ANS-C01 Hard Questions — 25 Difficult Questions

Question 1

A company is designing a network security architecture for a multi-account environment using AWS Transit Gateway. The security team needs to centralize inspection of all traffic between VPCs using a third-party firewall appliance in a shared services VPC. What is the most scalable and highly available design?

Accepted Answer

Create a Transit Gateway with separate route tables for each VPC and a shared route table for the inspection VPC. Use Gateway Load Balancer to distribute traffic to a fleet of firewall instances.. Option B is correct because it combines AWS Transit Gateway with separate route tables per VPC and a shared inspection route table, enabling centralized traffic steering to a fleet of firewall instances behind a Gateway Load Balancer (GWLB). GWLB transparently distributes traffic across multiple firewall instances at the network layer (GENEVE protocol), providing both horizontal scalability and high availability without introducing a single point of failure or requiring complex VPC peering meshes.

Answer

Use VPC peering to connect all VPCs to the shared services VPC and route traffic through the firewall.

Answer

Attach all VPCs to a Transit Gateway and configure a Network Load Balancer in the shared services VPC to distribute traffic to firewall instances.

Answer

Attach all VPCs to a Transit Gateway and use a single firewall instance with elastic network interfaces in each VPC.

Question 2

A company uses AWS Transit Gateway to connect multiple VPCs and on-premises networks via AWS Site-to-Site VPN. The security team wants to inspect all traffic between VPCs using a centralized inspection VPC with third-party firewall appliances. Which architecture ensures that traffic from VPC A to VPC B is routed through the inspection VPC?

Accepted Answer

Attach all VPCs to a Transit Gateway. Create separate route tables: one for inspection VPC attachments and one for others. In VPC A's route table, route to VPC B via the inspection VPC attachment.. Option A is correct because it uses separate Transit Gateway route tables to isolate the inspection VPC and other VPCs. By configuring the route table for VPC A to point to VPC B's CIDR via the inspection VPC attachment, all inter-VPC traffic is forced through the centralized firewall appliances. This leverages Transit Gateway's ability to route traffic between attachments based on route table associations and propagations, ensuring traffic flows through the inspection VPC without requiring VPC peering or additional per-VPC firewalls.

Answer

Create a VPC peering connection between VPC A and VPC B, and attach firewall appliances in both VPCs.

Answer

Use AWS PrivateLink to create VPC endpoints in VPC A and VPC B, and route traffic through the firewall VPC.

Answer

Deploy AWS Network Firewall in each VPC and configure VPC route tables to send traffic to the firewall endpoint.

Question 3

A global e-commerce company uses a hub-and-spoke network topology with a transit VPC in us-east-1. Each spoke VPC has an AWS Site-to-Site VPN connection to its respective on-premises office. Users report intermittent connectivity issues when accessing a web application hosted in a spoke VPC in eu-west-1 from an on-premises office in ap-southeast-1. The network engineer checks the VPN connection and finds it is up. Which design change would MOST likely resolve the issue?

Accepted Answer

Enable jumbo frames on the transit VPC's EC2-based virtual appliances.. The issue is intermittent connectivity between an on-premises office in ap-southeast-1 and a spoke VPC in eu-west-1, traversing a transit VPC in us-east-1. The VPN is up, so the problem is likely packet fragmentation or MTU mismatch across the long-haul path. Enabling jumbo frames on the transit VPC's EC2-based virtual appliances (e.g., firewall or router instances) increases the maximum transmission unit, reducing fragmentation and improving performance for large packets, which is a common cause of intermittent issues in hub-and-spoke topologies.

Answer

Change the VPN connection from static to dynamic routing.

Answer

Enable BGP route propagation on the transit VPC's route tables.

Answer

Deploy a new transit VPC in eu-west-1.

Question 4

A company has a Direct Connect connection with a private VIF attached to a virtual private gateway. The VPC has multiple subnets in two Availability Zones. The on-premises network advertises a default route (0.0.0.0/0) via BGP. The company wants all internet-bound traffic from the VPC to go through the on-premises network. Which THREE actions are required to achieve this?

