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					Serial WAN Link
Author: James Saunders Date: Wed, 8 Jun 2005 01:31:00 BST For the Cisco CCNA (Cisco Certified Network Associate) exam you are expected to know about WAN (Wide Area Network) links, the protocols used and how to configure a serial WAN link on a router. This tutorial will discuss these topics and end with a simple WAN lab which can be setup with minimal equipment and cost. WAN Connection through an ISP A serial WAN link is a dedicated Point to Point link, mainly used to connect two sites through a telco ISP. A serial link would appear to the routers at each end to be always on. The way in which the data gets between the two routers may be very different depending on the protocol and encapsulation methods (e.g. it may be Packet-switched or Leased Line).

As can be seen in the diagram above in a serial WAN setup there are a few bits of equipment with different terminologies; DCE and DTE, these terms relate to where in the end to end setup the equipment is. A DTE (Data Terminating Equipment) is the start and end points in the serial setup, this is the Cisco routers at each end. A DCE (Data Communications Equipment) is the equipment which the router connects to, usually provided by the ISP, and provides the main link out of an office to the ISP (in the example above, BT, these are equipment also known as CSU/DSU's or Modem's). A DTE can only talk to a DCE, but a DCE can talk to other DCE's. The speed at which a serial WAN link runs is determined by the 'clock rate', this rate governs the number of bits that can be transferred per second, for example, a clock rate of 64000 would give an overall serial link bandwidth of 64k. This clock rate can not be set at the DTE's (routers) at either end but is controlled by the telco ISP via the DCE's. If you require more bandwidth you must contact your ISP and request the clock rate to be increased and pay more money for the privilege! There is NOTHING you can add to the Cisco router configuration to increase the bandwidth above the clock rate!

Cables There are different types of cables used when setting up a serial WAN link. As discussed previously, the DTE (router) can only talk to a DCE, a cable called a DTE/DCE cable should be used for such a link. There are a variety of different flavor of plug that can be used at each end of a serial DTE/DCE cable but they all conform to the same standard which governs the Physical (OSI Layer 1) details, electrical signaling, voltages etc. A X.21 cable can be seen in the diagram above linked to s0/0 of the routers at each end. The following image shows some of the different types of serial cable connectors:

There are different ways in which a serial cable can be wired up, depending on which pins are connected to which and the gender of plug each end could either be a DTE or DCE connection, this is very important when setting up a serial link, if the wrong plug is connected at one end or the other you will not get a link, because transmit pins at one end will be sending data to transmit pins at the other. When you connect a serial cable to a Cisco router you can use the "show controller s 0/0" command which will tell you the hardware (OSI Layer 1) specifications including what type of cable you have plugged into the router, including the plug type.
router1#sh cont s 0/0 HD unit 0, ibd = 0xCE160, driver structure at 0xD2EE0 buffer size 1524 HD unit 0, X.21 DTE cable

The router can detect the cable type because particular pins are joined, this command will work even if the cable is not plugged in at the other end.

WAN Connection without an ISP (The cheap way to do a lab!) When studying for a CCNA you are hardly wanting to pay for a leased line from your ISP, or purchase expensive DSU/CSU's. You can connect two routers using a DTE serial cable and a DCE serial cable connected back-to-back. One additional configuration command must be entered onto one of the routers serial interfaces to enable it it to provide a clock-rate for the link, simulating a DCE.

The first thing to do is to ascertain which is the DTE and which is the DCE interface. This can be done by physically looking at the cable connector type. If you are using X21 then the female connector is the DCE end. The other (and CCNA exam preferred!) way of achieving this is to issue the "show controller s 0/0" command as discussed earlier. Once you have found the DCE end you next need to add the clock rate to this interface. DCE devices provide the clock rate and because you are emulating this you have to manually get the router to provide the clocking source. All you need to do is use the "clock rate" interface configuration command, which tells the router to supply clocking (synchronization) on the link which in normal conditions the CSU/DSU would provide. The following configuration commands are needed to set the clock-rate on Router 2 in the example diagram above.
router2#config t router2(config)int s0/0 router2(config-if)encapsulation hdlc router2(config-if)clock rate 64000 router2(config-if)bandwidth 64 router2(config-if)no shutdown

The commands above would set the clock rate to roughly 64K. You now need to setup Router 1 with the equivalent commands (note there is no need for the clock rate command on the DTE router, if you attempt to enter it it will simply display an error and not let you set it):
router1#config t router1(config)int s0/0 router1(config-if)encapsulation hdlc router1(config-if)bandwidth 64 router1(config-if)no shutdown

All that is needed now is to configure the Network Layer IP Address's such as 172.16.1.1 for the DCE and 172.16.1.2 for the DTE and check the status of a serial link with the "sh int s0/0" command...
router1#sh int s 0/0 Serial0 is up, line protocol is up ... Encapsulation HDLC, loopback not set, keepalive set (10 sec) ...

and maybe ping each other to check all is well. WAN Protocols There are a number of different WAN protocols which can run on a Cisco Serial interface, each protocol has different features, advantages and disadvantages: Frame Relay Packet-switched service, Allows bursing past CIR, giving the perception of "free capacity. Is pervasive in the United States. Packet-switched service, As a WAN technology, is not as pervasively available X.25 as Frame Relay. Has an attractive built-in Quality of Service feature. Packet-switched service, Asynchronous Transfer Mode, Is available pervasively ATM in some parts if the world. Offers beneficial error-recovery features when links have higher error rates. Leased Line service, High Level Data Link Control, Default encapsulation on HDLC Cisco Serial interfaces. Requires a Cisco router at each end. Leased Line service, Link Access Procedure Balanced, Provides error recovery, LAPB but this can result in throttling (slowing) the data rate. Leased Line service, Synchronous Data Link Control, IBM developed the protocol in the mid-1970s for use in Systems Network Architecture (SNA) SDLC environments. SDLC was the first link layer protocol based on synchronous, bitoriented operation. Leased Line service, Can improve the speed of routing protocol convergence. PPP Allows multi vendor interoperability. Has an error-recovery option and authentication. The protocols above are also known as "encapsulation" protocols. When creating a serial WAN link between two routers, the encapsulation protocol set at both ends must match! The default WAN protocol which Cisco routers use on their Serial interfaces is HDLC. HDLC is a Cisco proprietary protocol and will only work if you have Cisco routers at both ends, no other manufacturers use HDLC. To change the WAN protocol used on the serial links you must use the "encapsulation" interface configuration command. Any of the various WAN commands above can be used over a serial WAN link. For example to set the WAN protocol to PPP you would perform the following configuration changes:

router1#config t router1(config)int s0/0 router1(config-if)encapsulation ppp router1(config-if)bandwidth 64 router1(config-if)no shutdown router1(config-if)ip address 172.16.1.1 router2#config t router2(config)int s0/0 router2(config-if)encapsulation ppp router2(config-if)clock rate 64000 router2(config-if)bandwidth 64 router2(config-if)no shutdown router2(config-if)ip address 172.16.1.2

and once again check the status with the "sh int s0/0" command...
router1#sh int s 0/0 Serial0 is up, line protocol is up ... Encapsulation PPP, loopback not set, keepalive set (10 sec) ...


				
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