VPLS Architecture

A VPLS service emulates a LAN or the functionality of an Ethernet switch. An Ethernet switch has the following characteristics:

■ Forwarding of Ethernet frames

■ Forwarding of unicast frames with an unknown destination MAC address

■ Replication of broadcast and multicast frames to more than one port

■ Loop prevention

■ Dynamic learning of MAC addresses

■ MAC address aging

VPLS should also have these characteristics. Ethernet frames receive two MPLS labels before they are forwarded across the MPLS backbone. This forwarding of Ethernet frames is the same as in Chapter 10, "Any Transport over MPLS." An imposed virtual circuit (VC) label always serves as a demultiplexing label and indicates the VC that the frame belongs to. The tunnel label is the top label that indicates how the frame is forwarded from the ingress PE to the egress PE router.

If the PE router receives a frame that has an unknown destination MAC address, the frame is replicated and forwarded to all ports that belong to that LAN segment. The LAN segment on an Ethernet switch might be a collection of ports belonging to the same VLAN. When configuring VPLS, you must specify which VPLS instance a particular port or VLAN belongs to. The frames with unknown destination MAC addresses are forwarded to all ports belonging to that VPLS instance. On a true Ethernet switch, the port would just be a physical interface. However, with VPLS, it might be a physical interface, but it could also be a pseudowire to another PE router. Look at Figure 11-2, which shows the PE routers involved with the VPLS instance named cust-one for one customer. The customer has several sites, all of which are connected to a PE router. The PE routers have pseudowires between them to carry the Ethernet frames. Each pseudowire consists of two label switched paths (LSPs), one for each direction.

Figure 11-2 VPLS Reference Model

Cust-one

Cust-one

AMS Metro Site

Cust-one

Pseudowires

VPLS

Cust-one

Cust-one

PAR Metro Site

PAR Metro Site

If the CE router or switch sends a broadcast frame to the PE router, the frame is replicated and forwarded to all physical ports on that PE router belonging to that VPLS instance, but also to all pseudowires associated with that VPLS instance. Multicast frames are replicated and forwarded to all physical ports that are part of the multicast group and to all pseudowires (the underlying WAN ports). When forwarding broadcast frames, it is important to flood the frame throughout the broadcast domain. If the PE routers are not fully meshed for one VPLS instance, a spanning tree protocol is required to keep the Layer 2 topology loop free. However, a simpler mechanism was chosen to keep the forwarding free of loops. The PE routers need to be in a full mesh of pseudowires, and the PE routers perform split-horizon in Layer 2 forwarding. Split-horizon here means that a flooded frame that is received on one pseudowire will never be forwarded to other pseudowires.

NOTE Split-horizon is on by default. You can turn it off if needed. See the section "Hierarchical VPLS" for such an example.

As with Ethernet switches, the PE routers of the VPLS network should perform MAC address learning and aging. This means that the PE routers will notice the source address of received frames and associate them with a physical port or pseudowire. Similar to an Ethernet switch, the MAC addresses are aged out after a certain period of not receiving a frame from that MAC address. The aging time is refreshed after receiving a frame.

VPLS Data Plane

In the data plane, the transported frames look the same as Ethernet frames in the AToM model. Two labels are imposed on the Ethernet frame. The top label or tunnel label identifies the tunnel (LSP) that the frame belongs to. In other words, it forwards the frame from the local or ingress PE to the remote or egress PE. The bottom label is the VC label, and it identifies the pseudowire. In other words, the egress PE looks at the VC label; it uses the VC label to determine to which attachment circuit (Ethernet port or VLAN interface) the frame should be forwarded onto. Figure 11-3 shows the Ethernet frame with two MPLS labels as it is transported across the MPLS network.

Figure 11-3 VPLS Data Plane

Cust-one

Figure 11-3 VPLS Data Plane

Cust-one

Ethernet Frame

Tunnel Label

VC Label

The transported frame is the Ethernet frame without an 802.1Q tag; this tag is stripped before the frame is forwarded into the MPLS network. The PE router builds a MAC table as any regular Ethernet switch. This MAC table forwards Ethernet frames to and from the physical Ethernet ports and to and from the pseudowires. In Cisco IOS, each customer who is connected to the MPLS backbone has a virtual forwarding instance (VFI). VFI is the collection of data structures that

Cisco IOS uses to forward Ethernet frames onto the Ethernet Attachment Circuits (the physical Ethernet ports) and the virtual circuits or pseudowires. Control plane and data plane information feed the VFI. The control plane information is the configuration on the PE router and the signaling protocol LDP that signals the pseudowires. It can populate the VFI with VC membership and VC label information. The data plane information is the data derived from frame forwarding, such as the MAC address learning information. Look at Figure 11-4 to see the VPLS PE router with physical Ethernet ports and pseudowires into the MPLS network.

Figure 11-4 VPLS PE Router

MPLS

Physical Ethernet Frames

MPLS

Physical Ethernet Frames

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