Troubleshoot IPv4 Static and Default Route Configuration
Troubleshoot IPv4 Static and Default Route Configuration

Troubleshoot IPv4 Static and Default Route Configuration

Troubleshoot Route Configuration
5

Summary

This topic troubleshoot common static and default route configuration issues. Start learning CCNA 200-301 for free right now!!

Note: Welcome: This topic is part of Module 16 of the Cisco CCNA 2 course, for a better follow up of the course you can go to the CCNA 2 section to guide you through an order.

Network Changes

No matter how well you set up your network, you will have to be ready to troubleshoot some problem. Networks are frequently subject to events that can cause their status to change. For example, an interface can fail, or a service provider drops a connection. Links can become oversaturated, or an administrator may enter a wrong configuration.

When there is a change in the network, connectivity may be lost. Network administrators are responsible for pinpointing and solving the problem. To find and solve these issues, a network administrator must be familiar with tools to help isolate routing problems quickly.

Common Troubleshooting Commands

Common IOS troubleshooting commands include the following:

  • ping
  • traceroute
  • show ip route
  • show ip interface brief
  • show cdp neighbors detail

The figure shows the topology used to demonstrate these commands.

Troubleshoot IPv4 Route Configuration
Troubleshoot IPv4 Route Configuration

Click each button for an example and explanation of these common troubleshooting commands.

The example displays the result of an extended ping from the source interface of R1 to the LAN interface of R3. An extended ping is an enhanced version of the ping utility. Extended ping enables you to specify the source IP address for the ping packets.

R1# ping 192.168.2.1 source 172.16.3.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.3.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/5 ms
R1#

This example displays the result of a traceroute from R1 to the R3 LAN. Note that each hop route returns an ICMP reply.

R1# traceroute 192.168.2.1
Type escape sequence to abort.
Tracing the route to 192.168.2.1
VRF info: (vrf in name/id, vrf out name/id)
  1 172.16.2.2 1 msec 2 msec 1 msec
  2 192.168.1.1 2 msec 3 msec *
R1#

The show ip route command in this example displays the routing table of R1.

R1# show ip route | begin Gateway
Gateway of last resort is not set
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
S        172.16.1.0/24 [1/0] via 172.16.2.2
C        172.16.2.0/24 is directly connected, Serial0/1/0
L        172.16.2.1/32 is directly connected, Serial0/1/0
C        172.16.3.0/24 is directly connected, GigabitEthernet0/0/0
L        172.16.3.1/32 is directly connected, GigabitEthernet0/0/0
S     192.168.1.0/24 [1/0] via 172.16.2.2
S     192.168.2.0/24 [1/0] via 172.16.2.2
R1#

A quick status of all interfaces on the router is shown using the show ip interface brief command in this example.

R1# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0/0   172.16.3.1      YES manual up                    up
GigabitEthernet0/0/1   unassigned      YES unset  up                    up
Serial0/1/0            172.16.2.1      YES manual up                    up
Serial0/1/1            unassigned      YES unset  up                    up
R1#

The show cdp neighbors command provides a list of directly connected Cisco devices. This command validates Layer 2 (and therefore Layer 1) connectivity. For example, if a neighbor device is listed in the command output, but it cannot be pinged, then Layer 3 addressing should be investigated.

R1# show cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
                  D - Remote, C - CVTA, M - Two-port Mac Relay
Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
Switch           Gig 0/0/1         129              S I   WS-C3560- Fas 0/5
R2               Ser 0/1/0         156             R S I  ISR4221/K Ser 0/1/0
R3               Ser 0/1/1         124             R S I  ISR4221/K Ser 0/1/0
Total cdp entries displayed : 3
R1#

Solve a Connectivity Problem

Finding a missing (or misconfigured) route is a relatively straightforward process if the right tools are used in a methodical manner.

For instance, the user at PC1 reports that he cannot access resources on the R3 LAN. This can be confirmed by pinging the LAN interface of R3 using the LAN interface of R1 as the source. Again, we will use the topology in the figure to demonstrate how to troubleshoot this connectivity problem.

Troubleshoot IPv4 Route Configuration
Troubleshoot IPv4 Route Configuration

Click each button to see how troubleshooting commands are used to solve a connectivity problem.

Ping the Remote LAN

The network administrator can test connectivity between the two LANs from R1 instead of PC1. This can be done by sourcing the ping from the G0/0/0 interface on R1 to the G0/0/0 interface on R3, as shown in the example. The ping results show that there is no connectivity between these LANs.

R1# ping 192.168.2.1 source g0/0/0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.3.1
.....
Success rate is 0 percent (0/5)

Ping the Next-Hop Router

Next, a ping to the S0/1/0 interface on R2 is successful. This ping is sourced from the S0/1/0 interface of R1. Therefore, the issue is not loss of connectivity between R1 and R2.

R1# ping 172.16.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/4 ms

Ping R3 LAN from S0/1/0

A ping from R1 to the R3 interface 192.168.2.1 is successful as well. This ping is sourced from the S0/1/0 interface on R1. R3 has a route back to the network between R1 and R2, 172.16.2.0/24. This confirms that R1 can reach the remote LAN on R3. However, packets sourced from the LAN on R1 cannot. This indicates that either R2 or R3 may have an incorrect or missing route to the LAN on R1.

