BGP Large Communities are a 96-bit community format designed to carry routing-policy tags cleanly in a world where Autonomous System numbers can be 32 bits. They were standardised because the older community format could not naturally encode a full four-octet ASN plus an operator-defined value.
Why the original 32-bit community ran out of room
A Large Community contains three 32-bit fields and is written as three colon-separated integers: Global Administrator:Local Data Part 1:Local Data Part 2. The first field is normally the ASN of the organisation defining the meaning. The other two provide space for policy values. Because all three fields are large enough for modern ASNs and structured data, operators can build conventions such as location plus action without squeezing information into the legacy 16-bit halves of a classic community.
The three-field format
The format reduces ambiguity and workarounds. When four-octet ASNs became common, operators used awkward mappings or documentation schemes to fit them into classic communities. Large Communities provide a simple, extensible attribute that routers can propagate and match using familiar policy tools. They are now widely useful for transit, internet exchange and customer traffic-engineering policies.
More expressive, not more secure
Large Communities do not replace every classic community. Existing networks have years of automation and policy built around the older attribute, and well-known communities remain relevant. They also provide no cryptographic authenticity: a route can carry a Large Community that was added, removed or modified by another network. Their purpose is expressive routing metadata, not security.
Related reading
- RPKI vs IRR: What Is the Difference Between the Two Routing Databases?
- What Is ASPA? The New RPKI System for Checking BGP Paths