Multicast BIER - Bit Indexed Explicit Replication

Multicast BIER - RFC8279 Bit Index Explicit Replication - BIER is an architecture that provides optimal multicast forwarding through a "BIER domain" without requiring intermediate routers to maintain any multicast-related per-flow state. BIER also does not require any explicit tree-building protocol for its operation. So, it removes the need for PIM, MLDP, P2MP LSPs RSVP, etc.

A multicast data packet enters a BIER domain at a "Bit-Forwarding Ingress Router" (BFIR), and leaves the BIER domain at one or more "Bit-Forwarding Egress Routers" (BFERs).

The BFIR router adds a BIER header to the packet. The BIER header contains a bit-string in which each bit represents exactly one BFER to forward the packet to. The set of BFERs to which the multicast packet needs to be forwarded is expressed by setting the bits that correspond to those routers in the BIER header.


Multicast BIER Advantages

The obvious advantage of BIER is that there is no per-flow multicast state in the core of the network and there is no tree building protocol that sets up trees on-demand based on users joining a multicast flow. In that sense, BIER is potentially applicable to many services where Multicast is used. Many Service Providers currently investigating how BIER would be applicable to their network, what would be their migration process and which advantages they can get from BIER deployment. By using the BIER header, multicast is not sent to the nodes that do not need to receive the multicast traffic.

That’s why multicast follows an optimal path within the BIER domain. Transit nodes don’t maintain the per-flow state and as it is mentioned above, no other multicast protocol is needed. BIER simplifies multicast operation as no dedicated multicast control protocol for BIER is needed while the existing protocols such as IGP (IS-IS, OSPF) or BGP can be leveraged.

BIER uses a new type of forwarding lookup (Bit Index Forwarding Table). It can be implemented by software or hardware changes. Hardware upgrade requirements can be a challenge for BIER but when it is solved, BIER can be the single de-facto protocol for Multicast.

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Orhan Ergun

Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

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