Route Target Constraint for BGP Flow Spec(BGP Flow) and BGP Segment Routing Policies(BGP SR-Policy)

BGP has been used to distribute different types of routing and policy information. In some scenarios, the distributed routing information is specific for certain services, such as BGP/MPLS IP VPNs. Route Target Constraints (RTC), extends Outbound Route Filtering (ORF), describes how route targets are exchanged through the BGP RTC address family on a BGP/MPLS IP VPN network to generate egress policies. This feature enables the BGP/MPLS IP VPN network to control the advertisement of VPN routing information in a more refined manner. This document introduces an extension to the application scenarios of Route Target Constraints (RTC) to control the distribution of routing information to one or a group of network nodes, which could implement precise control of services such as BGP Flow Spec and BGP Segment Routing Policies.

This document introduces the following terms: Route Target Constraints [RFC 4684] Outbound Route Filtering BGP Flow Specification BGP Segment Routing Policy Network Layer Reachability Information

The encapsulation of Route Target membership NLRI is defined in Route Target Constraints (RTC), the NLRI is advertised in BGP UPDATE messages using the MP_REACH_NLRI and MP_UNREACH_NLRI attributes. The (AFI, SAFI) value pair used to identify this NLRI is (AFI=1, SAFI=132). The route-target field in the NLRI indicates a network node and is encoded as a IPv4 Address Specific Extended Community, as shown blow:

While encoding these fields: Global Administrator: 4 octets, indicates the router identifier of the node. If the Global Administrator is set to 0.0.0.0, it means that the peer node accepts all policy rules from the RR. Local Administrator: 2 octets, reserved for future use, MUST be set to 0 upon the sender and MUST be ignored upon the receiver.

This section describes a few use-case scenarios.

In the topology above, the Controller, PE1, and PE2 establish IBGP peer relationships with the RR respectively. PE1 and PE2 are clients of the RR. The Controller distributes Flowspec rules through the RR, and the RR reflects the Flowspec rules to PE1 and PE2. PE1 sends route target membership NLRI{100, 1.1.1.1:0} to the RR, and PE2 sends route target membership NLRI{100, 2.2.2.2:0} to the RR. After receiveing the UPDATE messages with Route Target Membership NLRI, the RR will trigger the RIB-OUTS of the Flowspec route to match the egress policies and update the route to PEs. If hierarchical RRs are deployed, the RRs need to advertise all received route target membership NLRI routes to the upper-layer RRs.

It is described in BGP Segment Routing Policies that one or more route targets SHOULD be attached to the advertisement, where each route target identifies one or more intended headends for the advertised SR Policy update. In the topology above, when the controller needs to deliver SR policies to PE1 and PE2, it will advertises SR policies with route target extended communities, SR Policy1 with {1.1.1.1:0} and SR Policy2 with {2.2.2.2:0}, to RR. The RR will reflect SR Policies to both PE1 and PE2. PEs need to do an ingress filtering, by matching route target extended community with its own router-id. In this case, PE1 will keep SR Policy1 and drop SR Policy2, as well as PE2 will keep SR Policy2 and drop SR Policy1. During this process, even though SR policies are correctly provisioned, the RR advertises all routes to all peers, which may cause network congestion. The ORF operations described in this document work as an egress filter on RR. PE1 sends route target membership NLRI{100, 1.1.1.1:0} to the RR, and PE2 sends route target membership NLRI{100, 2.2.2.2:0} to the RR. After receiving the Route Target Membership NLRI from the PE, the RR generates a PE-specific egress filter. Before advertising routes to PEs, the RR matches routes with egress policies, and will only deliver SR policy1 to PE1 and SR policy1 to PE2 respectively. In this way, services could be correctly deployed and network bandwidth could be saved.

TBD