isis-doc: Simple config and protocol description

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+.. include:: ../_include/need_improvement.txt
+
+.. _isis:
+
+#####
+IS-IS
+#####
+
+:abbr:`IS-IS (Intermediate System to Intermediate System)` is a link-state interior gateway routing protocol.
+Like OSPF, IS-IS runs the Dijkstra shortest-path first (SPF) algorithm to create a database of the network’s
+topology and, from that database, to determine the best (that is, shortest) path to a destination.
+The routers exchange topology information with their nearest neighbors.
+IS-IS runs directly on the data link layer (Layer 2).
+IS-IS addresses are called :abbr:`NETs (Network Entity Titles)` and can be 8 to 20 bytes long, but are generally 10 bytes long.
+
+For example :abbr:`NET (Network Entity Title)`
+
+.. code-block:: none
+
+  49.0001.1921.6800.1002.00
+
+The IS-IS address consists of three parts:
+
+  :abbr:`AFI (Address family authority identifier)`
+    ``49`` The AFI value 49 is what IS-IS uses for private addressing.
+
+  Area identifier:
+    ``0001`` IS-IS area number (Area1)
+
+  System identifier:
+    ``1921.6800.1002`` For system idetifier we recommend to use IP address or MAC address of the router.
+
+  NET selector:
+    ``00`` Must always be 00, to indicate "this system".
+
+Simple IS-IS configuration using 2 nodes and redistributing connected interfaces.
+
+**Node 1:**
+
+.. code-block:: none
+
+  set interfaces dummy dum0 address '203.0.113.1/24'
+  set interfaces ethernet eth1 address '192.0.2.1/24'
+
+  set policy prefix-list EXPORT-ISIS rule 10 action 'permit'
+  set policy prefix-list EXPORT-ISIS rule 10 prefix '203.0.113.0/24'
+  set policy route-map EXPORT-ISIS rule 10 action 'permit'
+  set policy route-map EXPORT-ISIS rule 10 match ip address prefix-list 'EXPORT-ISIS'
+
+  set protocols isis FOO interface eth1
+  set protocols isis FOO net '49.0001.1921.6800.1002.00'
+  set protocols isis FOO redistribute ipv4 connected level-2 route-map 'EXPORT-ISIS'
+
+**Node 2:**
+
+.. code-block:: none
+
+  set interfaces ethernet eth1 address '192.0.2.2/24'
+
+  set protocols isis FOO interface eth1
+  set protocols isis FOO net '49.0001.1921.6800.2002.00'
+
+Show ip routes on Node2:
+
+.. code-block:: none
+
+  vyos@r2:~$ show ip route isis 
+  Codes: K - kernel route, C - connected, S - static, R - RIP,
+         O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
+         T - Table, v - VNC, V - VNC-Direct, A - Babel, D - SHARP,
+         F - PBR, f - OpenFabric,
+         > - selected route, * - FIB route, q - queued route, r - rejected route
+
+  I   203.0.113.0/24 [115/10] via 192.0.2.1, eth1, 00:03:42