Flow-Aware Transport for EVPN over MPLS
Overview
Flow-Aware Transport (FAT) Labels introduce entropy into the MPLS label stack to improve Equal-Cost Multipath (ECMP) forwarding in EVPN networks. By inserting a FAT label as the bottom-of-stack (BOS) below the EVPN service label, per-flow hashing is enabled across core routers, ensuring efficient traffic distribution without requiring transport-layer changes. This label is independent of any modifications in the Transport Label, ensuring end-to-end entropy delivery.
BGP-based FAT label signaling, as defined in RFC 7432, is not supported.
Feature Characteristics
| • | FAT Label can be configured on Provider Edge (PE) routers for Unicast EVPN traffic, but validation occurs only on the ingress or egress interfaces of Transit (P) nodes. It does not apply to BUM traffic. |
| • | Supports both single-homed and multihomed deployments. |
| • | Applies to EVPN E-LAN, E-LINE, and E-Tree services. |
| • | Maintains compatibility with existing Control Word (CW) support. |
Benefits
| • | Improved Load Balancing: Ensures even distribution of traffic across ECMP paths. |
| • | Higher Throughput: Maximizes available bandwidth utilization by avoiding congestion hotspots. |
| • | Lower Latency: Reduces queuing and congestion delays in the core. |
| • | Data Center and Large-Scale EVPN Ready: Addresses east-west traffic patterns in leaf-spine and redundant mesh topologies. |
| • | No Transport Dependency: Entropy is provided by FAT label insertion, not by core transport capabilities. |