Day 9 - Switch Interfaces

Question 1

A legacy network is using an Ethernet hub. All devices connected to this hub are in the same:

A. Broadcast domain only
B. Collision domain only
C. Both a single broadcast domain and a single collision domain
D. Separate collision domains

Answer 1

✗ A. Broadcast domain only → Incorrect. Hubs don’t break up either broadcast or collision domains.

✗ B. Collision domain only → Incorrect. They also share a single broadcast domain.

✅ C. Both a single broadcast domain and a single collision domain → Correct. A hub acts as one big shared medium.

✗ D. Separate collision domains → Incorrect. Switches create separate collision domains, not hubs.

Question 2

Two PCs are connected to the same switch. Are they in the same collision domain?

A. Yes, because all devices on a switch share the same collision domain
B. No, each switch port is its own collision domain
C. Yes, if the switch is operating at half-duplex only
D. No, switches eliminate both collision and broadcast domains

Answer 2

✗ A. Yes, because all devices on a switch share the same collision domain → Incorrect. This describes a hub.

✅ B. No, each switch port is its own collision domain → Correct. Switches isolate collisions per port.

✗ C. Yes, if the switch is operating at half-duplex only → Incorrect. Duplex does not change collision domain boundaries.

✗ D. No, switches eliminate both collision and broadcast domains → Incorrect. Switches eliminate collisions, not broadcasts.

Question 3

A network engineer wants to configure interface FastEthernet0/1 on a Cisco switch to run at 100 Mbps. Which command is correct?

A. SW1(config-if)# duplex 100
B. SW1(config-if)# speed 100
C. SW1(config)# interface speed 100
D. SW1(config-if)# bandwidth 100

Answer 3

✗ A. SW1(config-if)# duplex 100 → Incorrect. duplex expects “full” or “half.”

✅ B. SW1(config-if)# speed 100 → Correct. Sets the port speed to 100 Mbps.

✗ C. SW1(config)# interface speed 100 → Incorrect. Not valid syntax.

✗ D. SW1(config-if)# bandwidth 100 → Incorrect. bandwidth is informational, not actual speed.

Question 4

Which command configures FastEthernet0/1 on a switch for full duplex?

A. SW1(config-if)# duplex full
B. SW1(config-if)# mode full
C. SW1(config-if)# set duplex full
D. SW1(config-if)# duplex 100

Answer 4

✅ A. SW1(config-if)# duplex full → Correct. Sets the port to full-duplex mode.

✗ B. SW1(config-if)# mode full → Incorrect. Not a valid command.

✗ C. SW1(config-if)# set duplex full → Incorrect. Not Cisco IOS syntax.

✗ D. SW1(config-if)# duplex 100 → Incorrect. duplex only accepts “full” or “half.”

Question 5

A switch port connected to a modern NIC supports full-duplex operation. What does this mean?

A. It can send but not receive at the same time
B. It can receive but not send at the same time
C. It can send and receive simultaneously
D. It avoids all collisions by disabling CRC checks

Answer 5

✗ A. It can send but not receive at the same time → Incorrect. That’s simplex.

✗ B. It can receive but not send at the same time → Incorrect. Same as above.

✅ C. It can send and receive simultaneously → Correct. Full-duplex allows bidirectional communication.

✗ D. It avoids all collisions by disabling CRC checks → Incorrect. CRC checks still occur.

Question 6

Devices connected to a legacy hub must operate in which duplex mode?

A. Full-duplex
B. Half-duplex
C. Auto-duplex
D. Either half or full

Answer 6

✗ A. Full-duplex → Incorrect. Hubs cannot support it.

✅ B. Half-duplex → Correct. Hubs share bandwidth and require half-duplex with CSMA/CD.

✗ C. Auto-duplex → Incorrect. Negotiation won’t work with a hub.

✗ D. Either half or full → Incorrect. Only half is supported.

Question 7

On half-duplex interfaces, which protocol is used to detect and recover from collisions?

A. STP
B. CSMA/CD
C. CSMA/CA
D. ARP

Answer 7

✗ A. STP → Incorrect. Spanning Tree prevents loops, not collisions.

✅ B. CSMA/CD → Correct. Carrier Sense Multiple Access with Collision Detection.

✗ C. CSMA/CA → Incorrect. Used in wireless (802.11).

✗ D. ARP → Incorrect. Resolves IP to MAC, unrelated.

Question 8

Which OSI layer do Ethernet hubs operate at?

A. Physical
B. Data Link
C. Network
D. Transport

Answer 8

✅ A. Physical → Correct. Hubs simply regenerate electrical signals.

✗ B. Data Link → Incorrect. Switches operate here.

✗ C. Network → Incorrect. Routers operate here.

✗ D. Transport → Incorrect. TCP/UDP operate here.

