Troubleshooting with show ip route

Course Objectives:

By the end of this course, you will be able to:

  • Understand the structure of the routing table in Cisco routers.

  • Use the show ip route command to analyze and troubleshoot network connectivity issues.

  • Identify missing, incorrect, or suboptimal routes in a network topology.

  • Implement corrective measures to resolve routing issues.

Module 1: Introduction to show ip route

1.1 What is the Routing Table?

  • Definition of a routing table

  • How routers forward packets based on routes

  • Types of routes:

    • Directly connected routes

    • Static routes

    • Dynamic routes (RIP, EIGRP, OSPF, BGP)

1.2 Understanding the show ip route Command

  • Syntax:

  • Explanation of output fields:

    • Route codes (C = Connected, S = Static, R = RIP, etc.)

    • Network destination and subnet mask

    • Administrative distance and metric

    • Next-hop and outgoing interface

Module 2: Practical Scenario - Troubleshooting a Routing Issue

2.1 Network Topology

Scenario: A company has three routers (R1, R2, R3) connected in a triangle using OSPF. However, hosts in R1's LAN cannot reach hosts in R3's LAN.

Topology Diagram:

Topology

  • R1 to R2: 10.0.0.0/30

  • R2 to R3: 10.0.0.4/30

  • R1's LAN: 192.168.1.0/24

  • R3's LAN: 192.168.2.0/24

  • Routing Protocol: OSPF

2.2 Configuring the Routers

  • Step 1: Basic Configuration (R1)

  • Step 2: Basic Configuration (R2)

  • Step 3: Basic Configuration (R3)

  • Step 4: Basic Configuration (PC1)

  • Step 5: Basic Configuration (PC2)

2.3 Verify OSPF Neighbor Relationships

Check that R1 and R2 have formed an adjacency:

  • R1

  • R2

  • R3

Module 3: Troubleshooting Using show ip route

3.1 Identifying the Issue

  • On R1, check if a route to 192.168.2.0/24 exists:

If the route is missing, OSPF may not be learning it from R2/R3.

  • On R2, check if it has a route to 192.168.2.0/24:

If R2 sees the route, but R1 does not, the issue is between R1 and R2.

  • On R3, check if it has a route to 192.168.1.0/24:

If missing, R2 might not be redistributing routes correctly.

3.2 Common Troubleshooting Steps

Step 1: Check OSPF Configuration

  • On R2, verify OSPF settings:

Ensure the correct networks are advertised.

Step 2: Verify OSPF Neighbor Adjacency

If adjacency is missing, check interface settings.

Step 3: Debug Routing Updates

Look for OSPF update failures.

Keywords

show ip route, Cisco troubleshooting, routing table, OSPF troubleshooting, static routes, dynamic routes, administrative distance, next-hop address, subnet mask, connected routes, route codes, network topology, packet forwarding, IP routing, missing routes, debug ip route, show ip ospf neighbor, route filtering, routing protocols, routing errors, سیسکو

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