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:

Router# show ip route

• 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)

R1#configure terminal
R1(config)#interface fastEthernet 1/0
R1(config-if)#ip address 192.168.1.1 255.255.255.0
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface fastEthernet 0/0
R1(config-if)#ip address 10.0.0.1 255.255.255.252
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#router ospf 1
R1(config-router)#network 192.168.1.0 0.0.0.255 area 0
R1(config-router)#network 10.0.0.0 0.0.0.3 area 0

• Step 2: Basic Configuration (R2)

R2#configure terminal
R2(config)#interface fastEthernet 0/0
R2(config-if)#ip address 10.0.0.2 255.255.255.252
R2(config-if)#no shut
R2(config-if)#exit
R2(config)#interface fastEthernet 1/0
R2(config-if)#ip address 10.0.0.5 255.255.255.252
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#router ospf 1
R2(config-router)#network 10.0.0.0 0.0.0.3 area 0
R2(config-router)#network 10.0.0.4 0.0.0.3 area 0

• Step 3: Basic Configuration (R3)

R3#configure terminal
R3(config)#interface fastEthernet 1/0
R3(config-if)#ip address 10.0.0.6 255.255.255.252
R3(config-if)#no shut
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface fastEthernet 0/0
R3(config-if)#ip address 192.168.2.1 255.255.255.0
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#router ospf 1
R3(config-router)#network 10.0.0.4 0.0.0.3 area 0
R3(config-router)#network 192.168.2.0 0.0.0.255 area 0

• Step 4: Basic Configuration (PC1)

PC1> ip 192.168.1.100/24 192.168.1.1
Checking for duplicate address...
PC1 : 192.168.1.100 255.255.255.0 gateway 192.168.1.1

PC1> show ip

NAME        : PC1[1]
IP/MASK     : 192.168.1.100/24
GATEWAY     : 192.168.1.1
DNS         :
MAC         : 00:50:79:66:68:00
LPORT       : 20020
RHOST:PORT  : 127.0.0.1:20021
MTU         : 1500

PC1> save
Saving startup configuration to startup.vpc
.  done

PC1>

• Step 5: Basic Configuration (PC2)

PC2> ip 192.168.2.100/24 192.168.2.1
Checking for duplicate address...
PC2 : 192.168.2.100 255.255.255.0 gateway 192.168.2.1

PC2> show ip

NAME        : PC2[1]
IP/MASK     : 192.168.2.100/24
GATEWAY     : 192.168.2.1
DNS         :
MAC         : 00:50:79:66:68:01
LPORT       : 20022
RHOST:PORT  : 127.0.0.1:20023
MTU         : 1500

PC2> save
Saving startup configuration to startup.vpc
.  done

PC2>

2.3 Verify OSPF Neighbor Relationships

Check that R1 and R2 have formed an adjacency:

• R1

Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.5          1   FULL/BDR        00:00:39    10.0.0.2        FastEthernet0/0
R1#

• R2

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.2.1       1   FULL/BDR        00:00:37    10.0.0.6        FastEthernet1/0
192.168.1.1       1   FULL/DR         00:00:31    10.0.0.1        FastEthernet0/0
R2#

• R3

R3#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.5          1   FULL/DR         00:00:33    10.0.0.5        FastEthernet1/0
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:

R1#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C        10.0.0.0/30 is directly connected, FastEthernet0/0
L        10.0.0.1/32 is directly connected, FastEthernet0/0
O        10.0.0.4/30 [110/2] via 10.0.0.2, 00:12:08, FastEthernet0/0
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, FastEthernet1/0
L        192.168.1.1/32 is directly connected, FastEthernet1/0
O     192.168.2.0/24 [110/3] via 10.0.0.2, 00:08:48, FastEthernet0/0
R1#

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:

R2#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.0.0.0/30 is directly connected, FastEthernet0/0
L        10.0.0.2/32 is directly connected, FastEthernet0/0
C        10.0.0.4/30 is directly connected, FastEthernet1/0
L        10.0.0.5/32 is directly connected, FastEthernet1/0
O     192.168.1.0/24 [110/2] via 10.0.0.1, 00:13:45, FastEthernet0/0
O     192.168.2.0/24 [110/2] via 10.0.0.6, 00:10:15, FastEthernet1/0
R2#

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:

R3#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O        10.0.0.0/30 [110/2] via 10.0.0.5, 00:11:33, FastEthernet1/0
C        10.0.0.4/30 is directly connected, FastEthernet1/0
L        10.0.0.6/32 is directly connected, FastEthernet1/0
O     192.168.1.0/24 [110/3] via 10.0.0.5, 00:11:33, FastEthernet1/0
      192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.2.0/24 is directly connected, FastEthernet0/0
L        192.168.2.1/32 is directly connected, FastEthernet0/0
R3#

If missing, R2 might not be redistributing routes correctly.

3.2 Common Troubleshooting Steps

Step 1: Check OSPF Configuration

• On R2, verify OSPF settings:

R2#show running-config | section ospf
router ospf 1
 network 10.0.0.0 0.0.0.3 area 0
 network 10.0.0.4 0.0.0.3 area 0

Ensure the correct networks are advertised.

Step 2: Verify OSPF Neighbor Adjacency

R2#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.2.1       1   FULL/BDR        00:00:36    10.0.0.6        FastEthernet1/0
192.168.1.1       1   FULL/DR         00:00:36    10.0.0.1        FastEthernet0/0

If adjacency is missing, check interface settings.

Step 3: Debug Routing Updates

R1#debug ip ospf events
OSPF events debugging is on

Look for OSPF update failures.

Keywords

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