When connecting a PC's serial COM port to the console port of a Cisco router or switch using an RJ45 UTP cable, which type of cable should you use?

Introduction / Context:
Accessing the console port of a Cisco router or switch is a fundamental task for initial configuration and troubleshooting. The console connection typically uses a special RJ45 UTP cable that is wired differently from standard Ethernet patch cables. Knowing which cable to select is a common exam question.

Given Data / Assumptions:

• We are connecting a PC's serial COM port (often via an RJ45 to serial adapter) to the console port on a router or switch.
• The physical connector on the network device is RJ45, but the signaling is serial, not Ethernet.
• We have several cable types to choose from: straight-through, crossover, and rolled.

Concept / Approach:
A straight-through cable is used to connect devices of different types at Ethernet Layer 2, such as a switch to a router or a switch to a host. A crossover cable historically connected similar Ethernet devices, such as switch to switch or host to host. A rolled, or rollover, cable has one end wired as the reverse of the other and is specifically designed for console connections, not for standard Ethernet data traffic.

Step-by-Step Solution:
Step 1: Recognize that the console interface is a management port that uses serial signaling and does not act as a normal Ethernet port.Step 2: Cisco traditionally supplies a light blue RJ45 rollover cable that is used to connect the device console port to a PC via a serial adapter.Step 3: The rollover cable has pin 1 on one end wired to pin 8 on the other end, pin 2 to pin 7, and so on, effectively reversing the pinout.Step 4: Straight-through or crossover Ethernet cables would not provide the correct pin mapping for the console serial connection.

Verification / Alternative check:
In practice, when you purchase a Cisco router or switch, it usually comes with a blue rollover console cable. Cisco documentation and labs explicitly refer to this as a rollover cable for console access, confirming that this is the correct choice.

Why Other Options Are Wrong:
Option A is used for normal Ethernet data connections between dissimilar devices, not for console connectivity.Option B is historically used for Ethernet connections between similar devices, not for serial console connectivity.Option C introduces a CSU or DSU, which relates to WAN connections, not to local console cables.

Common Pitfalls:
Because all of these cables may have RJ45 connectors, it is easy to confuse them. Remember that console ports use a special rollover cable, typically color coded, and that Ethernet patch cables, whether straight-through or crossover, are reserved for data ports.

Final Answer:
You should use a Rolled (rollover) console cable to connect a PC COM port to a router or switch console port.