In a lab setup or in certain circumstances, you might find that you need to connect two switches or two routers to each other in a back-to-back fashion. Normally, some other active device is used to connect router interfaces. For example, an Ethernet hub or switch, a Token Ring media attachment unit (MAU), and the Public Switched Telephone Network (PSTN) all perform an active role in interconnecting routers. If these are not available, as in a lab environment, a special cable is needed to make the back-to-back connection. NOTE It is not possible to make a back-to-back cable to connect two Token Ring interfaces. Token Ring connections require an active device such as a MAU or a Token Ring switch to terminate the connection. Ethernet ConnectionsNormally, a 10BASE-T or a 10/100BASE-TX host network interface card (NIC) connects to a switch through a straight-through Category 5 UTP cable. RJ-45 pins 1 and 2 form one pair, and pins 3 and 6 form another pair. To connect two Ethernet switch ports directly, however, you need a crossover cable. A crossover cable connects the pair containing pins 1 and 2 on one end to the pair containing pins 3 and 6 on the other end. Likewise, pins 3 and 6 are connected to pins 1 and 2. Table A-4 lists the pinout connections for both RJ-45 ends of the crossover cable.
Asynchronous Serial ConnectionsAn asynchronous serial connection, such as the Aux port or a line on an access server, requires an RJ-45 connection. For a back-to-back link between two async ports on two different routers, you must use a rollover cable. Rollover cables are usually flat eight-conductor cables with RJ-45 connectors, fashioned so that pin 1 on one end goes to pin 8 on the other end, pin 2 goes to pin 7, and so forth. Cisco normally supplies a rollover cable with a console cable kit. Table A-5 shows the pinout connections for both ends of the rollover cable.
56/64-kbps CSU/DSU ConnectionsNormally, if a router has an integrated or external CSU/DSU for a 56/64-kbps serial interface, the CSU/DSU is connected to the service provider's termination box. The service provider or PSTN establishes the active circuit between two CSU/DSU units and routers. Back-to-back serial connections can usually be made between the serial interfaces of two routers using one data termination equipment (DTE) serial cable and one data communication equipment (DCE) serial cable. One router becomes the DTE end, and the other becomes the DCE end and must provide the clock. If you have two routers with integrated CSU/DSUs, however, there is no way to access the physical serial interface. In this case, you can make a back-to-back 56/64 cable by crossing the transmit and receive pairs. Table A-6 shows the pinout connections of both RJ-48 (an RJ-45 will do) ends of the cable.
T1/E1 CSU/DSU ConnectionsYou can also make back-to-back connections between two routers with integrated T1/E1 CSU/DSUs using a specially made cable. Again, the transmit and receive pairs are crossed in the cable. Table A-7 lists the pinout connections of both RJ-48 (an RJ-45 will do) ends of the cable.
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