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 PPPPPP 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.
The major features and benefits of PPP follow:
HDLCHDLC 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.
Frame RelayFrame 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.
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.
Additional EncapsulationsYou 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. |