Routing Issues — Diagnosing and Resolving Network Routing Problems¶

Routing Issues occur when network devices cannot correctly determine the path that packets should take to reach their destination. These problems can result in unreachable hosts, packet loss, high latency, routing loops, asymmetric routing, intermittent connectivity, and application failures. Troubleshooting routing requires understanding routing tables, static routes, dynamic routing protocols, default gateways, route selection, and packet forwarding. Every Network Engineer, Linux Administrator, DevOps Engineer, SRE, Cloud Architect, and Kubernetes Administrator should be proficient in diagnosing routing issues.

Learning Path¶

Networking Mastery → Module 12: Network Troubleshooting → Lesson 6

Difficulty: Advanced

Reading Time: 220 Minutes

Course Progress

Course: Networking Mastery

Module: Network Troubleshooting

Lesson: 6 of 10

What You'll Learn¶

After completing this lesson, you'll be able to:

Understand routing failures
Diagnose missing routes
Identify routing loops
Troubleshoot asymmetric routing
Analyze routing tables
Verify dynamic routing protocols
Troubleshoot routing in cloud and Kubernetes environments

Prerequisites¶

Complete:

IPv4
Routing Fundamentals
Static Routing
Dynamic Routing
Ping
traceroute

Basic understanding of:

Routers
Routing Tables
Default Gateway

Why Do Routing Issues Occur?¶

Imagine users report:

Server

Cannot

Be

Reached

Ping fails.

DNS works.

The server is online.

Possible causes:

Missing Route
Wrong Gateway
Routing Loop
Incorrect Route Advertisement
Firewall
Asymmetric Routing

What is Routing?¶

Routing is:

The

Process

Of

Forwarding

Packets

Between

Networks

Routers examine the destination IP address and choose the best path.

Normal Packet Flow¶

Client

↓

Gateway

↓

Router

↓

Router

↓

Server

Every router forwards the packet based on its routing table.

Routing Decision Process¶

For every incoming packet:

Destination IP

↓

Routing Table Lookup

↓

Best Route

↓

Forward Packet

If no matching route exists:

Packet

Dropped

Routing Table¶

Every router maintains a routing table.

Example:

Destination

↓

Next Hop

↓

Interface

Example entry:

10.0.0.0/24

↓

192.168.1.1

↓

eth0

View Routing Table¶

Linux:

ip route

Older command:

route -n

Windows:

route print

Default Route¶

When no specific route matches:

0.0.0.0/0

↓

Default Gateway

Example:

ip route

Output:

default via 192.168.1.1

Missing Route¶

Example:

Client

↓

Router

↓

Unknown Network

↓

No Route

↓

Packet Dropped

Symptoms:

Destination Unreachable
Timeouts

Wrong Default Gateway¶

Example:

Client

↓

Incorrect Gateway

↓

Packet Lost

The packet never reaches the intended network.

Verify:

ip route

Routing Loop¶

Example:

Router A

↓

Router B

↓

Router A

↓

Router B

Packets circulate until:

TTL

Expires

Symptoms:

High Latency
Packet Loss
ICMP Time Exceeded

Detecting Routing Loops¶

Run:

traceroute destination

Repeated routers indicate a routing loop.

Example:

A

↓

B

↓

A

↓

B

Asymmetric Routing¶

Forward path:

Client

↓

Router A

↓

Server

Return path:

Server

↓

Router B

↓

Client

Different paths can cause:

Firewall Problems
Session Failures
Load Balancer Issues

Route Advertisement Problems¶

Dynamic routing protocols advertise routes.

Possible issues:

Missing Advertisement
Incorrect Network
Route Filtering
Authentication Failure

Affected protocols:

Open Shortest Path First (OSPF)
Enhanced Interior Gateway Routing Protocol (EIGRP)
Border Gateway Protocol (BGP)

Static Route Problems¶

Example:

ip route add 10.0.0.0/24 via 192.168.1.10

Incorrect next-hop:

Packets

Lost

Always verify:

Destination Network
Next Hop
Interface

Longest Prefix Match¶

Routers always choose:

Most

Specific

Route

Example:

10.0.0.0/8

and

10.1.1.0/24

Destination:

10.1.1.50

The router selects:

10.1.1.0/24

because it is the most specific match.

Dynamic Routing Issues¶

Possible causes:

Neighbor Down
Authentication Failure
Route Flapping
Incorrect Metrics
Area Misconfiguration
Network Statement Errors

Always verify routing protocol health.

Cloud Routing Issues¶

Cloud platforms use virtual route tables.

Examples:

AWS Route Tables
Azure Route Tables
Google Cloud Routes

Verify:

Subnet Association
Internet Gateway
NAT Gateway
VPN Gateway
Peering Routes

Kubernetes Routing Issues¶

Verify:

Container Network Interface (CNI)
Pod CIDR
Node CIDR
Service CIDR
kube-proxy
Network Policies

Common commands:

kubectl get nodes

kubectl get pods -o wide

Enterprise Routing Workflow¶

Client

↓

Gateway

↓

Core Router

↓

Distribution Router

↓

Internet

↓

Server

Each hop must have a valid route to the destination.

