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RFC 1997 / START HERE

BGP Communities, before the details

BGP Communities Attribute

BGP means Border Gateway Protocol. Routers use it to exchange routes to destinations. A community is a tag that groups destinations for routing policy; one route can carry several values.

Start the guide See the 7 chapters ↓

The idea in one picture

  1. Attach route tags

  2. Match local policy

  3. Respect propagation limits

A simplified picture of the idea; the chapters explain the actual mechanisms.

Why it exists

Operators can match a group instead of maintaining a separate rule for every destination. This RFC defines the classic four-octet tags and three well-known propagation limits; later updates clarify policy behavior and malformed attributes.

A familiar example

Imagine a route tagged to stay inside a network’s routing boundary. NO_EXPORT restricts where peers advertise it, while NO_ADVERTISE stops every re-advertisement. These are route-policy tags, not authentication or encryption.

Helpful background

You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.

BGPRFC 4271 · beginner overview ↗Route reflectionRFC 4456 · beginner overview ↗CIDRRFC 4632 · beginner overview ↗

What you’ll learn next

Begin with chapter one, or choose a topic.

  1. Attach policy tags to routesBGP means Border Gateway Protocol. Its peers exchange routes to destinations. A community groups destinations with a shared property; a route can carry several community values.
  2. Read four-octet values and reserved rangesEach community is a 32-bit value. For the non-reserved range, the first two octets identify an AS and the final two have AS-defined meaning.
  3. Find the export boundaryA BGP confederation groups member ASes behind one external boundary. NO_EXPORT and NO_EXPORT_SUBCONFED stop advertisements at different boundaries. The illustrated peers understand and retain the communities.
  4. Stop every re-advertisement with NO_ADVERTISENO_ADVERTISE has a stricter scope than an export-boundary tag: the received route must not be advertised to any other BGP peer.
  5. Carry component communities into an aggregateAggregation combines more-specific routes into a broader route. RFC 1997 gives a conditional recommendation about the aggregate community set.
  6. Make local policy behavior explicitRFC 1997 allows local modification of communities. Its RFC 8642 update explains why a policy command named set must not be assumed to mean the same thing everywhere.
  7. Handle malformed attributes without inventing trustRFC 7606 updates community error handling. A recognized community attribute with an invalid value length is not the same as an unfamiliar, well-formed tag.
Start with chapter one →

This introduction explains RFC 1997 (1996). Read the original RFC ↗