N10-009 Hard Questions — 25 Difficult Questions

Question 1

A network engineer has successfully established an IPsec site-to-site VPN tunnel between a branch office (10.0.1.0/24) and the main office (192.168.1.0/24). The tunnel status shows as active, and both sides can ping each other's tunnel interface IP addresses. However, users at the branch office cannot ping the main office server at 192.168.1.10, and the main office cannot ping the branch office server at 10.0.1.10. The firewall rules on both sides permit IPsec traffic and all internal traffic. What should the engineer check NEXT?

Accepted Answer

Verify routing entries on both routers to ensure the remote internal subnets are reachable via the tunnel.. The tunnel is active and both sides can ping each other's tunnel interface IPs, confirming that IPsec phase 1 and phase 2 are established and the tunnel itself is functional. However, users cannot reach the remote internal subnets (10.0.1.0/24 and 192.168.1.0/24), which indicates a routing problem: the routers likely lack routes for those remote subnets pointing to the tunnel interface. Without proper routing entries, traffic destined for the remote LAN is sent out the wrong interface or dropped, even though the tunnel is up.

Answer

Check the IPsec security associations for encryption algorithm mismatch.

Answer

Disable the firewall on the internal interfaces temporarily.

Answer

Regenerate the pre-shared key on both sides.

Question 2

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?

Accepted Answer

The DNS server is reachable but not responding due to high load.. The user can ping external IP addresses (e.g., 8.8.8.8) but cannot resolve domain names like google.com, which indicates that IP connectivity is working but DNS resolution is failing. Since the DNS server address is 8.8.8.8 and the user can ping it, the server is reachable; however, intermittent failures suggest the server is overwhelmed and dropping or not responding to queries. This matches the symptom of a reachable but unresponsive DNS server due to high load, where ICMP (ping) succeeds but UDP/53 DNS queries time out.

Answer

The default gateway is down.

Answer

The corporate firewall is blocking UDP port 53.

Answer

The user's machine has a corrupted DNS cache.

Question 3

A network administrator is configuring OSPF on routers in a multi-area network. The administrator wants to ensure that a router in area 1 does not learn external routes (Type 5 LSAs) injected by an ASBR in area 0, but it must still learn inter-area routes (Type 3 LSAs). The administrator wants to reduce the routing table size. Which OSPF area type should be configured for area 1?

Accepted Answer

Stub area. A stub area blocks Type 5 LSAs (external routes) from entering the area while still allowing Type 3 LSAs (inter-area routes). This meets the requirement of preventing external routes from the ASBR in area 0 from being learned by routers in area 1, while still permitting inter-area routing and reducing the routing table size.

Answer

Totally stubby area

Answer

Not-so-stubby-area (NSSA)

Answer

Normal area

Question 4

A network administrator is configuring BGP between two autonomous systems. Which BGP attribute is primarily used to influence inbound traffic to a particular AS?

Accepted Answer

MED (Multi-Exit Discriminator). The Multi-Exit Discriminator (MED) is a BGP attribute used to influence inbound traffic from a neighboring AS when multiple entry points exist. A lower MED value is preferred, allowing an AS to advertise to its neighbor which path should be used to reach it, thereby influencing traffic entering the local AS.

Answer

AS_PATH

Answer

Next Hop

Answer

Local Preference

Question 5

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?

Accepted Answer

SNMP. SNMP (Simple Network Management Protocol) is the correct choice because it is specifically designed for network management and monitoring. It allows a management system to automatically discover devices (via SNMP walks or queries to MIBs) and retrieve detailed configuration and status information, such as interface descriptions and software versions, by reading OIDs from the device's MIB. This matches the requirement for automatic discovery and detailed data collection.

Answer

LLDP

Answer

NetFlow

Answer

Syslog

Question 6

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?

Accepted Answer

Use VRRP to create a virtual IP address. VRRP (Virtual Router Redundancy Protocol) allows multiple routers or servers to share a virtual IP address, with one acting as the active master and the others as backups. If the active device fails, a backup takes over the virtual IP, ensuring continuous reachability. Since the file server has only one NIC, VRRP can be configured on a pair of routers or Layer 3 switches to provide a virtual IP that points to the server's real IP, but more practically, VRRP is used on the server's default gateway to ensure the server can always reach the network; however, for the server itself to be reachable via a single IP despite NIC failure, you would typically use a load balancer or NIC teaming, but VRRP can also be used to create a virtual IP on a pair of firewalls or routers that forward traffic to the server, making the server appear reachable via that virtual IP even if one path fails.

Answer

Configure a load balancer in front of multiple servers

Answer

Implement NIC teaming

Answer

Use DNS round robin

Question 7

A network administrator must monitor network devices using SNMP. The security policy mandates strong encryption for both authentication and data integrity. Which SNMP version and security level should be implemented?

