2.3 Azure ExpressRoute

📘Microsoft Azure Networking Solutions (AZ-700)

When designing Azure ExpressRoute, one of the most important tasks for the AZ-700 exam is understanding route advertisement configuration.

This topic focuses on:

How routes are advertised between on-premises networks and Azure
How BGP works in ExpressRoute
How to control which routes are advertised
How to prevent routing issues
How to design secure and optimized routing

This guide explains everything in simple and clear language so that both IT and non-IT learners can understand.

1. What is Route Advertisement in ExpressRoute?

Route advertisement means:

Informing another network which IP address ranges (prefixes) you can reach.

In Azure ExpressRoute, route advertisement happens using BGP (Border Gateway Protocol).

Your on-premises router advertises your internal IP address ranges to Azure.
Azure advertises Azure virtual network (VNet) address spaces to your on-premises network.

This allows both environments to communicate properly.

2. How BGP Works in ExpressRoute

ExpressRoute uses eBGP (External BGP) between:

Your on-premises edge router
Microsoft Enterprise edge router (MSEE)

Important concepts for the exam:

Concept	Meaning
ASN	Autonomous System Number used in BGP
Prefix	IP address range (example: 10.10.0.0/16)
AS Path	Path that a route takes through networks
BGP Weight / Local Preference	Determines preferred path

3. What Routes Are Advertised?

There are two directions:

A. Routes Advertised FROM On-Premises TO Azure

You advertise:

Your internal network prefixes
Data center subnets
Branch office networks (if connected)

Azure learns these routes and sends traffic to your network correctly.

B. Routes Advertised FROM Azure TO On-Premises

Azure advertises:

VNet address spaces
Connected VNet prefixes (if using VNet peering)
Azure services (if Microsoft peering is enabled)

4. Peering Type and Route Advertisement

ExpressRoute supports two main peering types:

1️⃣ Azure Private Peering

Used for:

VNet connectivity
Private IP communication

Routes advertised:

VNet prefixes
Peered VNet prefixes

2️⃣ Microsoft Peering

Used for:

Microsoft public services
Microsoft 365
Azure PaaS services

Routes advertised:

Microsoft public IP prefixes

You must configure route filters for Microsoft peering.

5. Route Advertisement Configuration Recommendations (Exam-Focused)

Now let’s focus on what the AZ-700 exam expects you to know when recommending a configuration.

✅ 1. Advertise Only Required Prefixes

Do NOT advertise unnecessary IP ranges.

Bad design:

Advertising 0.0.0.0/0 (default route) to Azure unnecessarily
Advertising overlapping address spaces

Good design:

Advertise only specific on-premises subnets

Why?

Prevents routing conflicts
Improves performance
Reduces security risk

✅ 2. Avoid Overlapping IP Address Spaces

Azure and on-premises networks must NOT have overlapping IP ranges.

Example problem:

On-premises: 10.0.0.0/16
Azure VNet: 10.0.0.0/16

This will break routing.

Always design non-overlapping IP ranges.

✅ 3. Use Route Summarization (Aggregation)

Instead of advertising many small subnets:

❌ 10.1.1.0/24
❌ 10.1.2.0/24
❌ 10.1.3.0/24

Better:

✅ 10.1.0.0/16

Benefits:

Reduces number of BGP routes
Improves router performance
Simplifies management

The exam may test:

When should you use route summarization?

Answer: When reducing BGP route count and simplifying routing tables.

✅ 4. Understand Route Limits

ExpressRoute circuits have BGP route limits.

If you exceed the route limit:

BGP session may drop
Routes may not be accepted

Always:

Monitor number of advertised routes
Use summarization if needed

✅ 5. Control Routing with BGP Attributes

You can influence routing behavior using:

Attribute	Purpose
AS Path	Determines path length
Local Preference	Preferred outbound path
MED	Suggests preferred inbound path

Used in scenarios like:

Active/active ExpressRoute circuits
Disaster recovery
Multi-region connectivity

✅ 6. Design for Redundancy

ExpressRoute provides:

Dual BGP sessions (primary and secondary)
Redundant Microsoft edge routers

Best practice:

Use active/active configuration
Ensure on-premises routers support redundancy
Advertise routes consistently from both sides

✅ 7. Default Route Advertisement (0.0.0.0/0)

You must carefully decide whether to advertise a default route to Azure.

If you advertise 0.0.0.0/0 to Azure:

All internet-bound Azure traffic may go to on-premises
This creates forced tunneling

This is useful when:

You want centralized security inspection
All outbound traffic must pass through on-prem firewall

But it increases:

Latency
Bandwidth usage

The exam may ask:

When should you advertise default route?

Answer:

When implementing forced tunneling.

✅ 8. Use Route Filters (Microsoft Peering)

For Microsoft peering:

You must:

Create route filters
Select Microsoft service communities

Without route filters:

No routes are advertised

✅ 9. Understand ExpressRoute FastPath Impact

If using ExpressRoute FastPath:

Traffic bypasses ExpressRoute gateway data plane
Route advertisement still happens via BGP
Improves performance

But:

Only supported on certain gateway SKUs

✅ 10. Cross-Region Route Advertisement

If using:

ExpressRoute Global Reach
Multiple VNets in different regions

Ensure:

Proper route propagation
No asymmetric routing
Consistent summarization

6. Common Exam Scenarios

Here are typical AZ-700 questions related to route advertisement:

Scenario 1:

Company has too many BGP routes and session drops.

Correct recommendation:
✔ Use route summarization
✔ Reduce number of prefixes

Scenario 2:

Company wants all Azure outbound traffic to pass through on-prem firewall.

Correct recommendation:
✔ Advertise 0.0.0.0/0 to Azure
✔ Implement forced tunneling

Scenario 3:

Company uses Microsoft peering but no routes appear.

Correct recommendation:
✔ Configure route filters

Scenario 4:

Two ExpressRoute circuits for redundancy.

Correct recommendation:
✔ Use BGP attributes to influence path selection
✔ Advertise consistent prefixes

7. Security Considerations

Route advertisement affects security.

Best practices:

Advertise only required prefixes
Avoid advertising internal management networks unnecessarily
Monitor BGP sessions
Use route filtering
Avoid overlapping IP ranges

8. Monitoring and Troubleshooting

You should know:

How to check BGP session status
How to view effective routes in Azure
How to verify advertised routes
How to monitor route count

Tools:

Azure Portal
PowerShell
Network Watcher

9. Key Exam Points to Remember

For AZ-700, remember:

✔ ExpressRoute uses BGP
✔ Advertise only necessary routes
✔ Avoid overlapping IP ranges
✔ Use summarization
✔ Understand route limits
✔ Know when to advertise default route
✔ Use route filters for Microsoft peering
✔ Configure redundancy properly
✔ Control routing using BGP attributes

Final Summary

Recommending a route advertisement configuration in Azure ExpressRoute means:

Designing which prefixes are advertised
Ensuring efficient routing
Preventing route conflicts
Supporting redundancy
Maintaining security
Controlling traffic flow

For the AZ-700 exam, you must understand:

BGP behavior
Prefix management
Route filtering
Default route advertisement
Redundancy design
Route summarization
Forced tunneling

If you understand these clearly, you will confidently answer any ExpressRoute route advertisement question in the exam.