doc: add isisd flex-algo documentation

Add the isisd flex-algo documentation

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>

doc/user/isisd.rst

@@ -316,12 +316,12 @@ Showing ISIS information
    Show the ISIS database globally, for a specific LSP id without or with
    details.
 
-.. clicmd:: show isis topology [level-1|level-2]
+.. clicmd:: show isis topology [level-1|level-2] [algorithm (128-255)]
 
    Show topology IS-IS paths to Intermediate Systems, globally, in area
    (level-1) or domain (level-2).
 
-.. clicmd:: show isis route [level-1|level-2] [prefix-sid|backup]
+.. clicmd:: show isis route [level-1|level-2] [prefix-sid|backup] [algorithm (128-255)]
 
    Show the ISIS routing table, as determined by the most recent SPF
    calculation.
@@ -422,7 +422,7 @@ Known limitations:
    MPLS dataplane. E.g. for Linux kernel, since version 4.13 the maximum value
    is 32.
 
-.. clicmd:: segment-routing prefix <A.B.C.D/M|X:X::X:X/M> <absolute (16-1048575)|index (0-65535) [no-php-flag|explicit-null] [n-flag-clear]
+.. clicmd:: segment-routing prefix <A.B.C.D/M|X:X::X:X/M> [algorithm (128-255)] <absolute (16-1048575)|index (0-65535) [no-php-flag|explicit-null] [n-flag-clear]
 
    prefix. The 'no-php-flag' means NO Penultimate Hop Popping that allows SR
    node to request to its neighbor to not pop the label. The 'explicit-null'
@@ -431,11 +431,163 @@ Known limitations:
    clear the Node flag that is set by default for Prefix-SIDs associated to
    loopback addresses. This option is necessary to configure Anycast-SIDs.
 
-.. clicmd:: show isis segment-routing node
+.. clicmd:: show isis segment-routing node [algorithm (128-255)]
 
    Show detailed information about all learned Segment Routing Nodes.
 
