A highly secure multitier application in AWS that includes S3, RDS, and some additional private links requires specific routing and bucket policies to keep the traffic safe. The reasons are as follows:

Specific routing policies are needed to ensure that the traffic between the tiers is routed through the private links, which provide secure and low-latency connectivity between AWS services and on-premises resources12.The private links can also prevent the exposure of the data and the application logic to the public internet12.

Bucket policies are needed to control the access to the S3 buckets that store the application data34.Bucket policies can specify the conditions under which the requests are allowed or denied, such as the source IP address, the encryption status, the request time, etc.34.Bucket policies can also enforce encryption in transit and at rest for the data in S334.

1: AWS PrivateLink

2: AWS PrivateLink FAQs

3: Using Bucket Policies and User Policies

4: Bucket Policy Examples

In the given scenario, an engineer is configuring a site-to-site IPsec VPN connection between an on-premises Cisco IOS XE router and AWS. The correct command to complete the tunnel configuration is ''tunnel source 20.20.20.21''.This command specifies the source IP address for the tunnel, which is essential for establishing a secure connection between two endpoints over the internet or another network1.Reference:

Configure IOS-XE Site-to-Site VPN Connection to Amazon Web Services - Cisco Community

[Security for VPNs with IPsec Configuration Guide, Cisco IOS XE Release 3S - Config

This command redistributes the routes learned from BGP AS100 into OSPF Area 1, which allows router R2 to advertise those routes to router R1 and connect the on-premises network to the cloud provider.The other options are incorrect because they either redistribute the wrong routes or use the wrong syntax5.

I hope this helps you understand the question and the answer. If you have any other questions or requests, please let me know. I am always happy to help.

The connectivity model that meets the requirements of high availability, redundancy, and low latency for the company's business applications isExpressRoute with private peering using SDCI.

ExpressRoute is a service that provides a dedicated, private, and high-bandwidth connection between the customer's on-premises network and Microsoft Azure cloud network1.

Private peering is a type of ExpressRoute circuit that allows the customer to access Azure services that are hosted in a virtual network, such as virtual machines, storage, and databases2.

SDCI (Secure Data Center Interconnect) is a Cisco solution that enables secure and scalable connectivity between multiple data centers and cloud providers, using technologies such as MPLS, IPsec, and SD-WAN3.

By using ExpressRoute with private peering and SDCI, the company can achieve the following benefits:

High availability: ExpressRoute circuits are redundant and resilient, and can be configured with multiple service providers and locations for failover and load balancing1.SDCI also provides high availability by using dynamic routing protocols and encryption mechanisms to ensure optimal and secure path selection3.

Redundancy: ExpressRoute circuits can be paired together to form a redundant connection between the customer's network and Azure4.SDCI also supports redundancy by allowing multiple connections between data centers and cloud providers, using different transport technologies and service levels3.

Low latency: ExpressRoute circuits offer lower latency than public internet connections, as they bypass the congestion and variability of the internet1.SDCI also reduces latency by using MPLS and SD-WAN to optimize the performance and quality of service for the traffic between data centers and cloud providers3.

What is Azure ExpressRoute?

Azure ExpressRoute peering

Cisco Secure Data Center Interconnect

ExpressRoute circuit and routing domain

A fully meshed topology with SD-WAN technology using dynamic routing and prioritized traffic for QoS meets the network architecture requirements of the company. A fully meshed topology provides high availability by eliminating single points of failure and allowing multiple paths between branch offices. SD-WAN technology enables multihoming by supporting multiple transport options, such as MPLS, internet, LTE, etc. SD-WAN also provides QoS by applying policies to prioritize traffic based on application, user, or network conditions. Dynamic routing allows the SD-WAN solution to adapt to changing network conditions and optimize the path selection for each traffic type. A fully meshed topology with SD-WAN technology can also support specific routing needs, such as segment routing, policy-based routing, or application-aware routing.Reference:

Designing and Implementing Cloud Connectivity (ENCC) v1.0

[Cisco SD-WAN Design Guide]

[Cisco SD-WAN Configuration Guide]