Field Mapping

RFC 8092	this document
Global Administrator	ASN
Local Data Part 1	Function
Local Data Part 2	Parameter

A mapping table on the use of fields in BGP Large Communities between [RFC8092] and this document.

In contemporary deployments of both BGP Communities [RFC1997] and BGP Large Communities, the function of a community can be divided into two categories:

• Informational Communities
• Action Communities

        AS 65551
            |
            ^
            |
        AS 64497
          /  \
         ^    \
        /      ^
   AS 64498     \
       |        | 
       `<->- AS 64499

Throughout the document a topology of four ASs is used to illustrate the use of communities in the following configuration:

The opaque nature of BGP Large Communities allows for rapid deployment of new features or changes to their routing policy that perform an action. Operators are encouraged to publicly publish and maintain documentation on the purpose of each BGP Large Community, both informational and action, that they support or are visible in BGP RIBs.

2.1. Informational Communities

Informational Communities are labels for attributes such as the origin of the route announcement, the nature of the relation with an EBGP neighbor or the intended propagation audience. Informational Communities can also assist in providing valuable information for day-to-day network operations such as debugging or capacity planning.

The Global Administrator field is set to the ASN which labels the routes with the Informational Communities. For example, AS 64497 might add a community with the GA 64497 to a route accepted from an IBGP or EBGP neighbor as a means of signaling that it was imported in a certain geographical region.

In general, the intended audiences of Informational Communities are downstream networks and the Global Administrator itself, but any AS could benefit from receiving these communities.

2.2. Action Communities

Action Communities are added as a label to request that a route be treated in a particular way within an AS. The operator of the AS defines a routing policy that adjusts path attributes based on the community. For example, the route's propagation characteristics, the LOCAL_PREF (local preference), the next-hop, or the number of AS_PATH prepends to be added when it is received or propagated can be changed.

The Global Administrator field is set to the ASN which has defined the functionality of that BGP Large Community and is the ASN that is expected to perform the action. For example, AS 64499 might label a route with a BGP Large Community containing GA 64497 to request that AS 64497 perform a pre-defined action on that route.

In general, the intended audience of Action Communities are transit providers taking action on behalf of a customer or the Global Administrator itself, but any AS could take action if they choose and any AS could add an Action Community with the GA of a non-adjacent ASN. However, note that an Action Community could also be informational. Its presence is an indicator that the GA may have performed the action and that an AS in the AS_PATH requested it.

Operators are recommended to publish the relative order in which Action Communities (both BGP Communities and BGP Large Communities) are processed in their routing policy.

3. Examples of Informational Communities

3.1. Location

An AS, AS 64497 in these examples, may inform other networks about the geographical region where AS 64497 imported a route by labeling it with BGP Large Communities following one of the following schemes or a combination of them.

3.1.1. An ISO 3166-1 Numeric Function

AS 64497 could assign a value of 1 to the Function field to designate the content of the Parameter field as an ISO-3166-1 numeric country identifier.