Sample questions
CompTIA Network+ N10-009 practice questions
Which of the following network devices operates primarily at Layer 2 of the OSI model and uses MAC addresses to forward data?
Trap 1: Hub
A hub operates at Layer 1 (physical layer) and blindly repeats signals to all ports.
Trap 2: Router
A router operates at Layer 3 (network layer) and forwards packets based on IP addresses.
Trap 3: Firewall
A firewall can operate at multiple layers, but its primary function is security filtering, not MAC-based forwarding.
- A
Hub
Why wrong: A hub operates at Layer 1 (physical layer) and blindly repeats signals to all ports.
- B
Switch
A switch forwards frames based on MAC addresses, operating at the data link layer.
- C
Router
Why wrong: A router operates at Layer 3 (network layer) and forwards packets based on IP addresses.
- D
Firewall
Why wrong: A firewall can operate at multiple layers, but its primary function is security filtering, not MAC-based forwarding.
Which of the following is a characteristic of UDP when compared to TCP?
Trap 1: A) UDP uses sequence numbers for ordering
UDP does not use sequence numbers; TCP does to ensure ordered delivery.
Trap 2: B) UDP provides reliable data delivery
UDP is unreliable; it does not guarantee delivery. TCP provides reliability through acknowledgments and retransmissions.
Trap 3: D) UDP requires a three-way handshake to establish a connection
UDP is connectionless and does not use a handshake. TCP uses a three-way handshake (SYN, SYN-ACK, ACK) to establish a connection.
- A
A) UDP uses sequence numbers for ordering
Why wrong: UDP does not use sequence numbers; TCP does to ensure ordered delivery.
- B
B) UDP provides reliable data delivery
Why wrong: UDP is unreliable; it does not guarantee delivery. TCP provides reliability through acknowledgments and retransmissions.
- C
C) UDP has lower overhead due to minimal header
Correct. UDP has a smaller header (8 bytes) compared to TCP (20 bytes) and no connection setup, resulting in lower overhead.
- D
D) UDP requires a three-way handshake to establish a connection
Why wrong: UDP is connectionless and does not use a handshake. TCP uses a three-way handshake (SYN, SYN-ACK, ACK) to establish a connection.
Which of the following IPv6 addresses is a valid link-local address?
Trap 1: 2001:db8::1
2001:db8::/32 is reserved for documentation and is a global unicast address.
Trap 2: ff02::1
ff02::1 is the all-nodes multicast address, not a link-local unicast address.
Trap 3: 10.0.0.1
10.0.0.1 is an IPv4 address; it is not a valid IPv6 address.
- A
fe80::1
fe80::/10 is the prefix for link-local addresses; fe80::1 is a common link-local address.
- B
2001:db8::1
Why wrong: 2001:db8::/32 is reserved for documentation and is a global unicast address.
- C
ff02::1
Why wrong: ff02::1 is the all-nodes multicast address, not a link-local unicast address.
- D
10.0.0.1
Why wrong: 10.0.0.1 is an IPv4 address; it is not a valid IPv6 address.
Which of the following security mechanisms requires a user to authenticate before gaining access to the wired network at a switch port?
Trap 1: Port security
Port security limits based on MAC addresses but does not require authentication; MAC addresses can be spoofed.
Trap 2: ACL
Access control lists filter traffic after the connection is established, they do not authenticate the user before access.
Trap 3: MAC authentication
MAC authentication is a form of authentication based on the MAC address, but it is less secure than 802.1X and can be bypassed. 802.1X is the more robust standard.
- A
802.1X
802.1X is a Layer 2 protocol that blocks all traffic until the device authenticates, providing strong access control.
- B
Port security
Why wrong: Port security limits based on MAC addresses but does not require authentication; MAC addresses can be spoofed.
- C
ACL
Why wrong: Access control lists filter traffic after the connection is established, they do not authenticate the user before access.
- D
MAC authentication
Why wrong: MAC authentication is a form of authentication based on the MAC address, but it is less secure than 802.1X and can be bypassed. 802.1X is the more robust standard.
Which of the following network protocols operates at the Transport layer of the OSI model and provides connection-oriented, reliable data delivery?
Trap 1: UDP
UDP is a transport layer protocol, but it is connectionless and unreliable; it does not provide guaranteed delivery.
Trap 2: IP
IP operates at the Network layer (Layer 3) and handles logical addressing and routing, not reliable transport.
