WAN Data Link Encapsulations
After you have the Physical layer plugged in, you must move up to the Data Link WAN encapsulation. Just as with the Physical layer, a variety of standards are available for the data link connections. However, the choice of the Data Link protocol is usually much simpler. As long as your WAN connection supports the Data Link encapsulation you use and you are using the
same type of encapsulation on both ends of the connection, the WAN link will work.
Sometimes, the type of WAN connection you are using forces you to choose one, specific Data Link encapsulation. For example, if you sign up with a service provider for a Frame Relay connection, you must use Frame Relay Data Link encapsulation. Likewise, if you sign up for an ATM connection, you must use ATM encapsulation. Other times, there may be some flexibility on the choice of protocol you can use. For example, if you sign up for a point-to-point T1 connection, you can use Cisco HDLC, SLIP, or PPP for your data link encapsulation. Here is a brief description of each of the encapsulation types.
Serial Line Internet Protocol (SLIP)
SLIP is a standards-based protocol for point-to-point serial connections that use only TCP/IP This was primarily used for dial-up connections to the Internet back in the earlier days of the Internet. It has been widely replaced by PPP.
Point-to-Point Protocol (PPP)
This protocol has largely replaced SLIP connections for point-to-point WAN connections and dial-up networking. PPP was released as an improvement to SLIP and added support for non-TCP/IP protocols and encrypted authentication (among many other features). PPP is the most popular protocol for connecting point-to-point WAN connections.
Cisco High-Level Data Link Control (HDLC)
HDLC was originally designed as an open standard protocol, meaning all routers could support it. However, the open standard version of HDLC was pretty horrible. It did not support multiple network-layer protocols, which meant that you could support only one protocol (such as TCP/IP, IPX/SPX, or AppleTalk) over your WAN connection. In view of this shortcoming,
Cisco modified the standard HDLC to support this missing feature. However, anytime a standard is modified, the protocol becomes proprietary. In this case, you can use HDLC only on Cisco routers to connect to other Cisco routers. HDLC is the default encapsulation on all serial interfaces on Cisco routers. Although HDLC does not have as many features as PPP, it does
offer very low overhead, which makes your WAN connections very efficient.
X.25 Link Access Procedure, Balanced (LAPB)
This encapsulation is used on X.25-based networks, which is the predecessor to Frame Relay. Although X.25 is used rarely in well-developed countries, it has widespread use in countries not as technologically advanced.
This encapsulation relates directly to the Frame Relay WAN connection, which is the faster successor to X.25. Frame Relay increased its speed capabilities by removing much of the error correction that is no longer needed on the more reliable circuits of today. Frame Relay has widespread use in nearly all well-developed areas.
Asynchronous Transfer Mode (ATM)
This technology is very similar to frame relay, but chops packets into very small pieces (53 bytes each) called cells. Because all the frames are exactly the same size, routers are able to process them much quicker. ATM also has the capability to run at very fast speeds because it adapts to run over fiber optic cabling.
PPP over Ethernet (PPPoE) and PPP over ATM (PPPoA)
These technologies have been implemented to allow service providers to harness the features of PPP on an ethernet or ATM connection. This technology is primarily used in DSL highspeed Internet deployments.
The ICND1 exam requires you to be familiar with the configuration of HDLC and PPP. The CCNA exam requires you to be familiar with the configuration of HDLC, PPP, and Frame Relay encapsulation types.
As mentioned previously, HDLC in its truest form is an industry standard created by the International Organization for Standardization (ISO). These are the same folks who created the OSI Model (bless their hearts). Because the ISO version of HDLC lacked the support for multiple protocol use, Cisco modified it and caused HDLC on Cisco routers to become a proprietary protocol. The beauty of HDLC is that it is very simple and works out of the box. Typically, if you are
deploying a WAN connection with a Cisco router on each side of the link, it eliminates plenty of troubleshooting involved in trying to enable the connection with HDLC, even if you plan on using PPP in the long run.
Because HDLC is so simple, there are no options to negotiate and you can rule it out of any troubleshooting you may encounter. If the link is not coming up, it is usually something on the service provider side of the business. Because HDLC is enabled by default, you don’t need to perform any additional configuration for the data link configuration of your serial interfaces. However, if the data link encapsulation was changed to something other than HDLC, you can re-enable HDLC by moving into interface configuration mode for the serial interface you want to use and type the command encapsulation hdlc.