Bit Indexed Explicit Replication (BIER) architecture allows optimal forwarding of multicast packets without requiring a legacy multicast protocol to build multicast trees or for intermediate routers to maintain any per-multicast flow state. This provides a simplified control plane because BIER information is distributed using underlay IGP.
The following terms are used in BIER:
Bit Forwarding Router (BFR)
A BFR is a router supporting BIER with a unique BFR prefix and optionally, a BIER ID assigned by the operator. A BFR establishes BFR adjacencies (IGP or SDN-programmed), computes the BIER routing table, and forwards or replicates BIER packets.
BIER domain and sub-domain (SD)
A BIER domain is a connected set of BFRs, each with a unique BFR ID. A BIER domain can be divided into sub-domains for scalability without a linear increase in size of the BIER header. For example, in IS-IS, a BIER sub-domain is IS-IS multi topology, where ipv4-unicast is a single sub-domain and ipv4-multicast is another sub-domain.
Sub-domains provide minimum traffic engineering and separation of services.
Bit Forwarding Ingress Router (BFIR)
A BFIR is the first PE in a BIER domain entered by a multicast packet. The BFIR adds a BIER header and forwards the packet using the BIER routing table.
Bit Forwarding Egress Router (BFER)
A BFER is the last PE that processes a BIER packet in a BIER domain. The BFER removes the BIER header before forwarding the packet. This is the only PE that requires a BIER ID as it is a PE with receiver connectivity.
Transit Bit Forwarding Router (transit BFR)
A transit BFT is a router in the BFR domain that is not a BFIR or a BFER that forwards the packet using the best path.
SR OS does not support multicast MPLS packets over an IGP shortcut. This includes BIER MPLS encapsulation. IGP shortcuts can be configured on SR OS for unicast and installed in the RIB or FIB, but BIER is not resolved over the IGP shortcut. If an IGP shortcut is used for unicast resolution, an IPv4-multicast MT can be used to create a separate MT for BIER without the IGP shortcut.