-.. _debugging-isis:
+.. _isis-flex-algo:
+
+Flex-Algos (Flex-Algo)
+======================
+
+*isisd* supports some features of
+`RFC 9350 <https://tools.ietf.org/html/rfc9350>`_ on an MPLS Segment-Routing
+dataplane. The compatibility has been tested against Cisco.
+
+IS-IS uses by default the `Shortest-Path-First` algorithm that basically
+calculates paths based on the shortest total metric to the destinations.
+Flex-Algo allows new algorithms to run in parallel to compute paths in different
+manners, based on metrics (IGP metric or a new type of metrics such as Traffic
+Engineering (TE) metric and minimum delay...) and constraints. New metric types
+are not yet implemented but constraints are already operational. Constraints can
+restrict paths to links with specific affinities or avoid links with specific
+affinities. Combinations of these are also possible.
+
+The administrator can configure up to 128 Flex-Algos in an IS-IS area.
+To do so, it defines a set of Flex-Algo Definitions (FAD) which
+have the following characteristics:
+
+- a numeric identifier (ID) between 128 and 255 inclusive
+- a set of constraints (basically, include or exclude a certain given set of
+	links, designated by a admin-group)
+- the calculation type (only the `Shortest-Path-First` is currently supported)
+- the metric type (only the IGP inherited metric type is currently supported)
+- some additional flags (not supported for the moment).
+
+A subset of routers advertises the Flex-Algo Definitions (FAD) to the other
+routers within an area. In order to use a common set of FADs, each router runs a
+FAD election process for each locally configured algorithm, using the following
+rules:
+
+- If a locally configured FAD is not advertised to the area, the router does not
+	participate in the particular flex algorithm.
+- If a given flex algorithm is running, the participation in this particular
+	flex algorithm stops when its advertisements are over.
+- A router includes its own FAD in the election process if and only if it is
+	advertised to the other routers.
+- If only one router advertises the FAD, the FAD is elected.
+- If several FADs are advertised with different priorities, the one with the
+	highest priority value is selected.
+- If there are multiple advertisements of the FAD with the same highest
+	priority, the FAD of the router with the highest IS-IS system-ID is
+	selected.
+
+Routers only use the specifications of the elected FAD regardless of the locally
+configured definitions. If a router does not support one of the FAD
+characteristics, it stops participating in the Flex-Algo.
+
+For each running Flex-Algo, the Segment-Routing SIDs must be
+configured with values unique to the algorithm. It allows routers to identify
+which flex algorithm they must use for a given packet.
+
+The following commands configure Flex-Algo at the 'router isis' configuration
+level. Segment-Routing prefixes must be configured for the Flex-Algo.
+
+.. clicmd:: flexible-algorithm (128-255)
+
+   Add a Flex-Algo Definition (FAD) and enter the FAD configuration
+   level. The algorithm ID value is in the range of 128 to 255 inclusive.
+
+.. clicmd:: no flexible-algorithm (128-255)
+
+   Unconfigure a Flex-Algo Definition.
+
+.. clicmd:: affinity-map NAME bit-position (0-255)
+
+   Add the specified 'affinity-map'. Affinity-map definitions are used in
+   FADs and in interfaces admin-group definition.
+
+   Affinity-maps format in advertisement TLVs use the extended admin-group
+   format defined in the RFC7308 section 2.2. The extended admin-group uses a
+   256 bits field. If an affinity-map is set, the bit at the extended
+   admin-group 'bit-position' is set 1, else it is set to 0.
+
+The following commands configure Flex-Algo at the 'router isis' and
+'flexible-algorithm (128-255)' configuration level.
+
+.. clicmd:: advertise-definition
+
+   Advertise the current FAD to other IS-IS routers by using specific IS-IS
+   TLVs. By default, the definition is is not shared with other routers.
+
+   A router can advertise a FAD without participating in the Flex-Algo.
+
+.. clicmd:: priority (0-255)
+
+   Set the specified 'priority' in the current FAD advertisements .
+
+.. clicmd:: metric-type [igp|te|delay]
+
+   Set the 'metric-type' for the current FAD. 'igp' is
+   the default value and refers to the classic 'Shortest-Path-First' algorithm.
+   If the 'te' or the 'delay' metric is selected, the value is advertised but
+   the flex algorithm is disabled locally because these types are not currently
+   supported.
+
+.. clicmd:: no metric-type
+
+   Reset the 'metric-type' to the default 'igp' metric.
+
+.. clicmd:: affinity exclude-any NAME
+
+   Add the specified affinity to the list of exclude-any affinities. The
+   Flex-Algo will compute paths that exclude the segments with any of
+   the specified affinities.
+
+.. clicmd:: no affinity exclude-any NAME
+
+   Remove the specified affinity to the list of exclude-any affinities.
+
+.. clicmd:: affinity include-all NAME
+
+   Add the specified affinity to the list of include-all affinities. The
+   Flex-Algo will compute paths that include the segments with all
+   the specified affinities.
+
+.. clicmd:: no affinity include-all NAME
+
+   Remove the specified affinity to the list of include-all affinities.
+
+.. clicmd:: affinity include-any NAME
+
+   Add the specified affinity to the list of include-any affinities. The
+   Flex-Algo will compute paths that include the segments with any of
+   the specified affinities.
+
+.. clicmd:: no affinity include-any NAME
+
+   Remove the specified affinity to the list of include-any affinities.
+
+The following commands configure Flex-Algo at the 'interface' configuration
+level.
+
+.. clicmd:: isis affinity flex-algo NAME
+
+	Add the specified affinity to the interface.
+
+.. clicmd:: no isis affinity flex-algo NAME
+
+	Remove the specified affinity from the interface.
+
+The following command show Flex-Algo information:
+
+.. clicmd:: show isis flex-algo [(128-255)]
+
+	Show information about the elected FADs
+
+'show isis route', 'show isis topology' and 'show isis segment-routing node'
+includes an 'algorithm (128-255)' optional argument. See
+:ref:`showing-isis-information` and :ref:`isis-segment-routing`.
 
 Debugging ISIS
 ==============