Trap 3: ARP
ARP maps IP addresses to MAC addresses and operates at Layer 2/3, not at the Transport layer.
- A
UDP
Why wrong: UDP is a transport layer protocol, but it is connectionless and unreliable; it does not provide guaranteed delivery.
- B
TCP
TCP provides connection-oriented, reliable data delivery with features like flow control, error checking, and retransmission of lost packets.
- C
IP
Why wrong: IP operates at the Network layer (Layer 3) and handles logical addressing and routing, not reliable transport.
- D
ARP
Why wrong: ARP maps IP addresses to MAC addresses and operates at Layer 2/3, not at the Transport layer.
A user reports intermittent inability to access websites. When the issue occurs, the user can ping external IP addresses (e.g., 8.8.8.8) but cannot ping domain names like google.com. The user's IP configuration shows a DNS server address of 8.8.8.8. What is the most likely cause?
Trap 1: The default gateway is down.
If the default gateway were down, the user would not be able to ping any external IP addresses. The fact that pinging 8.8.8.8 works rules this out.
Trap 2: The corporate firewall is blocking UDP port 53.
A firewall block on UDP port 53 would cause DNS failures consistently, not intermittently. The user would never be able to resolve domain names.
Trap 3: The user's machine has a corrupted DNS cache.
A corrupted DNS cache typically causes failures for specific cached entries, but the issue is described as intermittent and affects all domain names. Flushing the cache might resolve it, but the intermittent nature points more to server load.
- A
The DNS server is reachable but not responding due to high load.
Correct. If the DNS server is overloaded, it may drop queries intermittently. The user can still ping the server (ICMP is different from DNS), but DNS queries time out.
- B
The default gateway is down.
Why wrong: If the default gateway were down, the user would not be able to ping any external IP addresses. The fact that pinging 8.8.8.8 works rules this out.
- C
The corporate firewall is blocking UDP port 53.
Why wrong: A firewall block on UDP port 53 would cause DNS failures consistently, not intermittently. The user would never be able to resolve domain names.
- D
The user's machine has a corrupted DNS cache.
Why wrong: A corrupted DNS cache typically causes failures for specific cached entries, but the issue is described as intermittent and affects all domain names. Flushing the cache might resolve it, but the intermittent nature points more to server load.
Which of the following is a characteristic of a connectionless protocol at the transport layer?
Trap 1: It establishes a session before sending data
Establishing a session is a characteristic of connection-oriented protocols like TCP.
Trap 2: It guarantees delivery using acknowledgments
Acknowledgments and guaranteed delivery are features of reliable, connection-oriented protocols.
Trap 3: It retransmits lost segments
Retransmission of lost segments is performed by reliable protocols like TCP, not connectionless protocols.
- A
It establishes a session before sending data
Why wrong: Establishing a session is a characteristic of connection-oriented protocols like TCP.
- B
It guarantees delivery using acknowledgments
Why wrong: Acknowledgments and guaranteed delivery are features of reliable, connection-oriented protocols.
- C
It does not require a virtual circuit
Connectionless protocols send data independently without setting up a virtual circuit or logical connection.
- D
It retransmits lost segments
Why wrong: Retransmission of lost segments is performed by reliable protocols like TCP, not connectionless protocols.
Which of the following uniquely identifies a hardware network interface on a device?
Trap 1: IP address
An IP address identifies a device on a network but can change (DHCP) and does not uniquely identify the hardware interface.
Trap 2: Subnet mask
The subnet mask is used for network division, not for identifying a specific hardware interface.
Trap 3: Default gateway
The default gateway is the IP address of a router used to reach remote networks, not a hardware identifier.
- A
MAC address
A MAC address is a 48-bit hardware address assigned to each network interface card (NIC) for local network communications.
- B
IP address
Why wrong: An IP address identifies a device on a network but can change (DHCP) and does not uniquely identify the hardware interface.
- C
Subnet mask
Why wrong: The subnet mask is used for network division, not for identifying a specific hardware interface.
- D
Default gateway
Why wrong: The default gateway is the IP address of a router used to reach remote networks, not a hardware identifier.
A company wants to allow inbound HTTPS traffic to a web server located in the DMZ from the Internet. The firewall has three interfaces: Inside (corporate network), Outside (Internet), and DMZ (web server). Which of the following firewall rules is required?
