IP Multicast FIB


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 (, that will replicate packets to GigEthernet0/0/0 and GigEthernet0/0/1, do:

$ ip mroute add via GigEthernet0/0/0 Forward
$ ip mroute add 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:

$ ip mroute add via GigEthernet0/0/2 Accept

A (*,G) entry is added by not specifying a source address:

$ ip mroute add via GigEthernet0/0/2 Forward

A (*,G/m) entry is added by not specifying a source address and giving the group address a mask:

$ ip mroute add via GigEthernet0/0/2 Forward

Entries are deleted when all paths have been removed and all entry flags (see below) are also removed.


There are a set of flags associated only with an entry, see:

$ show mfib route flags

only some of these are relevant over the API/CLI:

  1. 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)
  2. Connected - indicates that the control plane should be informed of connected sources (also retrieved via the signal dump API)
  3. Accept-all-itf - the entry shall accept packets from all interfaces, thus eliminating the RPF check
  4. Drop - Drop all packet matching this entry.

flags on an entry can be changed with:

$ 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:

$ 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.