Before VPLS, the only way you could connect Ethernet LAN sites together was to set up a non-VPLS Layer 2 VPN or install multiple Layer 2 Ethernet switches. Figure 70 shows how three switches can be connected to each other.
Figure 70: Ethernet Switching Example
A typical switch builds its Layer 2 switching table with MAC address and interface information learned from traffic received from other switches. If a switch does not know how to reach a particular destination, it floods traffic for that destination to all ports except the one where the traffic originated. When information about a previously unknown destination is received, this information is added to the switching table. If a destination is known, the switch sends the traffic directly to the intended recipient through the associated port listed in the switching table.
Figure 71 shows a VPLS network comparable to the switch example and explains how VPLS functions similarly to Ethernet switches (assuming a Spanning Tree Protocol is configured).
Figure 71: VPLS Introductory Example
Notice that Layer 2 information gathered by a switch (for example, MAC addresses and interface ports) is included in the VPLS instance table. However, instead of requiring all VPLS interfaces to be physical switch ports, the router allows remote traffic for a VPLS instance to be delivered across an MPLS label-switched path (LSP) and arrive on a virtual port. The virtual port emulates a local, physical port. Traffic can be learned, forwarded, or flooded to the virtual port almost identically to the way traffic is sent to a local port.
The VPLS table learns MAC address and interface information for both physical and virtual ports. If no activity is seen for a particular MAC address, it is purged from the table over time.
As shown in Figure 71, the main difference between a physical port and a virtual port is that the router captures additional information from a virtual port—an outgoing MPLS label used to reach the remote site and an incoming MPLS label for VPLS traffic received from the remote site.
When you configure VPLS on a routing platform, a virtual port is generated as a logical interface on a virtual loopback tunnel (vt) interface or a label-switched interface (LSI). On M-series and T-series routers, virtual ports are created dynamically on vt interfaces if you install a Physical Interface Card (PIC) that supports virtual tunnels. With VPLS, you must install at least one Tunnel Services, Link Services, or Adaptive Services PIC in each VPLS provider edge (PE) router. On MX-series routers, virtual ports are created dynamically on vt interfaces if you configure tunnel services on one of the four Packet Forwarding Engines (PFEs) included in each DPC. If your routing platform does not offer tunnel services through a PIC or PFE, you can configure VPLS to create virtual ports on LSI logical interfaces.
One property of 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. If a customer edge (CE) Ethernet switch has two connections or more to the same PE router, you must enable the Spanning Tree Protocol to prevent loops. For more information on configuring the Spanning Tree Protocol, see the JUNOS Routing Protocols Configuration Guide.
The paths carrying VPLS traffic between each PE router participating in a routing instance are called pseudowires. The pseudowires are signaled using either BGP or LDP.
 | Copyright © 2009, Juniper Networks, Inc. All rights reserved. Trademark Notice. | |