Trap 1: Allow traffic from DMZ to Outside on port 443
This rule permits outbound traffic from the DMZ to the Internet, which would allow the web server to initiate outbound connections, but does not allow inbound HTTPS requests.
Trap 2: Allow traffic from Inside to DMZ on port 443
This rule permits internal corporate users to access the web server, but not external users from the Internet.
Trap 3: Allow traffic from Outside to Inside on port 443
This would allow inbound traffic directly into the corporate network, bypassing the DMZ, which is not the intention and violates security best practices.
- A
Allow traffic from Outside to DMZ on port 443
This rule permits HTTPS traffic from the Internet (Outside) to the web server in the DMZ, which is the requirement.
- B
Allow traffic from DMZ to Outside on port 443
Why wrong: This rule permits outbound traffic from the DMZ to the Internet, which would allow the web server to initiate outbound connections, but does not allow inbound HTTPS requests.
- C
Allow traffic from Inside to DMZ on port 443
Why wrong: This rule permits internal corporate users to access the web server, but not external users from the Internet.
- D
Allow traffic from Outside to Inside on port 443
Why wrong: This would allow inbound traffic directly into the corporate network, bypassing the DMZ, which is not the intention and violates security best practices.
A network administrator wants to ensure that only authorized devices can access the network on a switch port. The administrator has a list of allowed MAC addresses. Which security feature should be enabled on the switch port?
Trap 1: 802.1X
802.1X is an authentication protocol that requires user credentials, not just MAC address filtering.
Trap 2: MAC address filtering
MAC address filtering is a general term; on switches it is implemented via port security.
Trap 3: VLAN hopping prevention
VLAN hopping prevention protects against VLAN attacks, not device authorization.
- A
802.1X
Why wrong: 802.1X is an authentication protocol that requires user credentials, not just MAC address filtering.
- B
MAC address filtering
Why wrong: MAC address filtering is a general term; on switches it is implemented via port security.
- C
Port security
Port security allows restricting access based on MAC addresses, preventing unauthorized devices.
- D
VLAN hopping prevention
Why wrong: VLAN hopping prevention protects against VLAN attacks, not device authorization.
A network administrator is configuring BGP between two autonomous systems. Which BGP attribute is primarily used to influence inbound traffic to a particular AS?
Trap 1: AS_PATH
AS_PATH influences outbound traffic by affecting path selection for routes advertised.
Trap 2: Next Hop
Next hop is used for reachability, not for inbound traffic influence.
Trap 3: Local Preference
Local preference influences outbound traffic selection from the router's AS.
- A
AS_PATH
Why wrong: AS_PATH influences outbound traffic by affecting path selection for routes advertised.
- B
Next Hop
Why wrong: Next hop is used for reachability, not for inbound traffic influence.
- C
Local Preference
Why wrong: Local preference influences outbound traffic selection from the router's AS.
- D
MED (Multi-Exit Discriminator)
The MED attribute is used to suggest to a neighboring AS the preferred path for inbound traffic when multiple entry points exist.
A network device receives a frame on one port and forwards it out to all other ports. The device does not examine the destination MAC address. Which type of device is being described?
Trap 1: Switch
Switches examine the destination MAC address and forward frames only to the specific port, not to all ports (except for broadcasts).
Trap 2: Bridge
Bridges operate at Layer 2 and use MAC addresses to segment traffic, forwarding only to the necessary segment.
Trap 3: Router
Routers operate at Layer 3 and use IP addresses to determine the best path, not forwarding blindly to all ports.
- A
Switch
Why wrong: Switches examine the destination MAC address and forward frames only to the specific port, not to all ports (except for broadcasts).
- B
Hub
Hubs are Layer 1 devices that forward signals to all ports without any intelligence. They do not read MAC addresses.
- C
Bridge
Why wrong: Bridges operate at Layer 2 and use MAC addresses to segment traffic, forwarding only to the necessary segment.
- D
Router
Why wrong: Routers operate at Layer 3 and use IP addresses to determine the best path, not forwarding blindly to all ports.
A network administrator is configuring a monitoring system to collect metrics from network devices. The administrator needs to ensure that the monitoring system can automatically discover the devices and obtain detailed information about their configuration and status, such as interface descriptions and software versions. Which protocol is best suited for this purpose?
Trap 1: LLDP
LLDP is used to advertise and discover directly connected neighbors (device ID, port ID, capabilities). It does not provide detailed configuration or status information about non-neighbor devices.