Accepted Answer

SNMPv3 with authPriv. SNMPv3 with authPriv is correct because it provides both authentication (to verify the source of messages) and encryption (to ensure data integrity and confidentiality). The security policy mandates strong encryption for both authentication and data integrity, which only the authPriv security level fulfills by using HMAC-SHA/MD5 for authentication and AES/DES for encryption.

Answer

SNMPv1

Answer

SNMPv2c

Answer

SNMPv3 with noAuthNoPriv

Question 8

A network security administrator is configuring authentication for network devices and wants to use a protocol that supports separate encryption of the entire authentication packet. Which of the following protocols is designed to encrypt the entire authentication packet and is commonly used with AAA services?

Accepted Answer

TACACS+. TACACS+ is the correct answer because it encrypts the entire authentication packet, including the username, password, and all other fields, using a shared secret key. This full-packet encryption is a key differentiator from RADIUS, which only encrypts the password field. TACACS+ is commonly used with AAA services to provide separate authentication, authorization, and accounting processes.

Answer

RADIUS

Answer

LDAP

Answer

Kerberos

Question 9

A security analyst discovers that an attacker is sending large numbers of incomplete TCP connection requests to a server, causing the server to run out of resources and stop responding to legitimate requests. Which type of attack is this, and which mitigation should be implemented?

Accepted Answer

SYN flood; enable TCP intercept or SYN cookies. This is a SYN flood attack, where the attacker sends a high volume of TCP SYN packets without completing the three-way handshake, exhausting the server's connection queue. Enabling TCP intercept (on Cisco devices) or SYN cookies (RFC 4987) allows the server to validate handshakes before allocating resources, mitigating the attack.

Answer

Ping flood; implement rate limiting

Answer

Smurf attack; disable IP-directed broadcasts

Answer

ARP poisoning; enable dynamic ARP inspection

Question 10

A network technician is troubleshooting an intermittent connectivity issue between two switches connected via fiber optic cable. The link status shows up/down flapping. The technician checks the optical power levels and finds they are within acceptable range. Which of the following is the most likely cause?

Accepted Answer

Dirty fiber connectors. Dirty fiber connectors cause intermittent connectivity by scattering or absorbing light, leading to bit errors and link flaps even when average optical power levels appear within acceptable range. The flapping occurs because transient contaminants (e.g., dust or oil) momentarily disrupt the optical signal, triggering link down events that recover when the connector is jostled or the contaminant shifts. Since the power meter measures average power, it may not detect brief attenuation spikes caused by dirt.

Answer

Electromagnetic interference

Answer

Incorrect VLAN configuration

Answer

Duplex mismatch

Question 11

A security administrator is configuring a firewall to allow remote employees to access the company's internal web server (port 443) from the internet. The web server has an internal IP address of 10.0.0.5. The firewall has a public IP of 203.0.113.10. Which type of firewall rule should be created?

Accepted Answer

A) Port forwarding (DNAT) rule. A port forwarding (DNAT) rule is required because the web server uses a private RFC 1918 IP address (10.0.0.5), which is not routable on the public internet. The firewall must translate the destination IP from its public address (203.0.113.10) to the internal server's private address, allowing inbound traffic on port 443 to reach the correct internal host.

Answer

B) Allow rule with source any, destination 10.0.0.5, port 443

Answer

C) Access control list on the internal interface

Answer

D) VPN rule to require remote access VPN

Question 12

A security analyst notices that a user's workstation is sending encrypted DNS queries to an external IP address over TCP port 853. This traffic is being used to establish a covert communication channel to bypass the company's security controls. Which technique is being employed?

Accepted Answer

DNS tunneling. DNS tunneling is the correct answer because the analyst observed encrypted DNS queries over TCP port 853 (DNS over TLS) being used to establish a covert communication channel. This technique encapsulates non-DNS data (e.g., command-and-control traffic) within DNS query and response packets, allowing the attacker to bypass security controls by hiding malicious traffic inside legitimate DNS traffic.

Answer

DNS poisoning

Answer

DNS amplification

Answer

DNS zone transfer

Question 13

A user at a branch office reports that they cannot connect to a file server at the main office. The branch office connects to the main office via a site-to-site VPN, and the VPN tunnel shows as active. The user can ping the main office's default gateway but not the file server. Which of the following should the technician check NEXT?

Accepted Answer

A) Check the routing tables on the branch router. The VPN tunnel is active and the user can ping the main office's default gateway, which confirms that Layer 3 connectivity exists across the tunnel. However, the file server is unreachable, indicating that the traffic destined for the file server's subnet is not being forwarded correctly. The next logical step is to check the routing tables on the branch router to ensure there is a specific route (or a default route) pointing to the VPN tunnel interface for the file server's network. Without this route, the branch router may be dropping the packets or sending them out the wrong interface.