Question 9

A router interface is not yet configured with an IP address. What state will it show in the output of show ip interface brief?

A. administratively down/down
B. up/up
C. up/down
D. down/down

Answer 9

✅ A. administratively down/down → Correct. Interfaces are shut down by default until enabled/configured.

✗ B. up/up → Incorrect. Requires no shutdown and an IP.

✗ C. up/down → Incorrect. Would mean Layer 1 is up but Layer 3 down.

✗ D. down/down → Incorrect. Could be cable issue, but by default it’s admin down.

Question 10

A switch port is connected to another device but has not received a signal. In show ip interface brief, which state will be shown?

A. administratively down/down
B. up/up
C. administratively down/up
D. down/down

Answer 10

✗ A. administratively down/down → Incorrect. That’s when it’s manually shut.

✗ B. up/up → Incorrect. That requires link and protocol up.

✗ C. administratively down/up → Incorrect. Not a valid state.

✅ D. down/down → Correct. Both physical and protocol show down if no signal is detected.

Question 11

A switch port shows many collisions. The admin checks and sees the port is operating in half-duplex. What is the most likely fix?

A. Replace the cable
B. Configure the port for full-duplex
C. Disable CRC checks
D. Reboot the switch

Answer 11

✗ A. Replace the cable → Incorrect. Cable issues usually show as CRC errors, not collisions.

✅ B. Configure the port for full-duplex → Correct. Full-duplex eliminates collisions by allowing simultaneous send/receive (eliminating duplex mismatch on switches).

✗ C. Disable CRC checks → Incorrect. CRC is for error detection, not collisions.

✗ D. Reboot the switch → Incorrect. Won’t fix duplex mismatch.

Question 12

A new switch port is left unconfigured. By default, what speed and duplex will it typically negotiate on modern hardware?

A. 100 Mbps, half-duplex
B. 100 Mbps, full-duplex
C. Auto speed, auto duplex
D. 1 Gbps, half-duplex

Answer 12

✗ A. 100 Mbps, half-duplex → Incorrect. That was common on very old gear.

✗ B. 100 Mbps, full-duplex → Incorrect. Not the default; it’s negotiated.

✅ C. Auto speed, auto duplex → Correct. Modern switches default to auto negotiation.

✗ D. 1 Gbps, half-duplex → Incorrect. Gigabit Ethernet only supports full-duplex.

Question 13

A network engineer wants to configure FastEthernet0/1 and FastEthernet0/2 with the same settings. Which command allows this?

A. interface multi fa0/1-2
B. interface range fa0/1 - 2
C. int fa0/1 & fa0/2
D. set interface fa0/1-2

Answer 13

✗ A. interface multi fa0/1-2 → Incorrect. Not Cisco syntax.

✅ B. interface range fa0/1 - 2 → Correct. Allows bulk configuration of multiple interfaces.

✗ C. int fa0/1 & fa0/2 → Incorrect. Invalid syntax.

✗ D. set interface fa0/1-2 → Incorrect. Not Cisco IOS command.

Question 14

In show ip interface brief, what does the status “administratively down, down” indicate?

A. Cable disconnected
B. No IP address configured
C. Interface is shut down in configuration
D. Protocol mismatch

Answer 14

✗ A. Cable disconnected → Incorrect. Would show down/down, not admin down.

✗ B. No IP address configured → Incorrect. IP doesn’t affect admin state.

✅ C. Interface is shut down in configuration → Correct. shutdown command sets the interface admin down.

✗ D. Protocol mismatch → Incorrect. Would show up/down.

Question 15

In show ip interface brief, what does the status “up, down” usually indicate?

A. Layer 1 is up, but Layer 2/3 protocol is not working
B. Interface is disabled in configuration
C. No cable connected
D. Port is in err-disabled state

Answer 15

✅ A. Layer 1 is up, but Layer 2/3 protocol is not working → Correct. Physical link is active, but protocol is down.

✗ B. Interface is disabled in configuration → Incorrect. That’s admin down.

✗ C. No cable connected → Incorrect. That would show down/down.

✗ D. Port is in err-disabled state → Incorrect. Err-disabled shows as administratively down.

Question 16

A port is generating a high number of “runt frames” in the output of show interfaces. What does this indicate?

A. Frames smaller than 64 bytes, often caused by collisions
B. Frames larger than 1518 bytes
C. Frames padded to minimum size
D. Frames failing CRC checks

Answer 16

✅ A. Frames smaller than 64 bytes, often caused by collisions → Correct. Runts = too small, usually due to errors or collisions.

✗ B. Frames larger than 1518 bytes → Incorrect. Those are giants.

✗ C. Frames padded to minimum size → Incorrect. Padding makes a valid frame, not a runt.

