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FAT Flow Labels Overview


A pseudowire is a Layer 2 circuit or service that emulates the essential attributes of a telecommunications service, such as a T1 line, over an MPLS packet-switched network (PSN). The pseudowire is intended to provide only the minimum necessary functionality to emulate the wire with the required resiliency requirements for the given service definition.

In an MPLS network, the flow-aware transport of pseudowires (FAT) flow label, as described in RFC 6391, Flow-Aware Transport of Pseudowires over an MPLS Packet Switched Network, is used for load-balancing traffic across LDP-signaled pseudowires for virtual private LAN service (VPLS) and virtual private wire service (VPWS).

When the pseudowire is configured to use the FAT flow labels for load balancing, packets arriving at the ingress router are processed in the following sequence across the path of the pseudowire:

  • The ingress router uses the contents of the inbound packet in the hash-key algorithm to calculate the flow-label value.

  • The ingress router pushes the flow label to the label stack of the packet.

  • The transit routers perform load balancing based only on the label stack.

  • The egress router pops the flow label and forwards the packet to its destination.

For load balancing to work based on a flow-label configuration, a version of LDP that supports signaling extensions to use the flow label with pseudowires must be enabled on all routers. The LDP-signaling configuration is identical for VPLS and VPWS pseudowires.

FAT flow labels are supported on the following LDP-signaled forwarding-equivalence classes (FECs) for VPWS and VPLS pseudowires:

  • FEC 128 for VPWS—LDP-signaled VPWS with neighbors that are statically configured (BGP autodiscovery is not supported).

  • FEC 128 for VPLS—LDP-signaled VPLS with neighbors that are statically configured (BGP autodiscovery is not supported).

  • FEC 129 for VPWS—LDP-signaled VPWS with BGP autodiscovery of neighbors.

  • FEC 129 for VPLS—LDP-signaled VPLS with BGP autodiscovery of neighbors.


Flow-aware transport of pseudowires (FAT) flow labels are not supported with Enhanced Dense Port Concentrators (DPCs), and flow labels should not be configured in a system with DPCs. Entropy labels are supported with DPCs with the following restrictions: entropy labels are stripped by DPCs, but entropy labels are not pushed by DPCs.

The interface parameter (Sub-TLV) is used both for FEC 128 and FEC 129 pseudowires. The sub-TLV defined for LDP contains the transmit (T) and receive (R) bits. The T bit advertises the ability to push the flow label. The R bit advertises the ability to pop the flow labels. By default, the signaling behavior of the provider edge (PE) router for any of these pseudowires is to advertise the T and R bits in the label set to 0.

The flow-label-transmit and flow-label-receive configuration statements provide the ability to set the T bit and R bit advertisement to 1 in the Sub-TLV field, which is part of the interface parameters of the FEC for the LDP label-mapping message. You can use these statements to control the pushing of the load-balancing label and the advertisement of the label to the routing peers in the control plane.

Alternatively, for FEC 128 VPWS pseudowires only, you can configure the following statements to statically configure flow label push and pop operations:

  • flow-label-receive-static to statically pop the flow label on the pseudowire packets received from the remote PE router.

  • flow-label-transmit-static to statically push the flow label on the pseudowire packets sent to the remote PE router.