Day 9 Switch Interfaces

Switch vs Router Interface Behavior: Switch interfaces are enabled by default and do not have the shutdown command applied, while router interfaces are administratively disabled by default and require the "no shutdown" command to be enabled.

Full-Duplex vs Half-Duplex: Full-duplex allows a device to send and receive data simultaneously, while half-duplex requires the device to wait before sending if it is currently receiving data.

Autonegotiation: Interfaces automatically negotiate speed and duplex settings with neighboring devices to determine the optimal configuration both devices support.

Collision Domains: Hubs create a single collision domain where all connected devices can experience collisions, while switches create separate collision domains for each interface, enabling full-duplex operation.

Interface Status Monitoring: Network administrators can monitor interface health through various show commands that display status, errors, and counters.

Interface Default States

• Switch interfaces: NOT shutdown by default (up/up when connected, down/down when disconnected)

• Router interfaces: Shutdown by default (administratively down/down)

• Switch interfaces are Layer 2 switchports and don't require IP addresses

Speed and Duplex Settings

• Default: Both set to "auto" (autonegotiation enabled)

• FastEthernet: Supports 10/100 Mbps

• Gigabit Ethernet: Supports 10/100/1000 Mbps

• Manual configuration: Use speed {10|100|1000|auto} and duplex {auto|full|half} commands

Status Indicators

• show ip interface brief: Status (Layer 1) / Protocol (Layer 2)

  • up/up = working

  • down/down = not connected

  • administratively down/down = shutdown command applied

• show interfaces status: Shows "connected," "notconnect," or "disabled"

1. Confusing status outputs:

   • "down/down" ≠ "administratively down/down"

   • down/down = not connected

   • administratively down/down = shutdown command applied

2. Duplex mismatch symptoms:

   • Don't assume poor performance is always a speed issue

   • Duplex mismatches cause collisions even on switches

3. Autonegotiation failure:

   • If speed is 10 or 100 Mbps → switch uses half-duplex

   • If speed is 1000 Mbps or greater → switch uses full-duplex

   • This can create mismatches!

4. Interface range syntax:

   • Can use comma to separate non-consecutive ranges

   • Example: interface range f0/5-6, f0/9-12

5. Show commands confusion:

   • show ip interface brief - basic Layer 1/2 status, IP addresses

   • show interfaces status - descriptions, VLAN, duplex, speed

   • show interfaces [interface] - detailed statistics and error counters

"SWITCHES START, ROUTERS REST"

• Switches are enabled by default

• Routers require "no shutdown"

"Full = Free, Half = Hold"

• Full duplex = freely send/receive simultaneously

• Half duplex = hold and wait

"Small is RUNT, Big is GIANT"

• Runts < 64 bytes

• Giants > 1518 bytes

"Auto Speed Rules of Thumb"

• 10 or 100 = Half duplex (if auto fails)

• 1000+ = Full duplex (if auto fails)

"CSMA/CD = Can't Send, Must Avoid Collisions Daily"

• Used in half-duplex environments

"Hub = Half, Switch = Separate"

• Hubs force half-duplex (one collision domain)

• Switches enable full-duplex (separate collision domains)

SW1(config)# interface f0/1

SW1(config-if)# speed 100

SW1(config-if)# duplex full

SW1(config-if)# description Connected to R1

SW1(config)# interface range f0/5-12

SW1(config-if-range)# description Unused Ports

SW1(config-if-range)# shutdown

SW1(config)# interface range f0/5-6, f0/9-12

SW1(config-if-range)# no shutdown

SW1# show ip interface brief

SW1# show interfaces status

SW1# show interfaces f0/1

Switch interfaces are NOT shutdown by default (up/up or down/down). Router interfaces ARE shutdown by default (administratively down/down).

First "up" = Layer 1 status (physical). Second "up" = Layer 2 status (protocol). Both operational.

Full-duplex: Can send AND receive simultaneously. Half-duplex: Must wait; cannot send and receive at the same time.

Carrier Sense Multiple Access with Collision Detection. Used in half-duplex networks to detect and avoid collisions by listening before transmitting and sending jamming signals when collisions occur.

Minimum: 64 bytes. Maximum: 1518 bytes.

Runts: Frames smaller than 64 bytes. Giants: Frames larger than 1518 bytes. Both are errors.

interface range [range] Example: interface range f0/5-12 or interface range f0/5-6, f0/9-12

The switch will use 100 Mbps speed but will default to HALF-duplex (because speed is 10 or 100 Mbps).

The switch will use 1000 Mbps speed and will default to FULL-duplex (because speed is 1000 Mbps or greater).

When one side uses full-duplex and the other uses half-duplex (often due to failed autonegotiation or manual misconfiguration). Results in collisions and poor performance.

show interfaces [interface-id] Example: show interfaces f0/1

Connected (interface is up and connected), notconnect (interface is enabled but not connected), disabled (shutdown command applied).

A network segment where frames can collide. Hubs create ONE collision domain for all ports. Switches create SEPARATE collision domains for each port.

Hub: Forces half-duplex (Layer 1 repeater, all ports in one collision domain). Switch: Enables full-duplex (Layer 2 device, separate collision domains per port).

10 Mbps and 100 Mbps (denoted as 10/100BASE-TX)

10 Mbps, 100 Mbps, and 1000 Mbps

"a" = autonegotiated, "full" = full-duplex. The interface automatically negotiated full-duplex with the neighbor.

"a" = autonegotiated, "100" = 100 Mbps. The interface automatically negotiated 100 Mbps speed with the neighbor.

VLAN 1

No. Layer 2 switchports operate at the Data Link layer and use MAC addresses. (Multilayer switches are different - covered later)

Frames that failed the Cyclic Redundancy Check done via the Frame Check Sequence (FCS) in the Ethernet frame trailer.

interface [interface-id]

speed {10|100|1000|auto}

duplex {auto|full|half}

To add documentation/label to identify what the interface connects to. Shown in "show interfaces status" under the "Name" column.

Security concern - enabled interfaces could be exploited. Best practice to shutdown unused ports.

Total of various error counters including runts, giants, CRC errors, and frame errors.

Frames the switch tried to send but failed due to an error.

Frames that have an incorrect or illegal format.

1) Device sends jamming signal to inform others. 2) Each device waits a random time. 3) Devices listen again before retransmitting.

The interface is configured as a trunk port (carries traffic for multiple VLANs - covered in VLAN lessons).

Uses the slowest supported speed (10 Mbps on a 10/100/1000 interface).

Scenario 1: You connect a new PC to switch port F0/5. The port was previously shutdown. What must you do? Answer: Enter interface config mode and use "no shutdown" command.

Scenario 2: Two switches are connected but experiencing poor performance. You check and find SW1 is using full-duplex and SW2 is using half-duplex. What's the problem? Answer: Duplex mismatch. This causes collisions. Enable autonegotiation on both or manually set both to the same duplex.

Scenario 3: You see many "giant" errors on an interface. What does this indicate? Answer: Frames larger than 1518 bytes are being received. Could indicate a misconfiguration or faulty device.