Accepted Answer

Add a default route pointing to the virtual private gateway in the private subnet route tables.. Option B is correct because adding a default route (0.0.0.0/0) pointing to the virtual private gateway in the private subnet route tables ensures that all internet-bound traffic from those subnets is forwarded over the Direct Connect private VIF to the on-premises network. This leverages the BGP-advertised default route from the on-premises network, allowing the company to enforce its security and routing policies for internet access.

Answer

Deploy a NAT Gateway in a public subnet.

Answer

Attach an internet gateway to the VPC.

Question 5

A network engineer is troubleshooting connectivity between two VPCs (VPC-A and VPC-B) connected via a VPC peering connection. Both VPCs have CIDR blocks: VPC-A = 10.0.0.0/16, VPC-B = 10.1.0.0/16. An EC2 instance in VPC-A (10.0.1.10) cannot ping an EC2 instance in VPC-B (10.1.1.10). Security groups and NACLs allow all traffic. The route tables are configured as follows: In VPC-A, a route to 10.1.0.0/16 via the peering connection. In VPC-B, a route to 10.0.0.0/16 via the peering connection. What is the most likely cause?

Accepted Answer

There is an intermediate VPC or on-premises network that routes traffic incorrectly due to the lack of transitive routing.. VPC peering connections do not support transitive routing. Even though the route tables in VPC-A and VPC-B correctly point to each other via the peering connection, if there is an intermediate VPC or on-premises network involved in the path, traffic cannot be forwarded through that intermediate point. The ping fails because the peering connection is a direct, one-to-one link and does not allow routing through a third network.

Answer

The VPCs have overlapping CIDR blocks.

Answer

Security groups are blocking ICMP traffic.

Answer

The route tables are missing routes to the peering connection.

Question 6

A company is using AWS Transit Gateway to interconnect multiple VPCs and on-premises networks. The network engineer needs to ensure that traffic between VPC A and VPC B follows a specific path through a Network Virtual Appliance (NVA) in VPC C. Which TWO actions should the engineer take?

Accepted Answer

Add a static route in the Transit Gateway route table for VPC A's CIDR pointing to the VPC B attachment with a blackhole.. Option B is correct because adding a static route in the Transit Gateway route table for VPC A's CIDR pointing to the VPC B attachment with a blackhole forces traffic destined to VPC B to be dropped at the Transit Gateway, preventing direct routing. Option D is correct because adding a specific route for VPC B's CIDR in the Transit Gateway route table used by VPC A, pointing to the VPC C attachment, ensures traffic from VPC A to VPC B is forwarded through the NVA in VPC C, enforcing the desired inspection path.

Answer

Associate VPC A's and VPC B's route tables with a different Transit Gateway route table that has a default route pointing to the VPC C attachment.

Answer

Enable route propagation for VPC A and VPC B attachments in the same Transit Gateway route table.

Answer

Create a VPC peering connection between VPC A and VPC B.

Question 7

A multinational corporation is using AWS Transit Gateway to connect multiple VPCs and on-premises networks via AWS Direct Connect and VPN. The network team is experiencing asymmetric routing for traffic between two VPCs that both have routes to the same on-premises network. Which feature should the team implement to resolve this issue?

Accepted Answer

Use AS_PATH prepending on the BGP advertisements from the on-premises router to influence route preference.. AS_PATH prepending allows the on-premises router to artificially lengthen the AS_PATH for specific BGP routes, making those routes less preferred. This influences route selection in the Transit Gateway and VPC route tables, ensuring that traffic from each VPC takes a consistent path and eliminating asymmetric routing.

Answer

Deploy a NAT Gateway in each VPC to force symmetric traffic.

Answer

Create a Transit Gateway peering attachment between the two VPCs.

Answer

Enable VPC Flow Logs to detect and alert on asymmetric flows.

Question 8

A company is using AWS Transit Gateway to connect multiple VPCs and on-premises networks. The network team observes that traffic between two VPCs (VPC A and VPC B) is not being forwarded correctly. The transit gateway route table is configured with static routes for the VPC CIDRs. Which THREE steps should the engineer take to troubleshoot this issue? (Choose THREE.)

Accepted Answer

Verify that the transit gateway route table contains the CIDR blocks of both VPCs.. Option A is correct because the transit gateway route table must contain the CIDR blocks of both VPCs for traffic to be forwarded between them. Without these static routes, the transit gateway has no destination to route the traffic, causing it to be dropped. Verifying the route table entries ensures the necessary paths exist.

