1 files changed, 88 insertions, 0 deletions
diff --git a/docs/gettingstarted/developers/fib20/multicast.rst b/docs/gettingstarted/developers/fib20/multicast.rst
new file mode 100644
index 00000000000..15bcddfec46
--- /dev/null
+++ b/docs/gettingstarted/developers/fib20/multicast.rst
@@ -0,0 +1,88 @@
+.. _mfib:
+
+IP Multicast FIB
+----------------
+
+Basics
+^^^^^^
+
+An IP multicast FIB (mFIB) is a data-structure that holds entries that
+represent a (S,G) or a (\*,G) multicast group. There is one IPv4 and
+one IPv6 mFIB per IP table, i.e. each time the user calls 'ip[6] table
+add X' an mFIB is created.
+
+
+A path describes either where a packet is sent to or where a packet is
+received from. mFIB entries maintain two sets of 'paths'; the
+forwarding set and the accepting set. Each path in the forwarding set
+will output a replica of a received packet. A received packet is only
+accepted for forwarding if it ingresses on a path that matches in the
+accepting set - this is the RPF check.
+
+
+To add an entry to the default mFIB for the group (1.1.1.1, 239.1.1.1)
+that will replicate packets to GigEthernet0/0/0 and GigEthernet0/0/1, do:
+
+.. code-block:: console
+
+   $ ip mroute add 1.1.1.1 239.1.1.1 via GigEthernet0/0/0 Forward
+   $ ip mroute add 1.1.1.1 239.1.1.1 via GigEthernet0/0/1 Forward
+
+the flag 'Forward' passed with the path specifies this path to be part of the replication set.
+To add a path from GigEthernet0/0/2 to the accepting (RPF) set do:
+
+.. code-block:: console
+
+   $ ip mroute add 1.1.1.1 239.1.1.1 via GigEthernet0/0/2 Accept
+
+A (\*,G) entry is added by not specifying a source address:
+
+.. code-block:: console
+
+   $ ip mroute add 232.2.2.2 via GigEthernet0/0/2 Forward
+
+A (\*,G/m) entry is added by not specifying a source address and giving
+the group address a mask:
+
+.. code-block:: console
+
+   $ ip mroute add 232.2.2.0/24 via GigEthernet0/0/2 Forward
+
+Entries are deleted when all paths have been removed and all entry flags (see below) are also removed.
+
+Advanced
+^^^^^^^^
+
+There are a set of flags associated only with an entry, see:
+
+.. code-block:: console
+
+   $ show mfib route flags
+
+only some of these are relevant over the API/CLI:
+
+#. Signal - packets that match this entry will generate an event that
+   is sent to the control plane (which can be retrieved via the signal
+   dump API)
+#. Connected - indicates that the control plane should be informed of
+   connected sources (also retrieved via the signal dump API)
+#. Accept-all-itf - the entry shall accept packets from all
+   interfaces, thus eliminating the RPF check
+#. Drop - Drop all packet matching this entry.
+
+flags on an entry can be changed with:
+
+.. code-block:: console
+
+   $ ip mroute <PREFIX> <FLAG>
+
+An alternative approach to the RPF check, that does check the
+accepting path set, is to give the entry and RPF-ID:
+
+.. code-block:: console
+
+   $ ip mroute <PREFIX> rpf-id X
+
+the RPF-ID is an attribute of a received packet's meta-data and is
+added to the packet when it ingresses on a given entity such as an
+MPLS-tunnel or a BIER table disposition entry.