FAT Pseudowire Support for BGP L2VPN and VPLS 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 (FAT) of pseudowires flow label, as described in draft-keyupdate-l2vpn-fat-pw-bgp, is used for load-balancing traffic across BGP-signaled pseudowires for the Layer 2 virtual private network (L2VPN) and virtual private LAN service (VPLS).
FAT flow label is configured only on the label edge routers (LERs). This causes the transit routers or label-switching routers (LSRs) to perform load balancing of MPLS packets across equal-cost multipath (ECMP) paths or link aggregation groups (LAGs) without the need for deep packet inspection of the payload.
FAT flow label can be used for LDP-signaled forwarding equivalence class (FEC 128 and FEC 129) pseudowires for VPWS and VPLS pseudowires. 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 label. 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 for BGP signaled pseudowires like L2VPN and VPLS.