Routing table errors

5.3 Given a scenario, troubleshoot common issues with network services

Routing Issues

📘CompTIA Network+ (N10-009)

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1. What is a Routing Table?

A routing table is like a map inside a router or layer 3 switch that tells the device where to send network traffic. Every router uses this table to decide the best path to reach a specific network.

A routing table typically contains:

• Destination network: The network you want to reach (e.g., 192.168.10.0/24)
• Subnet mask: Defines the size of the network (e.g., /24 = 255.255.255.0)
• Next hop: The IP address of the next device the packet should go through
• Interface: The local port on the router used to forward the packet
• Metric: A value used to choose the best route when there are multiple options

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2. What Are Routing Table Errors?

Routing table errors occur when the information in the routing table is incorrect or incomplete. This can prevent traffic from reaching its destination, causing network problems.

Common causes include:

1. Incorrect or missing routes
   • A network might not be listed in the routing table.
   • Example: The router doesn’t know how to reach 10.1.1.0/24 because the route is missing.
2. Incorrect next-hop IP addresses
   • If the next-hop IP is wrong, the router sends packets to the wrong device.
3. Incorrect subnet mask
   • A wrong mask can cause traffic to be sent outside the intended network or block traffic that should be allowed.
4. Routing loops
   • When two routers point to each other incorrectly, packets can keep bouncing back and forth.
5. Conflicting routes
   • Multiple routes exist for the same destination, but with the same metric. The router may choose the wrong path.

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3. Symptoms of Routing Table Errors

When routing table errors exist, you may notice:

• Cannot reach certain networks (ping fails, SSH fails to remote device)
• Intermittent connectivity (sometimes traffic works, sometimes not)
• Slow network performance (because packets are taking longer, inefficient paths)
• Routing loops (network congestion, high CPU on routers, duplicate packets)

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4. How to Troubleshoot Routing Table Errors

Here’s a step-by-step approach for troubleshooting:

Step 1: Check the routing table

• Use commands to view routing tables:
  • Cisco IOS: show ip route
  • Windows: route print
  • Linux: ip route or netstat -r
• Look for missing or incorrect routes.

Step 2: Check network connectivity

• Use ping to test if a network is reachable.
• Use tracert (Windows) or traceroute (Linux) to see the path the packet takes.
• If the packet doesn’t reach the destination, check the router’s routing table for missing paths.

Step 3: Verify IP addresses and masks

• Ensure the next-hop IP is correct.
• Ensure the subnet mask matches the network design.

Step 4: Check for routing loops or conflicts

• Look for duplicate routes with the same metric.
• Check if a static route is conflicting with a dynamic routing protocol.

Step 5: Update or correct routes

• Add missing static routes: ip route 192.168.10.0 255.255.255.0 10.1.1.2
• Correct wrong next-hop IPs or subnet masks.
• If using a dynamic routing protocol (RIP, OSPF, EIGRP), ensure neighbors are correctly configured.

Step 6: Test again

• After changes, retest connectivity with ping and traceroute.

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5. Examples of Routing Table Errors in IT

1. Static route missing:
   • A server at 10.1.5.50 cannot reach 10.2.0.0/24 because the router has no route for that network.
2. Wrong next-hop IP:
   • Router sends traffic for 192.168.20.0/24 to 192.168.30.1 instead of 192.168.20.1, so packets get lost.
3. Incorrect subnet mask:
   • Router thinks 192.168.10.0/24 is 192.168.0.0/16, sending traffic to the wrong network.
4. Routing loop in dynamic routing:
   • OSPF routers incorrectly advertise routes, causing packets to circulate endlessly.

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6. Key Points for the Exam

• Know what a routing table contains: destination, mask, next hop, interface, metric.
• Understand static vs dynamic routes: static routes are manually configured; dynamic routes are learned automatically.
• Be able to identify symptoms of routing table errors.
• Know basic troubleshooting steps:
  • Check routing table
  • Test connectivity (ping, traceroute)
  • Verify IPs and subnet masks
  • Correct or add routes
  • Retest connectivity

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✅ Tip for the Exam:
If a question mentions a device cannot reach a network, always think:

• “Is the route missing?”
• “Is the next hop correct?”
• “Is the subnet mask right?”

These three checks often solve routing table problems.