Answer

B) Check the DNS resolution for the file server

Answer

C) Check the VPN encryption settings

Answer

D) Check the file server's antivirus software

Question 14

A user on VLAN 10 reports that they cannot access a file server on VLAN 20. The user can ping the default gateway (interface on the router) and other devices within VLAN 10. The router has an 'ip helper-address' for DHCP but no static routes or dynamic routing configured for inter-VLAN routing. The file server is correctly configured with an IP address in VLAN 20. What is the most likely cause of the communication failure?

Accepted Answer

The router does not have a route to the VLAN 20 subnet. The router has no route to the VLAN 20 subnet. Even though the router is directly connected to VLAN 20 via its subinterface, inter-VLAN routing requires the router to have a connected route in its routing table for that subnet. Without a route (either connected or static), the router cannot forward packets from VLAN 10 to VLAN 20. The 'ip helper-address' only forwards DHCP broadcasts, not general traffic.

Answer

The router is configured with an ACL that blocks traffic from VLAN 10 to VLAN 20

Answer

The switch port connecting the user to the network is configured with port security limiting MAC addresses

Answer

The file server has a firewall blocking ICMP ping requests

Question 15

An IPv6 address has the prefix 2001:db8::/32. Which of the following IP addresses belongs to the same prefix?

Accepted Answer

2001:db8:1::1. The prefix 2001:db8::/32 means the first 32 bits of the address must be 2001:0db8. Option A (2001:db8:1::1) expands to 2001:0db8:0001:0000:0000:0000:0000:0001, which matches the first 32 bits exactly, so it belongs to the same /32 prefix.

Answer

2001:db9::1

Answer

2001:db7::1

Answer

fe80::1

Question 16

Users on a VLAN report intermittent network disconnections lasting a few seconds. The network technician checks the switch and notices a high number of CRC errors on the port connecting to the core switch. The cable test passes. What is the most likely cause?

Accepted Answer

Duplex mismatch between the two switches. A duplex mismatch causes one side to send frames while the other is still transmitting, leading to collisions that are interpreted as CRC errors on the receiving interface. Since the cable test passes, the physical layer is fine, and the intermittent nature (lasting seconds) matches the symptom of a duplex mismatch where the half-duplex side backs off after collisions, causing brief outages.

Answer

STP reconvergence due to topology change

Answer

Broadcast storm caused by a loop

Answer

Faulty SFP transceiver on the core switch

Question 17

A company wants to ensure that only authorized users can access the internal network by requiring both a password and a one-time code from a mobile app. This is an example of:

Accepted Answer

Two-factor authentication. Two-factor authentication (2FA) requires exactly two distinct authentication factors from different categories: something you know (password) and something you have (one-time code from a mobile app). This matches the scenario precisely, as the password is a knowledge factor and the mobile app-generated code is a possession factor, satisfying the definition of 2FA.

Answer

Single sign-on

Answer

Biometric authentication

Answer

Multifactor authentication with three factors

Question 18

An NOC technician observes that the CPU usage on a core switch has been consistently above 90% for the past hour. Which SNMP operation should the technician use to monitor the CPU load over time with minimal network overhead?

Accepted Answer

SNMP TRAP. D is correct because SNMP TRAP is an unsolicited notification sent from the agent (the switch) to the NMS, which allows the NOC to receive CPU load alerts only when a threshold is exceeded, minimizing network overhead by avoiding continuous polling. In this scenario, the technician wants to monitor CPU load over time with minimal overhead, and traps provide event-driven reporting rather than periodic requests, reducing bandwidth and processing load on both the switch and the network.

Answer

SNMP GET

Answer

SNMP GETNEXT

Answer

SNMP WALK

Question 19

A security engineer is configuring a site-to-site VPN between two branch offices. The requirement is to encrypt all traffic between the two networks using IPsec. Which IPsec mode should be used to encrypt the entire IP packet including the original header?

Accepted Answer

Tunnel mode. Tunnel mode is the correct choice because it encrypts the entire original IP packet, including the original header, and then encapsulates it within a new IP header. This is required for site-to-site VPNs where the original source and destination IP addresses must be hidden or protected, and the new header is used for routing between the two VPN gateways.

Answer

Transport mode

Answer

AH only

Answer

ESP only

Question 20

An engineer is configuring a network and needs to understand the default behavior of an IPv6 node. When an IPv6 interface is initialized, which type of address is automatically assigned without any configuration server?

Accepted Answer

Link-local address. When an IPv6 interface is initialized, it automatically generates a link-local address (fe80::/10) using Stateless Address Autoconfiguration (SLAAC) without requiring any external server (e.g., DHCPv6). This address is mandatory for neighbor discovery and other link-local communication, allowing the node to operate on the local network segment immediately.

Answer

Global unicast address

Answer

Unique local address

Answer

Multicast address