Answer

Check the Direct Connect virtual interface status.

Answer

Verify that the NAT Gateway in each VPC is properly configured.

Question 9

A global e-commerce company is migrating to AWS and plans to use a hub-and-spoke topology with AWS Transit Gateway. The network team wants to ensure high availability for the connection between the hub VPC and the on-premises data center using AWS Direct Connect with multiple virtual interfaces (VIFs). They need to be able to fail over quickly with minimal packet loss. Which design should meet these requirements?

Accepted Answer

Provision two Direct Connect connections, each with multiple private VIFs, and attach them to the same transit gateway. Use BFD to detect failures and rely on ECMP routing across the VIFs.. Option C is correct because it provides true high availability and fast failover by using two separate Direct Connect connections, each with multiple private VIFs, attached to the same transit gateway. BFD (Bidirectional Forwarding Detection) enables sub-second failure detection, and ECMP (Equal-Cost Multi-Path) routing across the VIFs allows active-active load balancing, minimizing packet loss during failover. This design eliminates single points of failure and meets the requirement for rapid convergence with minimal disruption.

Answer

Provision one Direct Connect connection with a single private VIF, and enable BFD on the VIF to detect failures quickly.

Answer

Provision one Direct Connect connection with two private VIFs, and use a second Direct Connect connection as backup with a single VIF. Configure route tables to prefer the primary.

Answer

Provision two Direct Connect connections, each with a single private VIF, and use AWS Site-to-Site VPN as a backup for each.

Question 10

A company has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to initiate outbound connections to the internet for software updates. The company wants to ensure that all outbound traffic goes through a single, highly available IP address for whitelisting purposes. Which solution should be used?

Answer

Launch a NAT instance in a public subnet and assign an Elastic IP. Configure the private subnet route table to point to the NAT instance.

Answer

Create an internet gateway and attach it to the private subnet, then configure the route table.

Answer

Create a VPC endpoint for Amazon S3 and route software update traffic through that endpoint.

Answer

Create a NAT gateway in each Availability Zone and assign the same Elastic IP to both.

Question 11

A company is designing a network security architecture for a VPC that hosts a multi-tier application. The security team requires that the web tier can only be accessed from the internet, the application tier can only be accessed from the web tier, and the database tier can only be accessed from the application tier. Additionally, the team needs to ensure that no traffic can bypass these controls. Which THREE actions should the team take?

Accepted Answer

Use security group rules that reference other security groups as sources.. Security group rules can reference other security groups as sources, which allows the web tier security group to permit traffic only from the application tier security group, and the application tier security group to permit traffic only from the database tier security group. This creates a logical, stateful firewall that enforces the required traffic flow between tiers without relying on IP addresses, ensuring that no traffic can bypass the controls even if subnet configurations change.

Answer

Enable VPC Flow Logs on all subnets and send logs to Amazon S3.

Answer

Attach an internet gateway to the VPC and add a default route to the internet gateway in the web tier subnet's route table.

Question 12

A company is migrating its on-premises data center to AWS and wants to use AWS Direct Connect for private connectivity. The network team plans to advertise the company's public IP prefixes to AWS via BGP. Which AWS resource must be configured to allow advertisement of these prefixes?

Accepted Answer

A public virtual interface on the Direct Connect connection. To advertise public IP prefixes over AWS Direct Connect, you must use a public virtual interface (VIF). A public VIF allows you to reach public AWS endpoints (such as S3, DynamoDB, or any public IP range) over the Direct Connect link, and it supports BGP peering where you can advertise your public IP prefixes to AWS. Private VIFs are only for private VPC connectivity and do not accept customer-originated public prefix advertisements.

Answer

A VPN CloudHub topology

Answer

A private virtual interface on the Direct Connect connection

Answer

A transit virtual interface on the Direct Connect connection

Question 13

A company is migrating a legacy on-premises application to AWS. The application uses a large number of short-lived TCP connections and requires low latency. The network team is considering using either a Network Load Balancer (NLB) or a Gateway Load Balancer (GWLB). Which of the following is a key advantage of using NLB over GWLB for this use case?