Trap 2: NetFlow
NetFlow is used for traffic flow analysis (IP traffic statistics). It does not provide device configuration or inventory data.
Trap 3: Syslog
Syslog is used for collecting log messages from devices. While it may contain some informational messages, it is not designed for structured inventory or configuration polling.
- A
SNMP
SNMP (especially with SNMPv2c or v3) allows a management station to query device MIBs for detailed information such as interface descriptions, software versions, and status. It can also be used for discovery by polling known community strings.
- B
LLDP
Why wrong: LLDP is used to advertise and discover directly connected neighbors (device ID, port ID, capabilities). It does not provide detailed configuration or status information about non-neighbor devices.
- C
NetFlow
Why wrong: NetFlow is used for traffic flow analysis (IP traffic statistics). It does not provide device configuration or inventory data.
- D
Syslog
Why wrong: Syslog is used for collecting log messages from devices. While it may contain some informational messages, it is not designed for structured inventory or configuration polling.
A network administrator connects a new access switch to the core switch via a trunk port. Both switches have the same VLAN database, and the trunk is configured to allow all VLANs. However, hosts on VLAN 10 connected to the new access switch cannot communicate with hosts on VLAN 10 on the core switch. The administrator verifies that the access ports for VLAN 10 are correctly configured and that the trunk link status is up/up. Which of the following is the most likely cause?
Trap 1: The trunk port is in an err-disabled state.
The trunk status is up/up, so it is not err-disabled. An err-disabled port would show as down.
Trap 2: The switchport mode is not set to trunk on one side.
If the trunk link is up/up, both sides are likely configured as trunk ports. The question states the trunk is configured, so this is not the issue.
Trap 3: The spanning-tree protocol is blocking VLAN 10 on the trunk.
Spanning tree can block specific VLANs if there is a loop, but for a simple two-switch topology this is unlikely. Moreover, the symptom is that hosts on the same VLAN cannot communicate, which is more indicative of a native VLAN issue.
- A
The trunk port is in an err-disabled state.
Why wrong: The trunk status is up/up, so it is not err-disabled. An err-disabled port would show as down.
- B
The native VLAN on the trunk port is different on the two switches.
A native VLAN mismatch can cause untagged frames to be placed into different VLANs on each switch. This can prevent communication even if the allowed VLAN list is correct. The administrator should verify that the native VLAN is the same on both ends.
- C
The switchport mode is not set to trunk on one side.
Why wrong: If the trunk link is up/up, both sides are likely configured as trunk ports. The question states the trunk is configured, so this is not the issue.
- D
The spanning-tree protocol is blocking VLAN 10 on the trunk.
Why wrong: Spanning tree can block specific VLANs if there is a loop, but for a simple two-switch topology this is unlikely. Moreover, the symptom is that hosts on the same VLAN cannot communicate, which is more indicative of a native VLAN issue.
A network administrator is creating a new VLAN that will contain 20 devices. The administrator wants to use the most efficient subnet that provides enough usable IP addresses while minimizing waste. Which of the following subnet masks should be used?
Trap 1: 255.255.255.240 (/28)
A /28 provides only 14 usable addresses, insufficient for 20 hosts.
Trap 2: 255.255.255.248 (/29)
A /29 provides only 6 usable addresses, far too few.
Trap 3: 255.255.255.192 (/26)
A /26 provides 62 usable addresses, which is more than needed and wastes address space.
- A
255.255.255.240 (/28)
Why wrong: A /28 provides only 14 usable addresses, insufficient for 20 hosts.
- B
255.255.255.224 (/27)
A /27 provides 30 usable addresses, which is the smallest subnet that can accommodate 20 hosts.
- C
255.255.255.248 (/29)
Why wrong: A /29 provides only 6 usable addresses, far too few.
- D
255.255.255.192 (/26)
Why wrong: A /26 provides 62 usable addresses, which is more than needed and wastes address space.
A network administrator is implementing a change management process. Which of the following is the PRIMARY benefit of following this process?
Trap 1: It reduces the cost of implementing new hardware
Cost reduction is not the primary purpose of change management; it focuses on minimizing risk.
Trap 2: It ensures that all network changes are automated
Change management does not imply automation; it is a procedural framework.
Trap 3: It documents the network topology for future reference
Documentation is part of change management but not its primary benefit; the main benefit is risk reduction.
