- Point-to-Point Protocol (PPP): a specific dedicated path through the TSP network that connects two LANs over a large geographic area. Typically leased lines.
- Circuit-switched: allows the client to create and close connections over the TSP network. Think of the operation as phone call. Examples are ISDN or dialup network access.
- Packet-switched: a client uses a software-managed virtual circuit over a shared connection. Examples are Frame Relay.
The Cisco default encapsulation for a serial interface is High-Level Data Link Control (HDLC). This is a Data Link layer protocol used to encapsulate and transmit packets over point-to-point links. It handles the transfer of data in full duplex, as well as link management functions. As an OSI standard, many vendors implement the HDLC protocol in their equipment. These are usually not interoperable. This is because the Cisco HDLC frame uses a proprietary "Type" field that may not be compatible with equipment of other vendors. When the HDLC frame format was defined, it did not enclude a field to identify the Network layer protocol that it was framing. As such, the OSI version of HDLC assumes that any link using HDLC is running only a single Network layer protocol like IP. This has led vendors to implement HDLC using a proprietary frame format that includes a type code field, thus allowing the Network layer protocol within a frame to be identified. Because of this proprietary nature, you should only use HDLC framing on point-to-point links when the routers at each end of a link is from the same vendor. In cases where you want to connect equipment from different vendors over a leased line, the Point-to-Point protocol (or PPP) should be used. Always remember that the router on both sides of a ppp link must be using the same data framing method in order to communicate. You can change the encapsulation to PPP as a more flexible, nonpropietary encapsulation. PPP supports authentication in clear-text Password Authentication Protocol (PAP) or encrypted Challenge Handshake Authentication Protocol (CHAP). A router can also use Frame Relay as an encapsulation. Frame Relay virtual circuits use HDLC encapsulation and each circuit is identified by a data link connection identifier (DLCI).
You can use the encapsulation command in interface configuration mode. An example is encapsulation ppp.
HDLC and PPP are layer 2 protocols. HDLC is actually the default protocol on all Cisco serial interfaces. If you do a show run on a Cisco router, your serial interfaces will have no encapsulation by default (it's HDLC)! Use a show interface serial ... to see that you're running HDLC.
You can select one of the following address types for a serial WAN connection:
- A static IP address allows the administrator to manually enter the IP address and subnet mask. Available for PPP, Frame Relay and HDLC.
- IP unnumbered sets the interface to match the IP address of another enabled interface on the router. Available for PPP, Frame Relay and HDLC.
- IP negotiated is available for PPP and allows the router to obtain address information automatically from the ISP.