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 routecommand 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
show ip route1.1 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
show ip route CommandSyntax:
Router# show ip routeExplanation 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:
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 0Step 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 0Step 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 0Step 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
show ip route3.1 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 0Ensure 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 onLook 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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