BGP Large Communities (RFC 8092)

Why Large Communities?

Standard BGP communities are 32 bits: ASN:value where each field is 16 bits. This was designed when ASNs were 16-bit numbers (0-65535).

The problem: 4-byte ASNs (up to 4,294,967,295) cannot fit in the 16-bit ASN field of standard communities.

Standard community:   ASN16:VALUE16    (4 bytes total)
  Example: 65000:100

Large community:      ASN32:VALUE32:VALUE32  (12 bytes total)
  Example: 399999:100:200

Large Community Structure

A large community is a 12-byte value with three 32-bit fields:

+-----------------------------------+
| Global Administrator (32 bits)    |  -> Typically your ASN
+-----------------------------------+
| Local Data Part 1 (32 bits)       |  -> Function/category
+-----------------------------------+
| Local Data Part 2 (32 bits)       |  -> Value/parameter
+-----------------------------------+

The three fields are written as GA:LD1:LD2:

399999:1:100
  |     |  |
  |     |  +-- Value: local-pref 100
  |     +----- Function: set local-pref
  +----------- ASN: 399999

Use Cases

Informational Tagging

# Tag routes by source type
399999:1:1    -- Learned from customer
399999:1:2    -- Learned from peer
399999:1:3    -- Learned from transit
399999:1:4    -- Originated internally

# Tag by geographic region
399999:2:840  -- Learned in US (ISO country code)
399999:2:276  -- Learned in Germany
399999:2:392  -- Learned in Japan

Action Communities (for customers)

# Set local-preference
399999:100:50    -- Set local-pref to 50
399999:100:100   -- Set local-pref to 100
399999:100:200   -- Set local-pref to 200

# Prepend to specific peers
399999:200:13335  -- Prepend 1x toward Cloudflare (AS 13335)
399999:201:13335  -- Prepend 2x toward Cloudflare
399999:202:13335  -- Prepend 3x toward Cloudflare

# Do not announce to specific peer
399999:300:32934  -- Do not announce to Facebook (AS 32934)

The extra field allows targeting specific remote ASes -- something impossible with standard communities.

Configuration Examples

# Cisco IOS-XR
route-policy SET-CUSTOMER-TAG
  set large-community (399999:1:1)
end-policy

route-policy MATCH-CUSTOMER
  if large-community matches-any (399999:1:1) then
    set local-preference 200
  endif
end-policy

# Junos
policy-statement TAG-CUSTOMER {
    then {
        community add CUSTOMER-LC;
    }
}
community CUSTOMER-LC members [ large:399999:1:1 ];

# BIRD 2
function tag_customer() {
    bgp_large_community.add((399999, 1, 1));
}

Migration from Standard Communities

You do not need to replace standard communities immediately. Run both in parallel:

1. Define your large community scheme mirroring your standard community assignments.
2. Add large communities to existing policy alongside standard ones.
3. Publish documentation for both formats.
4. Encourage peers to use large communities for new implementations.
5. Deprecate standard communities once adoption is sufficient.

Adoption Status

Large communities are supported by all modern BGP implementations:

If you operate a 4-byte ASN, large communities are essential. Even with a 2-byte ASN, the richer three-field structure makes policy design cleaner and more expressive.