WAN Encapsulation Protocols

You need to be familiar with three main WAN encapsulation protocols. Each protocol usually accompanies a certain connection type, although it is certainly not a 1:1 relationship. The main encapsulation protocols you need to know are Point-to-Point Protocol (PPP), High-Level Data Link Control (HDLC), and Frame Relay. Additional encapsulations you might encounter are Asynchronous Transfer Mode (ATM), X.25, and Serial Line Internet Protocol (SLIP). To configure an encapsulation on an interface, issue the following command in interface configuration mode:

 encapsulation {ppp,hdlc,frame-relay} Router(config-if)# encapsulation ppp 

PPP

PPP is most commonly associated with dial-up or ISDN connections to the Internet. Although PPP is well suited for this task, it is certainly not limited to dial-up. Many of the benefits of PPP do, however, have specific relevance to dial-up, as mentioned in the following list. You can use PPP over dedicated connections, circuit-switched connections, and, in some cases, packet-switched connections.

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Packet-switched connections typically don't use PPP because there are more robust protocols available for that type of connection.


The major features and benefits of PPP follow:

  • Multilink PPP offers the ability to aggregate multiple connections together for more bandwidth. Multilink is sometimes referred to as MP for multilink protocol.

  • Compression PPP allows for packets to be compressed before transmission. PPP supports all hardware and software compression standards and uses the Compression Control Protocol to manage compression on both the sending and receiving ends.

  • Callback PPP supports both dynamic and fixed callback. Callback allows the remote access server to call back the initiating party. You can use callback as a security mechanism by limiting where a user can request callback to. Callback can also be a means of controlling the cost of remote connections by centralizing billing.

  • Encrypted authentication Instead of sending the username and password in cleartext, PPP supports a number of methods that protect usernames and passwords during authentication.

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PPP encapsulation is only the beginning; you can expect to see ISDN and other configuration questions on encryption, callback, and multilink using PPP.


HDLC

HDLC is typically used only on dedicated connections. It is the default encapsulation on Cisco router serial interfaces. This version of HDLC, however, is not an industry standard. The two types of HDLC are Cisco HDLC and IEEE HDLC, and they are not compatible. The HDLC that a Cisco router speaks is the Cisco proprietary version.

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If you are not communicating with a Cisco device at the other end, it is suggested that you use PPP as the encapsulation for the connection.


Frame Relay

Frame Relay is more or less the standard for high-speed, nonconsumer networks and Internet connectivity. Frame Relay provides access rates up to T3 in a packet-switched environment. Frame Relay is typically used as an internetwork connection protocol over reliable WAN connections. Frame Relay has lower overhead than other encapsulations because it leaves the error checking and correction to the higher-layer protocols.

As with just about any encapsulation, you can have more than one type on a router, but only one type per interface. The "addressing" of a Frame Relay interface is called the data-link circuit identifier (DLCI). The DLCI is locally significant, meaning that you are the only one who cares what your end of the connection is called. Most service providers instruct you to use DLCI 16 on your side of the Frame Relay connection. As long as you only have one Frame Relay connection, you can use DLCI 16, but if you have more than one connection, you have to use unique numbers for each DLCI.

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Think of a DLCI as a doorway out of the router, and each doorway must have a unique name to go with its unique destination.


With Frame Relay, you should be aware of your committed information rate (CIR). The CIR is what determines the guaranteed speed of your connection to the network. The CIR becomes important when working with central sites and branch offices. Typically, a central site has substantially more bandwidth than a branch or home office does, and CIR becomes an important variable when setting up and tuning those connections.

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Frame Relay is tested very heavily on this exam because it is the most widely implemented packet-switched network technology. Be comfortable with the commands to configure, debug, and troubleshoot Frame Relay as well as the terminology.


Additional Encapsulations

You should also know that Cisco routers support SLIP, X.25, and ATM as encapsulation methods. X.25 is falling out of favor because it is a slower technology. It has substantially more overhead than Frame Relay because it has its own error checking and correction. SLIP can only use IP and has been replaced by PPP. ATM is still a viable choice but its full use is beyond the scope of this book.



CCNP BCRAN Remote Access Exam Cram 2 (Exam Cram 640 - XXX)
CCNP BCRAN Remote Access Exam Cram 2 (Exam Cram 640 - XXX)
ISBN: N/A
EAN: N/A
Year: 2003
Pages: 183

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