Back-to-Back Router Connections


In a lab setup or in certain circumstances, you might find that you need to connect 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 MAU, and the Public Switched Telephone Network (PSTN) all perform an active role to interconnect routers. If these are unavailable, 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 an MAU or a Token Ring switch to terminate the connection.


Asynchronous Serial Connections

An 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, a rollover cable must be used. 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 B-3 shows the pinout connections for both ends of the rollover cable.

Table B-3. RJ-45 Connector Pinouts for Rollover Cables
RJ-45 Pin End A Description End A Description End B RJ-45 Pin End B
1 RTS CTS 8
2 DTR DSR 7
3 TxD RxD 6
4 GND GND 5
5 GND GND 4
6 RxD TxD 3
7 DSR DTR 2
8 CTS RTS 1

Ethernet Connections

Normally, a 10BaseT or a 10/100BaseTX router interface connects to a hub or 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. However, to connect two Ethernet router interfaces directly, without a hub or switch, a crossover cable is needed.

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 B-4 lists the pinout connections for both RJ-45 ends of the crossover cable.

Table B-4. RJ-45 Connector Pinouts for Crossover Cables
RJ-45 Pin End A Description End A Description End B RJ-45 Pin End B
1 TX+ RX+ 3
2 TX- RX- 6
3 RX+ TX+ 1
4     4
5     5
6 RX- TX- 2
7     7
8     8

56/64kbps CSU/DSU Connections

Normally, if a router has an integrated or external CSU/DSU for a 56/64kbps 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 DTE serial cable and one DCE serial cable. One router becomes the DTE end, and the other becomes the DCE end and must provide the clock. However, if you have two routers with integrated CSU/DSUs, 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 B-5 shows the pinout connections for both RJ-48 (an RJ-45 will do) ends of the cable.

Table B-5. RJ-48 Connector Pinouts for Back-to-Back 56/64kbps CSU/DSU Connections
RJ-48 Pin End A Description End A Description End B RJ-48 Pin End B
1 TX Ring RX Ring 7
2 TX Tip RX Tip 8
3     3
4     4
5     5
6     6
7 RX Tip TX Tip 1
8 RX Ring TX Ring 2

T1/E1 CSU/DSU Connections

A back-to-back connection can also be made 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 B-6 lists the pinout connections of both RJ-48 (an RJ-45 will do) ends of the cable.

Table B-6. RJ-48 Connector Pinouts for Back-to-Back T1/E1 CSU/DSU Connections
RJ-48 Pin End A Description End A Description End B RJ-48 Pin End B
1 RX (input) TX (input) 4
2 RX (input) TX (input) 5
3     3
4 TX (output) RX (output) 1
5 TX (output) RX (output) 2
6     6
7     7
8     8


Cisco Field Manual[c] Router Configuration
Cisco Field Manual[c] Router Configuration
ISBN: 1587050242
EAN: N/A
Year: 2005
Pages: 185

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