Loop Prevention

On an Ethernet network, Spanning Tree Protocol (STP) is often used to prevent loops. However, VPLS users do not know the topology of the Internet Service Provider (ISP) network. Therefore, enabling STP on the private network cannot prevent loops on the ISP network. VPLS uses full-mesh PWs and split horizon to prevent loops.

• PEs in a VSI must be fully meshed. This means that each PE must create a tree to every other PE in the VSI.
• All PEs must support split horizon to avoid loops. Split horizon requires that packets sent on a PW in a VSI should not be forwarded over other PWs in the VSI. Any two PEs in a VSI must communicate directly over a single PW, without data being forwarded through an intermediary device. This is why full-mesh PWs are required between PEs in a VSI.

The full-mesh PEs and split horizon ensure route reachability and prevent loops on VPLS networks. If a CE is connected to multiple PEs, or multiple CEs on the same VPLS VPN are interconnected, VPLS cannot guarantee loop prevention. In this situation, other methods must be used to prevent loops.

STP can run on an L2VPN private network. All STP Bridge Protocol Data Units (BPDUs) are transparently transmitted over the ISP network.