- A
It reduces the cost of implementing new hardware
Why wrong: Cost reduction is not the primary purpose of change management; it focuses on minimizing risk.
- B
It ensures that all network changes are automated
Why wrong: Change management does not imply automation; it is a procedural framework.
- C
It minimizes the impact of changes on network operations and reduces errors
The core goal of change management is to manage changes in a controlled manner to prevent outages and errors.
- D
It documents the network topology for future reference
Why wrong: Documentation is part of change management but not its primary benefit; the main benefit is risk reduction.
A network administrator needs to power IP phones and wireless access points through the Ethernet cable. Which standard should be supported?
Trap 1: 802.11ac
802.11ac is a wireless networking standard, not related to power delivery over Ethernet.
Trap 2: 802.1X
802.1X is a port-based authentication standard, not for power delivery.
Trap 3: 802.3ab
802.3ab defines Gigabit Ethernet over twisted-pair copper, not PoE.
- A
802.3af
802.3af is the IEEE standard for Power over Ethernet, providing up to 15.4W to powered devices.
- B
802.11ac
Why wrong: 802.11ac is a wireless networking standard, not related to power delivery over Ethernet.
- C
802.1X
Why wrong: 802.1X is a port-based authentication standard, not for power delivery.
- D
802.3ab
Why wrong: 802.3ab defines Gigabit Ethernet over twisted-pair copper, not PoE.
A network administrator needs to collect detailed data about network traffic flows, including source/destination IP addresses, ports, and protocols, to analyze bandwidth usage patterns. Which technology should be used?
Trap 1: SNMP
SNMP retrieves device statistics like interface utilization, but does not provide per-flow details.
Trap 2: Syslog
Syslog is used for collecting log messages, not traffic flow data.
Trap 3: ICMP
ICMP is a protocol for error reporting and diagnostics, not for flow collection.
- A
SNMP
Why wrong: SNMP retrieves device statistics like interface utilization, but does not provide per-flow details.
- B
NetFlow
NetFlow captures metadata about each flow (conversation), including IP addresses, ports, and protocol, enabling detailed traffic analysis.
- C
Syslog
Why wrong: Syslog is used for collecting log messages, not traffic flow data.
- D
ICMP
Why wrong: ICMP is a protocol for error reporting and diagnostics, not for flow collection.
A network administrator wants to ensure that a critical file server is always reachable via a single IP address, even if the server's NIC fails. The server has a single NIC. Which technique should be used to provide high availability for this IP address?
Trap 1: Configure a load balancer in front of multiple servers
Load balancers distribute traffic but the question specifies a single server with a single NIC.
Trap 2: Implement NIC teaming
NIC teaming requires multiple network interfaces, but the server has only one NIC.
Trap 3: Use DNS round robin
DNS round robin distributes traffic among multiple IPs but does not provide automatic failover for a single server.
- A
Configure a load balancer in front of multiple servers
Why wrong: Load balancers distribute traffic but the question specifies a single server with a single NIC.
- B
Implement NIC teaming
Why wrong: NIC teaming requires multiple network interfaces, but the server has only one NIC.
- C
Use VRRP to create a virtual IP address
VRRP allows two devices to share a virtual IP, providing failover if the primary fails.
- D
Use DNS round robin
Why wrong: DNS round robin distributes traffic among multiple IPs but does not provide automatic failover for a single server.
A network administrator needs to connect 10 workstations in a way that each workstation's traffic does not collide with others. Which device should be used to connect these workstations?
Trap 1: Hub
Incorrect. A hub forwards all traffic to all ports, creating a single collision domain.
Trap 2: Router
Incorrect. A router primarily separates broadcast domains and routes between networks, but does not provide per-port collision domains.
Trap 3: Modem
Incorrect. A modem is used for converting digital signals to analog for transmission over telephone or cable lines, not for connecting local workstations.
- A
Hub
Why wrong: Incorrect. A hub forwards all traffic to all ports, creating a single collision domain.
- B
Switch
Correct. Each port on a switch is a separate collision domain, preventing collisions between different workstations.
- C
Router
Why wrong: Incorrect. A router primarily separates broadcast domains and routes between networks, but does not provide per-port collision domains.
- D
Modem
Why wrong: Incorrect. A modem is used for converting digital signals to analog for transmission over telephone or cable lines, not for connecting local workstations.
