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 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 (126.96.36.199, 188.8.131.52) that will replicate packets to Eth0/0 and Eth0/1, do:
ip mroute add 184.108.40.206 220.127.116.11 via Eth0/0 Forward ip mroute add 18.104.22.168 22.214.171.124 via Eth0/1 Forward
the flag 'Forward' passed with the path specifies this path to be part of the replication set. To add a path from Eth0/2 to the accepting (RPF) set do:
ip mroute add 126.96.36.199 188.8.131.52 via Eth0/2 Accept
A (*,G) entry is added by not specifying a source address:
ip mroute add 184.108.40.206 via Eth0/2 Forward
A (*,G/m) entry is added by not specifying a source address and giving the group address a mask:
ip mroute add 220.127.116.11/24 via Eth0/2 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:
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:
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.