Accepted Answer

NLB preserves the client source IP, while GWLB does not by default.. For an application using a large number of short-lived TCP connections requiring low latency, NLB is the optimal choice because it operates at Layer 4 and preserves the client source IP address by default, which is critical for applications that need to log or process the original client IP. GWLB, by design, uses GENEVE encapsulation (UDP port 6081) to tunnel traffic to backend appliances, which replaces the client source IP with the NLB's private IP unless explicit configuration (e.g., proxy protocol) is used. This makes NLB the better fit for preserving source IP without additional overhead.

Answer

NLB can forward traffic to third-party virtual appliances.

Answer

NLB can handle higher throughput than GWLB.

Answer

NLB supports TLS termination, while GWLB does not.

Question 14

A company is designing a hybrid network using AWS Transit Gateway. The company has three VPCs (VPC-A, VPC-B, VPC-C) all attached to the same Transit Gateway. The on-premises network connects to the Transit Gateway via a Direct Connect gateway. The company needs to ensure that VPC-C can communicate with the on-premises network but not with VPC-A or VPC-B. Which TWO actions should the network engineer take?

Accepted Answer

Associate the VPC-C route table with the Direct Connect gateway attachment.. Option B is correct because associating the VPC-C route table with the Direct Connect gateway attachment allows VPC-C to exchange routes with the on-premises network via the Transit Gateway, enabling communication. Option D is correct because creating a separate Transit Gateway route table for VPC-C and not propagating VPC-A and VPC-B routes ensures that VPC-C has no routes to those VPCs, effectively isolating it from them at the network layer.

Answer

Use security groups on VPC-C to deny traffic from VPC-A and VPC-B.

Answer

Establish VPC peering connections between VPC-C and VPC-A, and VPC-C and VPC-B.

Answer

Apply a network ACL on VPC-C subnets to block traffic from VPC-A and VPC-B.

Question 15

A company has a Direct Connect connection with two private virtual interfaces (VIFs) to two different VPCs in the same AWS Region. The company wants to use AWS Transit Gateway to simplify connectivity between these VPCs and their on-premises network. Which steps are required to integrate the existing Direct Connect connection with Transit Gateway?

Accepted Answer

Create a Direct Connect gateway, associate the existing VIFs, and attach the Direct Connect gateway to the Transit Gateway.. Option D is correct because AWS Direct Connect private VIFs cannot be attached directly to a Transit Gateway. Instead, you must create a Direct Connect Gateway, associate the existing private VIFs with it, and then attach the Direct Connect Gateway to the Transit Gateway. This architecture allows the Transit Gateway to route traffic between the on-premises network (via the Direct Connect connection) and the attached VPCs, while also enabling inter-VPC routing through the Transit Gateway.

Answer

Set up a VPN connection over the Direct Connect link and attach the VPN to the Transit Gateway.

Answer

Attach the existing private VIFs directly to the Transit Gateway.

Answer

Create a new private VIF on the Direct Connect connection and attach it to the Transit Gateway.

Question 16

A company runs a multi-tier web application on AWS. The web servers in public subnets need to send traffic to the application servers in private subnets. The application servers must only accept traffic from the web servers. Both tiers are in the same VPC. Which design meets these requirements without introducing a single point of failure or unnecessary complexity?

Accepted Answer

Place the web servers in a public subnet with a security group that allows inbound from 0.0.0.0/0 on port 443. Place the app servers in a private subnet with a security group that allows inbound from the web server security group on the application port.. Option A is correct because it uses security group referencing, which allows the app servers' security group to dynamically allow traffic from any instance associated with the web servers' security group, regardless of IP address changes. This design avoids a single point of failure by not introducing any load balancer or gateway, and it minimizes complexity by leveraging native VPC security group behavior within the same VPC.

Answer

Place the web servers and app servers in the same private subnet behind an internal Network Load Balancer. Route web traffic through the NLB.

Answer

Place the web servers in a public subnet with an Internet Gateway. Place the app servers in a private subnet with a NAT Gateway for outbound traffic. Use NACLs to allow inbound from the web subnet CIDR.

Answer

Place the web servers in a public subnet with a NACL allowing inbound on port 443. Place the app servers in a private subnet with a NACL allowing inbound from the web subnet CIDR on the application port.