Troubleshooting Workflow¶

Step 1

Verify connectivity.

ping

Step 2

View the path.

traceroute

Step 3

Check routing table.

ip route

Step 4

Capture traffic.

tcpdump

Step 5

Analyze packets.

Wireshark

Common Routing Problems¶

Problem	Symptom
Missing Route	Destination Unreachable
Wrong Gateway	No Connectivity
Routing Loop	TTL Expired
Asymmetric Routing	Session Problems
Incorrect Route Metric	Suboptimal Path
Route Flapping	Intermittent Connectivity

CLI Examples¶

View routing table.

ip route

Show interface addresses.

ip addr

Trace packet path.

traceroute example.com

Capture routing traffic.

sudo tcpdump icmp

Test gateway.

ping 192.168.1.1

Hands-on Lab¶

Task 1¶

Display the routing table.

ip route

Task 2¶

Identify:

Default Gateway
Local Routes
Static Routes

Task 3¶

Run:

traceroute google.com

Identify each router.

Task 4¶

Remove a static route in a lab environment.

Observe:

Packet Loss
Connectivity Failure

Restore the route and verify recovery.

Task 5¶

Capture packets while testing routing.

sudo tcpdump icmp

Task 6¶

Compare routing tables on two Linux servers.

Identify missing routes.

Task 7¶

Verify routing in a Kubernetes cluster by checking Pod-to-Pod communication across nodes.

Task 8¶

Draw the packet path:

Client

↓

Gateway

↓

Core Router

↓

ISP

↓

Internet

↓

Cloud Router

↓

Server

Explain how each router selects the next hop using the routing table.

Production Troubleshooting¶

Problem:

Server

Unreachable

Check:

Gateway
Routing Table
Static Routes
Dynamic Routes
Route Advertisements
Firewall
Network Address Translation (NAT)
Packet Capture

Workflow:

Ping

↓

Traceroute

↓

Routing Table

↓

Packet Capture

↓

Root Cause

Static Routing vs Dynamic Routing¶

Static Routing	Dynamic Routing
Manual Configuration	Automatic Learning
Simple Networks	Large Networks
Low Overhead	Protocol Overhead
Manual Updates	Automatic Updates
Limited Scalability	Highly Scalable

Routing Issues vs DNS Issues¶

Routing Issue	DNS Issue
IP Unreachable	Hostname Fails
Ping by IP Fails	Ping by IP Works
traceroute Shows Failure	DNS Query Fails
Route Table Problem	Resolver Problem
Network Layer	Application Support Layer

Common Mistakes¶

❌ Assuming every connectivity problem is a routing issue.

✅ Verify DNS and firewall configuration first.

❌ Ignoring the default route.

✅ Confirm the default gateway is correct.

❌ Overlooking asymmetric routing.

✅ Verify both forward and return paths.

❌ Trusting traceroute alone.

✅ Validate routing tables and packet captures.

❌ Forgetting longest prefix match.

✅ Check for more specific routes overriding expected paths.

Best Practices¶

Verify the routing table before changing routes.
Check the default gateway first.
Use traceroute to identify routing failures.
Document static routes.
Monitor routing protocol neighbors.
Validate cloud route tables after infrastructure changes.
Capture packets when routing behavior is unclear.
Test both forward and return paths.

Interview Questions¶

Beginner¶

What is routing?
What is a routing table?
What is a default route?
What is a routing loop?

Intermediate¶

Explain longest prefix matching.
What is asymmetric routing?
How do you troubleshoot missing routes?
Compare static and dynamic routing.

Architect Level¶

Design a routing troubleshooting workflow for an enterprise network.
Explain how routing loops occur and how to identify them.
How would you troubleshoot intermittent routing problems across multiple cloud regions?

Summary¶

In this lesson, you learned:

Routing Issues
Routing Tables
Default Routes
Missing Routes
Routing Loops
Asymmetric Routing
Longest Prefix Match
Static and Dynamic Routing
Cloud Routing
Kubernetes Routing

Routing is the foundation of IP networking. When routing problems occur, applications may experience connectivity failures, packet loss, or high latency even though the underlying hosts are healthy. By systematically verifying routing tables, gateways, traceroute output, and packet captures, engineers can efficiently isolate and resolve routing issues across enterprise networks, cloud platforms, and Kubernetes clusters.

Key Takeaways¶

Every router forwards packets using its routing table.
The default route is used when no more specific route exists.
Longest prefix match determines the preferred route.
Common routing problems include missing routes, routing loops, and asymmetric routing.
Use Ping, traceroute, ip route, tcpdump, and Wireshark together for comprehensive troubleshooting.
Cloud and Kubernetes environments rely on correct routing just as traditional networks do.

What's Next?¶

MTU Problems

In the next lesson, you'll learn about MTU Problems.

You'll explore:

What MTU is
Maximum Transmission Unit
IP Fragmentation
Path MTU Discovery (PMTUD)
Jumbo Frames
MTU Mismatch
Production MTU Troubleshooting

By the end of the lesson, you'll understand how MTU affects network performance and how to diagnose fragmentation, black-hole connections, and MTU mismatches in enterprise, cloud, and Kubernetes environments.