Pseudowire (PW) / Any Transport over MPLS (AToM) is an IETF L2VPN mechanism that emulates point-to-point Layer 2 services (such as Ethernet, 802.1Q VLANs, HDLC, or PPP) over a packet-switched MPLS network backbone. It allows enterprise customers to link geographically separated LANs transparently as if connected directly by an Ethernet cable.
Key Components of AToM L2VPN
xconnect Command: Cisco IOS command configured under an attachment interface or sub-interface to map customer L2 traffic directly to a remote PE loopback via a unique Virtual Circuit ID (VC ID).
Virtual Circuit (VC) Label: Demux label assigned via Directed LDP signaling. It identifies the specific customer attachment circuit on the egress LER.
Transport Label: Outer MPLS IGP label used to route the frame across the provider core to the egress PE loopback.
Control Word (CW): Optional 4-byte header inserted between the VC label and L2 payload to preserve sequence ordering and pad small frames.
Pseudowire Encapsulation Stack
Outer Label (20 Bits): Transport Label (LDP/TE) -> Forwarded by Core P Routers.
Inner Label (20 Bits): VC Label (Signaled by Targeted LDP) -> Processed only by PE Routers.
Control Word (32 Bits): Optional sequence/flags field.
Layer 2 Payload: Original Customer Ethernet Frame (MAC Header + IP Packet + FCS).