Understanding VPLS

This topic describes VPLS functions on provider edge (PE) routers.

Because a VPLS carries Ethernet traffic across a service provider network, it must mimic an Ethernet network in some ways. When a PE router configured with a VPLS routing instance receives a packet from a customer edge (CE) device, it first determines whether it has the destination of the VPLS packet in the appropriate routing table. If it does, it forwards the packet to the appropriate PE router or CE device. If it does not, it broadcasts the packet to all other PE routers and CE devices that are members of that VPLS routing instance. In both cases, the CE device receiving the packet must be different from the one sending the packet.

When a PE router receives a packet from another PE router, it first determines whether it has the destination of the VPLS packet in the appropriate routing table. If it does, the PE router either forwards the packet or drops it depending on whether the destination is a local or remote CE device:

• If the destination is a local CE device, the PE router forwards the packet to it.
• If the destination is a remote CE device (connected to another PE router), the PE router discards the packet.

If the PE router cannot determine the destination of the VPLS packet, it floods the packet to all attached CE devices.

Figure 83 illustrates this process.

A VPLS can be directly connected to an Ethernet switch. Layer 2 information gathered by an Ethernet switch, for example, MAC addresses and interface ports, is included in the VPLS routing instance table.

An MPLS label-switched interface (LSI) label is used as the inner label for VPLS. This label maps to a VPLS routing instance. On the PE router, the LSI label is stripped and then mapped to a logical LSI interface. The Layer 2 Ethernet frame is then forwarded using the LSI interface to the correct VPLS routing instance.

One restriction on flooding behavior in VPLS is that traffic received from remote PE routers is never forwarded to other PE routers. This restriction helps prevent loops in the core network. However, if a CE Ethernet switch has two or more connections to the same PE router, you must enable the Spanning Tree Protocol (STP) on the CE switch to prevent loops.

JUNOS Software allows standard Bridge Protocol Data Unit (BPDU) frames to pass through emulated Layer 2 connections, such as those configured with Layer 2 VPNs, Layer 2 circuits, and VPLS routing instances. However, CE Ethernet switches that generate proprietary BPDU frames might not be able to run STP across Juniper Networks routing platforms configured for these emulated Layer 2 connections.

Note: Under certain circumstances, VPLS PE routers might duplicate an Internet Control Message Protocol (ICMP) reply from a CE device when a PE router has to flood an ICMP request because the destination MAC address has not yet been learned. The duplicate ICMP reply can be triggered when a CE device with promiscuous mode enabled is connected to a PE router. The PE router automatically floods the promiscuous mode enabled CE device, which then returns the ICMP request to the VPLS PE routers. The VPLS PE routers consider the ICMP request to be new and flood the request again, creating a duplicate ping reply.

Related Topics

• Understanding VPLS Routing Instances
• Understanding VPLS Interfaces
• VPLS Exceptions on J Series and SRX Series devices