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Each WAN connection uses an encapsulation protocol to encapsulate traffic while it is crossing the WAN link. To ensure that you use the correct encapsulation protocol, you must configure the Layer 2 encapsulation type to use. The choice of encapsulation protocol depends on the WAN technology and the communicating equipment. Typical WAN protocols include:
■ PPP: PPP originally emerged as an encapsulation protocol for transporting IP traffic over point-to-point links. PPP also established a standard for the assignment and management of IP addresses, asynchronous (start/stop) and bit-oriented synchronous encapsulation, network protocol multiplexing, link configuration, link quality testing, and error detection. In addition, PPP established option negotiation for such capabilities as network-layer address negotiation and data-compression negotiation. PPP supports these functions by providing an extensible link control protocol (LCP) and a family of Network Control Protocols (NCPs) to negotiate optional configuration parameters and facilities. The broadband connection type that is used will determine the use of Point-to-Point Protocol over Ethernet (PPPoE) or Point-to-Point Protocol over ATM (PPPoA).
■ High-Level Data Link Control (HDLC): HDLC is the default encapsulation type for Cisco routers on point-to-point dedicated links. It is a bit-oriented synchronous data-link layer protocol. HDLC specifies a data encapsulation method on synchronous serial links using frame characters and checksums. HDLC is a standard that is open for interpretation. As a result, there are different versions of HDLC. If you are communicating with a device from another vendor, synchronous PPP is a more viable option.
Frame Relay: Frame Relay is a high-performance packet-switched WAN protocol that operates at the physical and data-link layers of the OSI reference model. Frame Relay was originally designed for use across ISDN interfaces. Today, it is used over a variety of other network interfaces and typically operates over WAN facilities that offer more reliable connection services and a higher degree of reliability.
■ ATM: ATM is the international standard for cell relay in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media such as E3, SONET, and T3.
This topic describes PPP encapsulation.
PPP is an international standard encapsulation that is used for these types of connections: Asynchronous serial
■ ISDN Synchronous serial Broadband
PPP (RFC 1331) provides a standard method of encapsulating higher-layer protocols across point-to-point connections. PPP extends the HDLC packet structure with a 16-bit protocol identifier that contains information on the content of the packet.
Because it is standardized, PPP supports vendor interoperability. PPP uses its NCP component to encapsulate multiple protocols.
PPP uses another of its major components, the LCP, to negotiate and set up control options on the WAN data link. Some of the PPP LCP features covered in this course are:
■ Authentication Compression Multilink
PPPoE provides the ability to connect a network of hosts to an access concentrator over a simple bridging access device. With this model, a host uses its own PPP stack, and the user is presented with a familiar user interface. Access control, billing, and type of service can be done on a per-user, rather than a per-site, basis.
PPPoA was primarily implemented as part of asymmetric DSL (ADSL) technology. It relies on RFC 1483 (now RFC 2686), operating in either logical link control/Subnetwork Access Protocol (LLC/SNAP) or virtual circuit multiplexing (VC mux) mode. Customer premises equipment (CPE) will encapsulate a PPP session based on this RFC for transport across the ADSL loop and the digital subscriber line access multiplexer (DSLAM).
In these architectures, IP address allocation is based on IP Control Protocol (IPCP) negotiation, which follows the same principle as PPP in dial mode.
In PPPoE, the source of IP address allocation depends on the type of service to which the subscriber has subscribed and where the PPP sessions are terminated. PPPoE makes use of the dial-up networking feature of Microsoft Windows, and the IP address assigned is reflected within the PPP adapter. PPPoE can be used on existing CPE (that cannot be upgraded to PPP or that cannot run PPPoA), extending the PPP session over the bridged Ethernet LAN to the PC. PPPoE can also be configured on the CPE to terminate the PPP session and use Network Address Translation (NAT) for workstation access to the Internet.
Although PPPoA does not require host-based software, it does require that each CPE device have a username and password for authentication to a central site. The PPP sessions initiated by the subscriber are terminated at the service provider that authenticates users via a local database on the router or through a RADIUS server. The PPPoA session authentication is based on Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP). The service provider must assign only one IP address for the CPE, and the CPE can be configured for NAT.
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