Question 17

A company is designing a multi-Region architecture using AWS Transit Gateway and Direct Connect. They have VPCs in us-east-1 and eu-west-1, each with an attached Transit Gateway. The Direct Connect gateway is associated with the Transit Gateway in us-east-1. They need to enable communication between VPCs across Regions using the Direct Connect gateway. What is the correct design to achieve this?

Accepted Answer

Create a Transit Gateway peering attachment between the Transit Gateways in us-east-1 and eu-west-1, and route traffic over the Direct Connect gateway via the peering.. Option C is correct because Transit Gateway peering attachments allow transitive routing between Transit Gateways in different AWS Regions. By establishing a peering attachment between the Transit Gateways in us-east-1 and eu-west-1, traffic from the VPC in eu-west-1 can be routed through its local Transit Gateway, across the peering attachment to the Transit Gateway in us-east-1, and then over the Direct Connect gateway to on-premises networks. This design enables cross-region VPC communication via Direct Connect without requiring a single Transit Gateway or direct VPC peering.

Answer

Create a single Transit Gateway that spans both Regions and attach VPCs in both Regions.

Answer

Use VPC peering between the VPCs and route Direct Connect traffic through the VPC peering connection.

Answer

Associate the Direct Connect gateway with both Transit Gateways in us-east-1 and eu-west-1.

Question 18

A company has a VPC with multiple subnets spanning three Availability Zones. They have deployed an Application Load Balancer (ALB) in the VPC and need to ensure high availability and scalability for a web application. Which of the following are design considerations for implementing the ALB in this environment? (Choose THREE.)

Accepted Answer

Enable the ALB in at least two Availability Zones to provide high availability.. Option A is correct because AWS requires an Application Load Balancer to be enabled in at least two Availability Zones to achieve high availability. If one AZ fails, the ALB can continue routing traffic to healthy targets in the remaining AZs, ensuring fault tolerance and compliance with the ALB's regional service model.

Answer

Configure the ALB to use subnets that span multiple Availability Zones.

Answer

Use VPC peering to connect the ALB to subnets in other VPCs.

Question 19

A company has a Direct Connect connection with a private VIF to a VPC. The on-premises router is advertising a default route (0.0.0.0/0) via BGP. The VPC has an internet gateway attached, and the route table has a default route to the internet gateway. The network team notices that traffic from on-premises to the internet is not working as expected. What is the most likely cause?

Accepted Answer

The on-premises router is advertising a default route, causing a routing conflict.. The on-premises router advertising a default route (0.0.0.0/0) via BGP over the private VIF causes a routing conflict because the VPC route table already has a default route pointing to the internet gateway. When traffic from on-premises destined for the internet enters the VPC, it matches the more specific BGP-learned default route and is forwarded back toward the on-premises router (or black-holed), rather than being sent to the internet gateway. This creates a routing loop or asymmetric routing, breaking internet connectivity from on-premises.

Answer

The BGP session is not established.

Answer

The VPC route table does not have a route to the internet gateway for the on-premises traffic.

Answer

The private VIF is associated with the wrong VLAN.

Question 20

A company is using an AWS Transit Gateway to connect multiple VPCs and on-premises networks via Direct Connect. The network team notices that traffic from an on-premises network (CIDR 172.16.0.0/12) to a VPC (CIDR 10.0.0.0/16) is being dropped. The transit gateway route table shows a static route for 10.0.0.0/16 pointing to the VPC attachment. The Direct Connect virtual interface (VIF) is associated with the transit gateway and the on-premises router is advertising 172.16.0.0/12 via BGP. What is the most likely cause of the traffic being dropped?

Accepted Answer

The transit gateway route table does not have a route for the on-premises CIDR (172.16.0.0/12) pointing to the Direct Connect attachment.. The transit gateway route table must contain a route for the on-premises CIDR (172.16.0.0/12) pointing to the Direct Connect attachment for return traffic to be forwarded correctly. Without this route, the transit gateway has no path for traffic destined to the on-premises network, causing it to be dropped. The static route for 10.0.0.0/16 only handles traffic toward the VPC, not the return direction.

Answer

The VPC has a route that points to the transit gateway for the on-premises CIDR, causing asymmetric routing.

Answer

The Direct Connect VIF is not configured with BFD.

Answer

The VPC route table does not have a route for the on-premises CIDR pointing to the transit gateway.