A network administrator needs to ensure that data sent from a host arrives at the correct destination on a different network. Which of the following provides the logical address used for this purpose in IPv4?
Trap 1: MAC address
MAC addresses are used for local delivery on the same network segment, not for routing across networks.
Trap 2: Port number
Port numbers identify specific applications or services on a host, not the host itself.
Trap 3: Default gateway
The default gateway is the router that sends packets to other networks; it is not the destination address.
- A
MAC address
Why wrong: MAC addresses are used for local delivery on the same network segment, not for routing across networks.
- B
IP address
IP addresses are hierarchical and routable, allowing packets to be forwarded across multiple networks to the destination.
- C
Port number
Why wrong: Port numbers identify specific applications or services on a host, not the host itself.
- D
Default gateway
Why wrong: The default gateway is the router that sends packets to other networks; it is not the destination address.
A network administrator wants to centrally collect and analyze event logs from routers, switches, and firewalls. Which protocol is most commonly used for sending log messages from network devices to a central log server?
Trap 1: SNMP
SNMP is used for polling and receiving traps for device status and performance metrics, but it is not typically used for sending detailed log messages.
Trap 2: NetFlow
NetFlow collects traffic flow data (IP addresses, ports, protocols) but does not provide system logs or events.
Trap 3: ICMP
ICMP is used for diagnostic tools like ping and traceroute, not for logging.
- A
SNMP
Why wrong: SNMP is used for polling and receiving traps for device status and performance metrics, but it is not typically used for sending detailed log messages.
- B
Syslog
Syslog is the de facto standard for logging messages from network devices. It allows administrators to forward logs to a central server for analysis and archiving.
- C
NetFlow
Why wrong: NetFlow collects traffic flow data (IP addresses, ports, protocols) but does not provide system logs or events.
- D
ICMP
Why wrong: ICMP is used for diagnostic tools like ping and traceroute, not for logging.
A network administrator is configuring an IEEE 802.1Q trunk between two switches. Which of the following must match on both ends for the trunk to function correctly?
Trap 1: The trunk port speed and duplex
Speed and duplex must match for any link, but this is not a trunk-specific requirement.
Trap 2: The encapsulation type
Both ends must use 802.1Q, but this is a standard setting and less likely to be misconfigured than the native VLAN.
Trap 3: The allowed VLAN list
The allowed VLAN list can be pruned differently on each end; the trunk will still pass traffic for VLANs allowed on both sides.
- A
The native VLAN ID
A mismatched native VLAN can cause traffic on the untagged VLAN to be placed in the wrong VLAN on the other switch.
- B
The trunk port speed and duplex
Why wrong: Speed and duplex must match for any link, but this is not a trunk-specific requirement.
- C
The encapsulation type
Why wrong: Both ends must use 802.1Q, but this is a standard setting and less likely to be misconfigured than the native VLAN.
- D
The allowed VLAN list
Why wrong: The allowed VLAN list can be pruned differently on each end; the trunk will still pass traffic for VLANs allowed on both sides.
A network administrator is troubleshooting an intermittent link between two switches connected by single-mode fiber. The interface log shows "Link up / Link down" events multiple times per hour. Which of the following is the most likely cause?
Trap 1: Incorrect VLAN configuration on the switch ports
VLAN mismatches typically result in connectivity issues or traffic errors, but they do not usually cause the physical link to flap (repeatedly lose carrier).
Trap 2: Crossed fiber pairs
Crossed pairs (transmit/receive swapped) would prevent the link from ever establishing, not cause intermittent flapping.
Trap 3: Duplex mismatch between the switches
Duplex mismatch usually results in high error rates, collisions, and poor performance, but the link state remains up. It does not typically cause link flapping.
- A
Incorrect VLAN configuration on the switch ports
Why wrong: VLAN mismatches typically result in connectivity issues or traffic errors, but they do not usually cause the physical link to flap (repeatedly lose carrier).
- B
Crossed fiber pairs
Why wrong: Crossed pairs (transmit/receive swapped) would prevent the link from ever establishing, not cause intermittent flapping.
- C
Dirty fiber connectors
Dirty connectors can cause intermittent signal loss as the light is partially blocked. This is a common cause of flapping fiber links. Cleaning the connectors often resolves the issue.
- D
Duplex mismatch between the switches
Why wrong: Duplex mismatch usually results in high error rates, collisions, and poor performance, but the link state remains up. It does not typically cause link flapping.
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