✗ D. Frames failing CRC checks → Incorrect. That’s CRC errors, not runts.

Question 17

Which show interfaces counter records frames larger than the maximum standard size (1518 bytes)?

A. CRC errors
B. Giants
C. Runts
D. Input errors

Answer 17

✗ A. CRC errors → Incorrect. Detects corruption, not oversize frames.

✅ B. Giants → Correct. Frames >1518 bytes are counted as giants.

✗ C. Runts → Incorrect. Frames <64 bytes.

✗ D. Input errors → Incorrect. Summary, but not specific.

Question 18

An engineer runs show interfaces and sees a growing number of CRC errors. What is the most likely cause?

A. Duplex mismatch or cabling problem
B. Frame padding
C. Broadcast storm
D. VLAN misconfiguration

Answer 18

✅ A. Duplex mismatch or cabling problem → Correct. These are the classic causes of CRC errors.

✗ B. Frame padding → Incorrect. Padding is valid.

✗ C. Broadcast storm → Incorrect. Causes congestion, not CRC errors.

✗ D. VLAN misconfiguration → Incorrect. Would block traffic, not cause CRC errors.

Question 19

Which counter in show interfaces is a summary that includes runts, giants, CRC errors, and other errors received?

A. Output errors
B. Input errors
C. Total errors
D. Collisions

Answer 19

✗ A. Output errors → Incorrect. Tracks transmission failures, not received errors.

✅ B. Input errors → Correct. Includes all Layer 2 receive errors (runts, giants, CRC, etc.).

✗ C. Total errors → Incorrect. Not an IOS counter name.

✗ D. Collisions → Incorrect. Only counts collisions, not all errors.

Question 19

In the output of show interfaces, which counter records frames the switch tried to send but failed due to an error?

A. Output errors
B. Input errors
C. Runts
D. CRC errors

Answer 19

✅ A. Output errors → Correct. Tracks errors when the switch transmits.

✗ B. Input errors → Incorrect. Tracks errors received.

✗ C. Runts → Incorrect. Too-small frames.

✗ D. CRC errors → Incorrect. Corruption detection, not transmission errors.

Question 20

An engineer sees a switch port with increasing CRC errors and “late collisions” in show interfaces. What is the most likely cause?

A. Duplex mismatch
B. VLAN misconfiguration
C. No IP address on the interface
D. Broadcast storm

Answer 20

✅ A. Duplex mismatch → Correct. Duplex mismatches cause late collisions and CRC errors.

✗ B. VLAN misconfiguration → Incorrect. Causes connectivity issues, not collisions.

✗ C. No IP address on the interface → Incorrect. Switchports don’t require IP to forward frames.

✗ D. Broadcast storm → Incorrect. Would cause congestion, not late collisions.

Question 21

A PC is connected to a switch port. The switch is set to full-duplex, but the PC is set to half-duplex. What will most likely occur?

A. No communication is possible
B. Excessive collisions and CRC errors
C. The PC will automatically correct its duplex
D. The switch port will shut down immediately

Answer 21

✗ A. No communication is possible → Incorrect. Traffic will still pass, but poorly.

✅ B. Excessive collisions and CRC errors → Correct. This is the classic symptom of a duplex mismatch.

✗ C. The PC will automatically correct its duplex → Incorrect. Auto-negotiation is disabled when forced settings are applied.

✗ D. The switch port will shut down immediately → Incorrect. Ports stay up, but errors occur.

Question 22

Which duplex setting is supported by Gigabit Ethernet ports?

A. Half-duplex only
B. Full-duplex only
C. Both half and full duplex
D. Simplex only

Answer 22

✗ A. Half-duplex only → Incorrect. Not supported at all.

✅ B. Full-duplex only → Correct. Gigabit Ethernet requires full-duplex.

✗ C. Both half and full duplex → Incorrect. Only applies to 10/100 Mbps Ethernet.

✗ D. Simplex only → Incorrect. Ethernet is not simplex.

Question 23

A switch port is configured for speed 100 and duplex half. Which kind of device is most likely connected?

A. A legacy hub
B. A modern switch
C. A wireless access point
D. A router with Gigabit interface

Answer 23

✅ A. A legacy hub → Correct. Hubs only support half-duplex operation.

✗ B. A modern switch → Incorrect. Defaults to full-duplex.

✗ C. A wireless access point → Incorrect. Uses full-duplex switch connections.

✗ D. A router with Gigabit interface → Incorrect. Gigabit doesn’t support half-duplex.

