The libyang library can be installed from third-party packages available `here
<https://ci1.netdef.org/browse/LIBYANG-YANGRELEASE/latestSuccessful/artifact>`_.

Note: the libyang dev/devel packages need to be installed in addition
to the libyang core package in order to build FRR successfully.

For example, for CentOS 7.x:

.. code-block:: shell

   wget https://ci1.netdef.org/artifact/LIBYANG-YANGRELEASE/shared/build-1/CentOS-7-x86_64-Packages/libyang-0.16.46-0.x86_64.rpm
   wget https://ci1.netdef.org/artifact/LIBYANG-YANGRELEASE/shared/build-1/CentOS-7-x86_64-Packages/libyang-devel-0.16.46-0.x86_64.rpm
   sudo rpm -i libyang-0.16.46-0.x86_64.rpm libyang-devel-0.16.46-0.x86_64.rpm

or Ubuntu 18.04:

.. code-block:: shell

   wget https://ci1.netdef.org/artifact/LIBYANG-YANGRELEASE/shared/build-1/Ubuntu-18.04-x86_64-Packages/libyang-dev_0.16.46_amd64.deb
   wget https://ci1.netdef.org/artifact/LIBYANG-YANGRELEASE/shared/build-1/Ubuntu-18.04-x86_64-Packages/libyang_0.16.46_amd64.deb
   sudo apt install libpcre3-dev
   sudo dpkg -i libyang-dev_0.16.46_amd64.deb libyang_0.16.46_amd64.deb

Alternatively, libyang can be built and installed manually by following
the steps below:

.. code-block:: shell

   git clone https://github.com/opensourcerouting/libyang
   cd libyang
   git checkout -b tmp origin/tmp
   mkdir build; cd build
   cmake -DENABLE_LYD_PRIV=ON ..
   make
   sudo make install

When building libyang on CentOS 6, it's also necessary to pass the
``-DENABLE_CACHE=OFF`` parameter to cmake.

Note: please check the `libyang build requirements
<https://github.com/CESNET/libyang/blob/master/README.md#build-requirements>`_
first.

Libyang uses loadable libraries an YANG modules.  It supports
environment variables to allow overriding the load paths for each of
these. With FRR, this override currently must be done at the time of
running FRR's configure command using new options. The new options are:

.. code-block:: shell

   --with-yangmodelsdir=DIR
                          yang models directory (${datarootdir}/yang)
   --with-libyang-pluginsdir=DIR
                          yangmodule plugins directory
                          (${libdir}/frr/libyang_plugins)

an example which uses the compile directory is:

.. code-block:: shell

   ./configure --with-libyang-pluginsdir="`pwd`/yang/libyang_plugins/.libs" \
          --with-yangmodelsdir="`pwd`/yang"