Chapter 2. Troubleshooting Layer Two Forwarding Protocol VPNs

The Layer Two Forwarding (L2F) Protocol is a Cisco proprietary protocol that facilitates the transparent forwarding of Point-to-Point Protocol (PPP) and Serial Line Internet Protocol (SLIP) across an IP backbone.

L2F, which is defined in RFC 2341, allows the separation of the functionality of the traditional Network Access Server (NAS), with call reception on a NAS, but termination of the PPP or SLIP connection on a device called a Home Gateway. The Home Gateway is geographically separated from the NAS. This means that an enterprise can outsource call reception to an Internet service provider (ISP) but still terminate PPP/SLIP connections within the corporate network. This allows the enterprise to save or minimize on call charges because remote access clients are no longer required to dial directly to the enterprise. They are instead able to dial in to the ISP's nearest Point-of-Presence (POP). Figure 2-1 illustrates this concept.

Figure 2-1. L2F Topology

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This logical separation of call reception and PPP connection termination presupposes a mechanism for tunneling the PPP frames across the backbone to the Home Gateway. This mechanism is provided by L2F.

Only compulsory tunneling is supported with L2F. This means that tunneling directly from the remote access client to the Home Gateway is not supported.

Note that L2F is also used to support the tunneling of multilink PPP (MP) links between NASs in a multichassis multilink PPP (MMP) environment.