BGP routing policies control route distribution and behavior across networks. Communities, optional transitive BGP attributes associate operational metadata with routes, like origin location or specific routing actions. However, standard and extended BGP community attributes fall short in environments using 4-byte Autonomous System Numbers (ASNs).

To solve this issue, BGP introduced the Large Community (LC) attribute. It consists of an unordered set of one or more twelve-octet values, including: A 4-byte Global Administrator field (typically the ASN) and two 4-byte operator-defined fields.

These attributes enable operators to encode information pertinent to their routing policies and operational needs. Operators must not transmit duplicate values, and receiving BGP speakers must silently discard any redundant entries.

OcNOS enhanced support for BGP Large Communities by:

Scalable Policy Management: Named community-lists simplify reuse and maintenance across multiple route-maps.

Enhanced Flexibility: Additive and deletion capabilities allow policy controls without impacting other community values.

Future-Proofing: Prepares networks for advanced policy frameworks as multi-ASN and operator-specific routing needs evolve.

This section illustrates the use of BGP Large Communities for scalable and consistent route policy enforcement in a service provider or multi-tenant environment. It highlights how route tagging at the source (Router A), centralized policy processing at a Route Reflector (Router B), and controlled propagation to downstream routers (Router C) enable flexible traffic engineering, service differentiation, or route filtering, without the need for policy replication on every node.

The figure below consists of three BGP routers in a route-reflector setup. Router B acts as the Route Reflector (RR), while routers A and C are its RR clients. There are no direct iBGP sessions between A and C; all BGP routing information exchange flows through the RR.

Each client establishes iBGP sessions with B for the following address families:

In this setup, Router A originates prefixes with large communities. Router B uses a policy engine to match, strip, and overwrite these communities using a route-map. It then advertises the modified routes to Router C. Router C applies a large-community filter to accept only those routes that match the expected community values. This demonstrates policy-based route control using BGP Large Communities in an RR-based MPLS network.

Tag outbound BGP routes with two large community values (1:2:3 and 4:5:6) to influence downstream routing behavior.

Match received BGP routes containing large community 1:2:3 and 4:5:6, strip them, and apply new values 40:50:60 and 10:20:30.

Participate in BGP flow and MPLS forwarding. No large community changes or filters. Acts as a transit or receiving node.

Verify that the routes are being advertised correctly to Router B and are carrying the intended community tags (in this case, Large Communities 1:2:3 and 4:5:6).

The show ip bgp command confirms that the IPv4 unicast route (e.g. 150.150.150.150/32) exists in the local BGP table. The BGP session to B (neighbor 5.5.5.5) is active and routes are sent.

The show ip bgp 150.150.150.150 command confirms that the route is locally originated and advertised to B. It is useful for verifying if the prefix is a candidate for applying the Large Community policy.

The command show ip bgp vpnv4 vrf A confirms that the routes are originated and tagged with route targets, and the redistribution from VRF A to BGP VPNv4 is occurring.

The command show ip bgp vpnv4 vrf A 190.10.10.0 confirms that Router A is advertising VPNv4 routes (e.g. 190.10.10.0/24) to Router B and VRF-specific community values like RTs.

The command show bgp ipv6 confirms that IPv6 prefixes (e.g., 1234::4500/120) are present, valid, and advertised to B. It verifies that the IPv6 session to 5555::5555 is working and that the route-map is applied.

Use Case: A transit provider assigns unique BGP Large Community values to each customer. These values are utilized in route maps to influence outbound policy decisions, such as local preference or AS-path modification.

Define large-community values associated with Customer A:

Apply policy using a route-map:

This configuration applies a higher local preference to prefixes received from Customer A. By using a named large-community list, the setup remains scalable-community values can be updated in one place without editing the route map directly. This approach also simplifies policy management across multiple route maps or peers.

The following are the newly added and existing commands updates as part of the BGP LC enhancements.

Use this command to define a set of BGP Large Community values for use in policy configuration (e.g., route-maps). It functions similarly to standard or extended community-lists and allows reuse of defined large-community patterns.

Use no parameter of this command to remove the configured large community values.

None

Configure mode

Introduced in OcNOS version 6.6.1.

The following example demonstrates how to configure different types of BGP Large Community lists in OcNOS and verify them using the show command.

Use this command to match BGP routes based on Large Community values. This allows selective policy actions (e.g., setting local preference, AS path, AIGP) for routes carrying specific BGP Large Community attributes.

Use no parameter of this command to remove the configured Large Community values.

None

Route-map mode

Introduced in OcNOS version 6.6.1.

The following example shows how to reference large-community in route-maps for policy enforcement.

Use this command within a route-map mode to delete the large-community-list values from matched routes (if present).

Use no parameter of this command to add the specified large-community values to matched routes.

None

Route-map mode

Introduced in OcNOS version 6.6.1.

The following example shows how to remove (delete) selected communities from matched routes using the set large-comm-list command.

Use this command to display the configured BGP IPv4 Large Community Lists. This includes both standard and expanded lists, named or numbered. The command helps verify large-community filters applied in route-maps for matching or modifying BGP route attributes.

None

Exec mode and Privileged Exec mode

Introduced in OcNOS version 6.6.1.

Below are the revised commands. For more command details, refer to the Route-Map Commands section.

For more details, refer to the set large-community command in the Route-Map Commands section in the OcNOS Layer 3 Guide.

The following provides definitions for key terms or abbreviations and their meanings used throughout this document: