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Layer 2 Again: VFIs
VFI is a service-specific partition on a switch that associates attachment circuits in the form of VLANs with virtual switched interfaces (VSIs).

If that did not make much sense, it is useful to have some background on the service itself, namely Virtual Private LAN Services (VPLS), to understand VFIs.

VPLS is a Layer 2 LAN service offered by service providers (SPs) to connect Ethernet devices over a WAN. The customer devices (call them customer edges [CDs] for now; we review this in more detail in Chapter 5, "Infrastructure Segmentation Architectures") are all Ethernet switches. However the SP uses a Layer 3 network running Multiprotocol Label Switching (MPLS) to provide this service. The device on the edge of the SP network is called a provider edge (PE). Its role is to map Ethernet traffic from the customer LAN to MPLS tunnel s that connect to all the other PEs that are part of the same service instance. The PEs are connected with a full mesh of tunnels and behave as a logical switch, called a VSI. Another way to think about this is to see the VPLS service as a collection of Ethernet ports connected across a WAN. A VSI is a set of ports that forms a single broadcast domain.

In many ways, a VSI behaves just as you would expect a regular switch to. When a PE receives an Ethernet frame from a customer device, it first learns the source address, as would any switch, before looking at the destination MAC address and forwarding the frame. If the port mapping for the destination MAC address is unknown, or is a broadcast, the frame is sent to all PEs that are part of the VSI. The PEs use split horizon to avoid creating loops, which in turn means that no spanning tree is needed across the SP network.

Obviously, the previous explanation hides a fair amount of detail, but it should be enough to give a high-level view of what is going on.

Once again, there is a need to define and manage groups of isolated ports and tunnels on a switch. The VLAN construct is too limited, and a VRF is strictly a Layer 3 affair, so it is necessary to come up with a new virtual device structure for VPLS, called a VFI.

The VFI lists addresses of all the PEs that form a VSI. Recall that VPLS uses a full mesh of point-to-point tunnels for inter-PE connectivity, so there will be connections to each PE listed. The customer-facing ports map VLANs to a VFI name. Example 6 shows a short configuration extract that will make this clearer. Figure 3 shows the corresponding network topology. The thick line represents the VLAN that runs across the MPLS backbone and connects the VSIs on the PE devices. The CE switches "think" thy re connected by a 802.1q trunk on VLAN100. The thin lines between each PE are the actual pseudowires defined in the 12 vfi statement of Example 6.

Note
A pseudowire is a tunnel. The term is often used in the context of a Layer 2 service.

Figure3. VPLS Topology

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