Question 24

An engineer uses show interfaces and notices incrementing “output errors.” What does this indicate? A. Errors detected while receiving frames B. Frames the switch failed to transmit properly C. CRC mismatches D. Collisions caused by hubs

Answer 24

✗ A. Errors detected while receiving frames → Incorrect. That’s input errors. ✅ B. Frames the switch failed to transmit properly → Correct. Output errors reflect transmission failures. ✗ C. CRC mismatches → Incorrect. CRC errors are counted on input, not output. ✗ D. Collisions caused by hubs → Incorrect. Collisions are counted separately.

Question 25

On a switch, a port shows “giants” in its interface statistics. What does this mean? A. Frames smaller than 64 bytes were received B. Frames larger than 1518 bytes were received C. Frames had invalid VLAN tags D. Frames were corrupted by duplex mismatch

Answer 25

✗ A. Frames smaller than 64 bytes were received → Incorrect. That’s runts. ✅ B. Frames larger than 1518 bytes were received → Correct. Giants = oversize frames beyond Ethernet standard. ✗ C. Frames had invalid VLAN tags → Incorrect. VLAN tag errors don’t show as giants. ✗ D. Frames were corrupted by duplex mismatch → Incorrect. Would cause CRC errors, not giants.

Question 26

Which command would you use to manually re-enable a switch interface that has been shut down? A. interface restart B. no shutdown C. enable interface D. restore port

Answer 26

✗ A. interface restart → Incorrect. Not Cisco syntax. ✅ B. no shutdown → Correct. Re-enables an administratively shut port. ✗ C. enable interface → Incorrect. Not a valid IOS command. ✗ D. restore port → Incorrect. Not Cisco syntax.

Question 27

In show interfaces, which counter tracks frames dropped due to buffer overflow? A. Output errors B. Input errors C. Overruns D. Collisions

Answer 27

✗ A. Output errors → Incorrect. Transmission failures. ✗ B. Input errors → Incorrect. Summary of inbound errors, not buffer overflow. ✅ C. Overruns → Correct. Frames dropped when the buffer can’t keep up. ✗ D. Collisions → Incorrect. Separate counter for half-duplex.

Question 28

A switch port is configured for speed auto and duplex auto. It connects to a device forced to speed 100 duplex full. What is the likely result? A. Both sides match full-duplex automatically B. Duplex mismatch leading to collisions and errors C. The link will not come up at all D. The switch will force half-duplex

Answer 28

✗ A. Both sides match full-duplex automatically → Incorrect. Auto vs forced creates mismatches. ✅ B. Duplex mismatch leading to collisions and errors → Correct. Auto-negotiation fails to detect forced full-duplex. ✗ C. The link will not come up at all → Incorrect. Speed will match, but duplex will not. ✗ D. The switch will force half-duplex → Incorrect. It negotiates but falls back incorrectly.

Question 29

Which situation would cause a port to show “up, up” in show ip interface brief but still fail to pass traffic? A. IP address not in the correct subnet B. Interface administratively shut down C. Cable disconnected D. Duplex set to half on both ends

Answer 29

✅ A. IP address not in the correct subnet → Correct. Physical and protocol layers are up, but L3 connectivity fails. ✗ B. Interface administratively shut down → Incorrect. Would show admin down. ✗ C. Cable disconnected → Incorrect. Would show down/down. ✗ D. Duplex set to half on both ends → Incorrect. Half-duplex works, just with collisions.

Question 30

A user reports extremely slow network performance. You check show interfaces on the switch and see increasing CRC errors and late collisions. The port is configured as speed 100 duplex auto on the switch, and the server NIC is speed 100 duplex full. What is the cause? A. Cable too long B. Duplex mismatch C. Broadcast storm D. IP addressing error

Answer 30

✗ A. Cable too long → Incorrect. Cable issues show as CRC or input errors, but not late collisions. ✅ B. Duplex mismatch → Correct. One side is forced full, the other side auto → auto side defaults to half-duplex. Causes CRC errors + late collisions. ✗ C. Broadcast storm → Incorrect. Would affect the whole VLAN, not just one port. ✗ D. IP addressing error → Incorrect. Wrong addressing prevents connectivity, but doesn’t cause collisions.

Question 31

A switch port is configured for duplex full. The connected PC is left at default (auto). The link is up but users report frequent retransmissions and poor throughput. What is the most likely issue? A. The PC defaulted to half-duplex, creating a duplex mismatch B. The PC auto-negotiated full-duplex correctly C. The switch port should be administratively shut down D. The cable is wired incorrectly

Answer 31

✅ A. The PC defaulted to half-duplex, creating a duplex mismatch → Correct. Forced full vs auto = mismatch. ✗ B. The PC auto-negotiated full-duplex correctly → Incorrect. Auto fails when the other side is forced. ✗ C. The switch port should be administratively shut down → Incorrect. That would kill the link. ✗ D. The cable is wired incorrectly → Incorrect. Cabling errors show as link down, not CRC/throughput problems.