configexamples: Add PPPoE IPv6 basic setup

Add new document on basic PPPoE IPv6 setup for home network.

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diff --git a/docs/configexamples/index.rst b/docs/configexamples/index.rst
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@@ -17,3 +17,4 @@ This chapter contains various configuration examples:
    tunnelbroker-ipv6
    ha
    wan-load-balancing
+   pppoe-ipv6-basic
diff --git a/docs/configexamples/pppoe-ipv6-basic.rst b/docs/configexamples/pppoe-ipv6-basic.rst
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+.. _examples-pppoe-ipv6-basic:
+
+#######################################
+PPPoE IPv6 Basic Setup for Home Network
+#######################################
+
+This document is to describe a basic setup using PPPoE with DHCPv6-PD +
+SLAAC to construct a typical home network. The user can follow steps described
+here to quickly setup a working network and use this as a starting point to
+further configure or fine tune other settings.
+
+To achieve this, your ISP is required to support DHCPv6-PD. If you're not sure,
+please contact your ISP for more information.
+
+Network Topology
+================
+
+.. image:: /_static/images/pppoe-ipv6-pd-diagram.png
+   :width: 60%
+   :align: center
+   :alt: Network Topology Diagram
+
+Configurations
+==============
+
+PPPoE Setup
+-----------
+
+.. code-block:: none
+
+   set interfaces pppoe pppoe0 authentication password <YOUR PASSWORD>
+   set interfaces pppoe pppoe0 authentication user <YOUR USERNAME>
+   set interfaces pppoe pppoe0 service-name <YOUR SERVICENAME>
+   set interfaces pppoe pppoe0 source-interface 'eth0'
+
+* Fill ``password`` and ``user`` with the credential provided by your ISP.
+* ``service-name`` can be an arbitrary string.
+
+DHCPv6-PD Setup
+---------------
+
+During address configuration, in addition to assigning an address to the WAN
+interface, ISP also provides a prefix to allow router to configure addresses of
+LAN interface and other nodes connecting to LAN, which is called prefix
+delegation (PD).
+
+.. code-block:: none
+
+   set interfaces pppoe pppoe0 ipv6 address autoconf
+   set interfaces pppoe pppoe0 dhcpv6-options pd 0 interface eth1 address '100'
+
+* Here we use prefix to configure the address of eth1 (LAN) to form ``<prefix>::64``,
+  where ``64`` is hexadecimal of address 100.
+* For home network users, most of time ISP only provides /64 prefix, hence
+  there is no need to set SLA ID and prefix length. See :ref:`pppoe-interface`
+  for more information.
+
+Router Advertisement
+--------------------
+
+We need to enable router advertisement for LAN network so that PC can receive
+the prefix and use SLAAC to configure address automatically.
+
+.. code-block:: none
+
+   set service router-advert interface eth1 link-mtu '1492'
+   set service router-advert interface eth1 name-server <NAME SERVER>
+   set service router-advert interface eth1 prefix ::/64 valid-lifetime '172800'
+
+* Set MTU in advertisement to 1492 because of PPPoE header overhead.
+* Set DNS server address in advertisement so that clients can obtain it by using
+  RDNSS option. Most operating systems (Windows, Linux, Mac) should
+  already support it.
+* Here we set the prefix to ``::/64`` to indicate advertising any /64 prefix
+  the LAN interface is assigned.
+* Since some ISPs disconnects continuous connection for every 2~3 days, we set
+  ``valid-lifetime`` to 2 days to allow PC for phasing out old address.
+
+Basic Firewall
+--------------
+
+To have basic protection while keeping IPv6 network functional, we need to:
+
+* Allow all established and related traffic for router and LAN
+* Allow all icmpv6 packets for router and LAN
+* Allow DHCPv6 packets for router
+
+.. code-block:: none
+
+   set firewall ipv6-name WAN_IN default-action 'drop'
+   set firewall ipv6-name WAN_IN rule 10 action 'accept'
+   set firewall ipv6-name WAN_IN rule 10 state established 'enable'
+   set firewall ipv6-name WAN_IN rule 10 state related 'enable'
+   set firewall ipv6-name WAN_IN rule 20 action 'accept'
+   set firewall ipv6-name WAN_IN rule 20 protocol 'icmpv6'
+   set firewall ipv6-name WAN_LOCAL default-action 'drop'
+   set firewall ipv6-name WAN_LOCAL rule 10 action 'accept'
+   set firewall ipv6-name WAN_LOCAL rule 10 state established 'enable'
+   set firewall ipv6-name WAN_LOCAL rule 10 state related 'enable'
+   set firewall ipv6-name WAN_LOCAL rule 20 action 'accept'
+   set firewall ipv6-name WAN_LOCAL rule 20 protocol 'icmpv6'
+   set firewall ipv6-name WAN_LOCAL rule 30 action 'accept'
+   set firewall ipv6-name WAN_LOCAL rule 30 destination port '546'
+   set firewall ipv6-name WAN_LOCAL rule 30 protocol 'udp'
+   set firewall ipv6-name WAN_LOCAL rule 30 source port '547'
+   set interfaces pppoe pppoe0 firewall in ipv6-name 'WAN_IN'
+   set interfaces pppoe pppoe0 firewall local ipv6-name 'WAN_LOCAL'
+
+Note to allow router to receive DHCPv6 response from ISP, we need to allow
+packets with source port 547 (server) and destination port 546 (client).