``2001:db8:3::/48`` in the router and configure Docker on Host3 with
+``--fixed-cidr-v6=2001:db8:3:1::/64``.
+
+Remember the subnet for Docker containers should at least have a size of
+``/80``. This way an IPv6 address can end with the container’s MAC address and
+you prevent NDP neighbor cache invalidation issues in the Docker layer. So if
+you have a ``/64`` for your whole environment, use ``/76`` subnets for the
+hosts and ``/80`` for the containers. This way you can use 4096 hosts with 16
+``/80`` subnets each.
+
+Every configuration in the diagram that is visualized below the dashed line
+across hosts is handled by Docker, such as the docker0 bridge IP address
+configuration, the route to the Docker subnet on the host, the container IP
+addresses and the routes on the containers. The configuration above the line
+across hosts is up to the user and can be adapted to the individual environment.
+
+----------
+References
+----------
+
+.. [1] https://docs.docker.com/v17.09/engine/userguide/networking/default_network/ipv6/#docker-ipv6-cluster
+.. [2] http://www.debug-all.com/?p=128