In Azure networking, a routing domain is a segment of the network where all the resources follow the same routing logic and policies. Routing domains can be used to segment traffic, apply specific routing policies, and ensure network isolation. Each routing domain typically has its own set of routing tables that determine how traffic should be forwarded within and between domains.

Boundaries of a Routing Domain

The boundaries of a routing domain in Azure are typically defined by the following components:

1. Virtual Networks (VNets): VNets are fundamental building blocks in Azure networking, and each VNet can be considered a routing domain.
2. Subnets: Subnets within a VNet can also be routing domains, depending on the routing policies applied.
3. Network Security Groups (NSGs): NSGs can define boundaries by controlling inbound and outbound traffic at the subnet or network interface level.
4. Route Tables: Route tables attached to subnets define the specific routing within that subnet.

Inter-Routing-Domain Routing Types

Inter-routing-domain routing types in Azure include:

1. Peering:

• VNet Peering: Allows you to connect VNets within the same Azure region or across regions. VNets can communicate as if they were on the same network, but they maintain their own routing tables and boundaries.
• Global VNet Peering: Extends VNet peering to allow communication across different Azure regions.

1. VPN Gateways:

• Site-to-Site VPN: Connects an on-premises network to an Azure VNet over a secure VPN tunnel.
• VNet-to-VNet VPN: Connects two or more VNets using VPN gateways.

1. ExpressRoute:

• ExpressRoute Circuits: Provides a private connection between Azure datacenters and on-premises infrastructure, bypassing the public internet.
• ExpressRoute Direct: Offers dedicated, private connections for high-throughput and latency-sensitive applications.

1. Azure Virtual WAN:

• Hub-and-Spoke Architecture: Uses Virtual WAN hubs to connect VNets and on-premises networks in a centralized manner.
• Any-to-Any Connectivity: Facilitates any-to-any connectivity through Virtual WAN.

Translations on the Networking Routing Domain Boundaries

When traffic crosses the boundaries of routing domains, certain translations and adjustments might occur to ensure proper routing and security. These include:

1. NAT (Network Address Translation):

• Public IP Addresses: When traffic exits an Azure VNet to the internet, it typically uses a public IP address, requiring NAT.
• Azure Firewall and NAT Gateway: These services provide NAT capabilities for outbound traffic.

1. Route Propagation:

• BGP (Border Gateway Protocol): Used in ExpressRoute and VPN gateways to propagate routes between Azure and on-premises networks.

1. Access Control:

• NSGs and Application Security Groups (ASGs): Apply security rules to control traffic entering and exiting a routing domain.
• Azure Firewall: Acts as a centralized security policy enforcement point, applying rules to traffic crossing routing domain boundaries.

1. Uptime and Performance Management:

• Traffic Manager and Front Door: These services help manage traffic routing for high availability and performance, ensuring that traffic is directed to the most appropriate endpoints.

Summary

In Azure networking, routing domains help segment and control network traffic. Their boundaries are defined by components like VNets, subnets, NSGs, and route tables. Inter-routing-domain routing can be achieved through VNet peering, VPN gateways, ExpressRoute, and Virtual WAN. Translations and adjustments such as NAT, route propagation, and access control are necessary to manage traffic crossing these boundaries.