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.. _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.
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