Physical Interfaces & Cabling

Lesson 2 gave you a checking order for faults and told you to start at Layer 1. This is that layer. A large share of real network outages are physical — a cable in the wrong port, a port that was never switched on, an optic that does not match the fiber. They are also the fastest faults to confirm or rule out, which is exactly why they come first.

Copper cabling in an office is almost always UTP — unshielded twisted pair — terminated in the familiar RJ-45 plug. It is cheap and easy to work with, and it has two limits worth memorising: a single run tops out at 100 metres, and because it carries electricity it is vulnerable to electromagnetic interference, the electrical noise thrown off by motors, fluorescent lighting, and power cabling.

Fiber sends pulses of light down a glass core instead. Light does not care about electrical noise, and it travels much further before it degrades. Two kinds matter for CCNA. Multimode fiber has a wider core, is cheaper to drive, and covers hundreds of metres — the usual choice inside a campus or data centre. Single-mode fiber has a very narrow core, costs more, and reaches many kilometres — the choice for long building-to-building runs and WAN links.

A straight-through cable connects pin 1 at one end to pin 1 at the other. A crossover cable deliberately swaps the transmit and receive pairs. The classic rule is about what sits at each end: unlike devices take a straight-through, like devices take a crossover.

Every other connection in this lesson carries network traffic. The console port does not. It is a dedicated management port that you reach with a rollover cable or a USB cable, and it works even when the device has no IP address, no working configuration, and no network at all. That property is what out-of-band means: the management path does not depend on the thing you are managing.

The default console settings are worth committing to memory, because you will be asked for them: 9600 baud, 8 data bits, no parity, 1 stop bit — written 9600 8-N-1 — with no flow control. This is how you talk to a switch fresh out of its box.

When you want to know whether an interface is working, one command answers it. The output shows two separate status columns, and they mean different things: the first is the physical link, the second is the protocol running over it. Layer 1, then Layer 2.

Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/1     10.1.1.1        YES manual up                    up
GigabitEthernet0/2     unassigned      YES unset  administratively down down
GigabitEthernet0/3     10.1.2.1        YES manual up                    down

Duplex describes whether an interface can send and receive at the same time. Full duplex can; half duplex must take turns, which is why the old shared-Ethernet world had collisions at all. Modern switched links run full duplex, and both ends have to agree — if one end is full and the other is half, the link still comes up, traffic still passes, and the connection is quietly broken.

The symptom is a slow link rather than a dead one, along with rising late collisions. A late collision is one detected after the point where a collision should ever be possible, which is the fingerprint of exactly this mismatch: the full-duplex side sends whenever it likes, and the half-duplex side sees that as a collision arriving too late to be normal. The fix is to make both ends match — either set speed and duplex explicitly at both ends, or let both autonegotiate.

• UTP copper: RJ-45, 100 m maximum, vulnerable to electrical interference.
• Fiber: multimode for campus distances, single-mode for kilometres; both immune to electrical noise.
• Straight-through for unlike devices, crossover for like devices — Auto-MDIX usually hides the difference.
• Console access is out-of-band: rollover or USB, 9600 8-N-1, works with no network at all.
• Status pairs: up/up healthy · administratively down needs no shutdown · down/down is Layer 1 · up/down is Layer 2.
• A duplex mismatch leaves the link up but slow, with late collisions. Match both ends.