Layer two Forwarding

Layer two forwarding is often abbreviated as L2F in the networking world. This layer two forwarding protocol was exclusively made and introduced to the world by the Cisco Systems. It is basically a tunneling protocol that works at the layer 2 of the OSI model. It was developed to establish a VPN or virtual private network connections over the internet for the clients and the other devices that accessed the internet. The biggest drawback of this technology is that it fails to provide any privacy or encryption for the data that is to be forwarded or exchanged. This job of privacy and encryption is totally left up to the protocol that is being tunneled through by this protocol.

This protocol was designed and made by the Cisco to provide a way to tunnel the PPP or point to point protocol over the layer 2, which is also considered as the data layer as all the data pass through here, before going on to another network or going to another part of the same network. One thing should be remembered, that this Layer two forwarding protocol is quite similar to the PPTP protocol that was developed by the Microsoft; however, these two technologies shouldn’t be mixed together.

The data packet structure of the layer two forwarding protocol is quite complex. The total header consists of 31 bits.  The 0-12 bits are responsible for holding a specific type of code. The bits 13 – 15 hold the version. And the bits 16-23 hold the protocol, the bits 24-31 holds a sequence which is optional. The Multiplex ID is present between the bits ranging from 0 to 15, while the client ID is held in the rest of the remaining bits. The length also occupies 15 bits, and the rest of the bits are consumed by the payload offset. And then there is packet key and payload (non-optional, that is, compulsory) and they take up 31 bits of space.