MPLS Layer 2 VPNs and Pseudowires (LDP, BGP-AD, VPLS)
What this domain tests
MPLS Layer 2 VPNs and Pseudowires tests how Ethernet or other Layer 2 frames are carried over MPLS. It covers point-to-point pseudowires signaled by LDP, autodiscovery with BGP-AD, and multipoint VPLS.
Core rules
- A pseudowire (PW) emulates a Layer 2 service between two PEs. It uses a VC label to identify the PW and an outer tunnel label to reach the remote PE.
- LDP PW signaling uses a targeted LDP session and a PWid/FEC 128. Each direction has its own VC label.
- BGP-AD advertises auto-discovery and PW signaling information using BGP extended communities and route targets; it reduces manual PW configuration.
- VPLS is a multipoint Ethernet service. PEs learn source MAC addresses per VSI and forward unknown unicast, broadcast, and multicast over the PW mesh.
- Split horizon prevents a PE from forwarding a frame received from one PW back out another PW in the same VPLS.
Worked example
PE1 and PE2 belong to VPLS "blue". PE1 learns MAC aa:bb:cc from its local CE. A frame to that MAC arrives at PE2. PE2 does not know the MAC, so it floods the frame over all blue PWs. PE1 receives it, sees the destination MAC is local, and forwards it to the CE. PE1 also learns the source MAC from the PW and installs it in the VSI MAC table. The VC label on the PW tells PE1 which VSI to use.
Traps
Do not call VPLS point-to-point; VPWS is point-to-point. Do not confuse the VC label with the transport label. BGP-AD needs RT policy, and LDP PW needs a targeted session, not just link LDP.