Tunneling

Encapsulating one protocol inside another (typically IP, but tunneling is used in other protocols as well).  

Tunneling can best be described with an anology.  Suppose you want to send a message across the U.S. from LA to DC: To do this you would do the following:

1. place the message inside an envelope (encapsulate it) 

2. send it across the U.S. within the envelope (tunneled)

3. it arrives in DC and the receiver removes the message from the envelope (decapsulates it)

Passenger protocol – the Layer 3 packets that are the protocol being tunneled

Encapsulation Protocol –  a field that identifies the passenger protocol

Transport Protocol – the protocol used to send the packets across the tunnel – IP typically

To configure, you must issue commands at each end of the tunnel.  For an example, at router A:

config t

int tunnel 0

tunnel source ethernet 0

tunnel destination 10.1.5.4

ipx network 3               (the passenger protocol)