1.3 Design and Implement VNet Connectivity and Routing
📘Microsoft Azure Networking Solutions (AZ-700)
1. What Is a Route Table in Azure?
In Azure, a route table is used to control how network traffic moves inside and outside a virtual network (VNet).
- It contains routes
- Each route tells Azure:
- Where traffic should go
- Which next hop should be used
Azure automatically creates system routes, but custom route tables allow you to override or extend this behavior.
2. What Does “Associate a Route Table with a Subnet” Mean?
A route table does nothing by itself.
It only starts working after it is associated with a subnet.
Important meaning:
- You do not associate route tables with VNets
- You associate route tables with subnets
- All resources inside that subnet must follow the routes in that route table
3. Why Do We Associate Route Tables with Subnets?
Associating a route table with a subnet allows you to:
- Control traffic flow for specific workloads
- Force traffic through:
- Firewalls
- Network virtual appliances (NVAs)
- Gateways
- Block or redirect traffic
- Apply different routing rules to different subnets
Example in an IT environment:
- Web servers in one subnet
- Application servers in another subnet
- Each subnet needs different traffic rules
- You associate different route tables to each subnet
4. Default Routing vs Custom Routing
Default (System Routes)
Azure automatically provides:
- Routes within the VNet
- Routes to the internet
- Routes to on-premises (if connected)
These routes cannot be deleted.
Custom Route Tables
When you associate a custom route table:
- Azure still keeps system routes
- Custom routes take priority
5. How Route Table Association Works
Key Rule (Exam Important):
One subnet can have only one route table associated with it
But:
- One route table can be associated with multiple subnets
Effect:
- All VMs, containers, and services inside that subnet follow the same routing rules
6. Steps to Associate a Route Table with a Subnet (Conceptual)
Step 1: Create a Route Table
- Create a user-defined route (UDR)
- Add routes if required
Step 2: Open the Route Table
- Go to Subnets section
Step 3: Select a Subnet
- Choose:
- VNet
- Subnet
Step 4: Save the Association
- The route table is now active
Once saved, routing behavior changes immediately
7. Common Next Hop Types Used After Association
When associating a route table, routes inside it can point traffic to:
| Next Hop Type | Purpose |
|---|---|
| Virtual appliance | Send traffic to firewalls or NVAs |
| Internet | Force traffic directly to internet |
| Virtual network gateway | Send traffic to VPN or ExpressRoute |
| VNet peering | Route traffic between VNets |
| None | Block traffic |
8. Important Rules and Limitations (Exam Favorites)
Association Rules
- Route tables are associated only at subnet level
- Not associated at:
- VM level
- NIC level
- VNet level
Priority Rules
- User-defined routes > System routes
- More specific routes > Less specific routes
Regional Rule
- Route table and subnet must be in the same region
9. What Happens If No Route Table Is Associated?
If a subnet has no custom route table:
- Azure uses only system routes
- Traffic follows default Azure routing behavior
10. Real IT Scenarios Where Association Is Used
Scenario 1: Firewall Enforcement
- Subnet with application servers
- Route table sends all outbound traffic to a firewall VM
- Route table is associated with that subnet
Scenario 2: Forced Tunneling
- Subnet needs internet traffic routed through on-premises network
- Route table is associated with the subnet to override internet routing
Scenario 3: Traffic Isolation
- Certain subnets should not reach the internet
- Route table associated with subnet uses Next hop = None
11. Exam Tips and Keywords to Remember
Memorize These Statements:
- Route tables are associated with subnets
- One subnet = one route table
- One route table = many subnets
- Custom routes override system routes
- Route table becomes active only after association
Common Exam Question Pattern:
“Traffic is not flowing as expected even though routes exist.”
Correct thinking:
- Check whether the route table is associated with the subnet
12. Summary (Quick Revision)
- A route table controls network traffic paths
- It must be associated with a subnet to work
- All resources in that subnet follow the same routes
- Custom routes override Azure system routes
- Subnet association is required for routing changes