R1# ping 192.168.2.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/4 ms

Verify the R2 Routing Table

The next step is to investigate the routing tables of R2 and R3. The routing table for R2 is shown in the example. Notice that the 172.16.3.0/24 network is configured incorrectly. The static route to the 172.16.3.0/24 network has been configured using the next-hop address 192.168.1.1. Therefore, packets destined for the 172.16.3.0/24 network are sent back to R3 instead of to R1.

R2# show ip route | begin Gateway
Gateway of last resort is not set
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
C        172.16.1.0/24 is directly connected, GigabitEthernet0/0/0
L        172.16.1.1/32 is directly connected, GigabitEthernet0/0/0
C        172.16.2.0/24 is directly connected, Serial0/1/0
L        172.16.2.2/32 is directly connected, Serial0/1/0
S        172.16.3.0/24 [1/0] via 192.168.1.1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, Serial0/1/1
L        192.168.1.2/32 is directly connected, Serial0/1/1
S     192.168.2.0/24 [1/0] via 192.168.1.1
R2#

Correct the R2 Static Route Configuration

Next, the running configuration does, in fact, reveal the incorrect ip route statement. The incorrect route is removed, and the correct route is then entered.

R2# show running-config | include ip route
ip route 172.16.3.0 255.255.255.0 192.168.1.1
ip route 192.168.2.0 255.255.255.0 192.168.1.1
R2#
R2# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)# no ip route 172.16.3.0 255.255.255.0 192.168.1.1
R2(config)# ip route 172.16.3.0 255.255.255.0 172.16.2.1
R2(config)#

Verify New Static Route is Installed

The routing table on R2 is checked once again to confirm the route entry to the LAN on R1, 172.16.3.0, is correct and pointing toward R1.

R2(config) # exit
R2#
*Sep 20 02:21:51.812: %SYS-5-CONFIG_I: Configured from console by console
R2# show ip route | begin Gateway
Gateway of last resort is not set
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
C        172.16.1.0/24 is directly connected, GigabitEthernet0/0/0
L        172.16.1.1/32 is directly connected, GigabitEthernet0/0/0
C        172.16.2.0/24 is directly connected, Serial0/1/0
L        172.16.2.2/32 is directly connected, Serial0/1/0
S        172.16.3.0/24 [1/0] via 172.16.2.1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, Serial0/1/1
L        192.168.1.2/32 is directly connected, Serial0/1/1
S     192.168.2.0/24 [1/0] via 192.168.1.1
R2#

Ping the Remote LAN Again

Next, a ping from R1 sourced from G0/0/0 is used to verify that R1 can now reach the LAN interface of R3. As a last step in confirmation, the user on PC1 should also test connectivity to the 192.168.2.0/24 LAN.

R1# ping 192.168.2.1 source g0/0/0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.3.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/4 ms

Syntax Checker – Troubleshoot IPv4 Static and Default Routes

Troubleshoot IPv4 Static and Default Routes based on the specified requirements

Troubleshoot IPv4 Route Configuration
Troubleshoot IPv4 Route Configuration
  • Send a ping from R1 to the G0/0/0 interface on R3.
R1#ping 192.168.2.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
U.U.U
Success rate is 0 percent (0/5)
  • Test the next hop gateway by sending a ping from R1 to the S0/1/0 interface of R2.
R1#ping 172.16.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 2/2/3 ms
  • Review the routing table on R1.
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
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR
Gateway of last resort is not set
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
S        172.16.1.0/24 \[1/0\] via 172.16.2.2
C        172.16.2.0/24 is directly connected, Serial0/1/0
L        172.16.2.1/32 is directly connected, Serial0/1/0
C        172.16.3.0/24 is directly connected, GigabitEthernet0/0/0
L        172.16.3.1/32 is directly connected, GigabitEthernet0/0/0
S     192.168.1.0/24 \[1/0\] via 172.16.2.2
S     192.168.2.0/24 \[1/0\] via 172.16.2.2
  • Review the routing table on R2.
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
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR
Gateway of last resort is not set
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
C        172.16.1.0/24 is directly connected, GigabitEthernet0/0/0
L        172.16.1.1/32 is directly connected, GigabitEthernet0/0/0
C        172.16.2.0/24 is directly connected, Serial0/1/0
L        172.16.2.2/32 is directly connected, Serial0/1/0
S        172.16.3.0/24 \[1/0\] via 172.16.2.1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, Serial0/1/1
L        192.168.1.2/32 is directly connected, Serial0/1/1
  • Enter configuration mode and configure a static route on R2 to reach the R3 LAN.
R2#configure terminal
R2(config)#ip route 192.168.2.0 255.255.255.0 192.168.1.1
  • Exit configuration mode and review the routing table on R2.
R2(config)#exit
\*Sep 20 03:10:34.913: %SYS-5-CONFIG\_I: Configured from console by console
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
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR
Gateway of last resort is not set
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
C        172.16.1.0/24 is directly connected, GigabitEthernet0/0/0
L        172.16.1.1/32 is directly connected, GigabitEthernet0/0/0
C        172.16.2.0/24 is directly connected, Serial0/1/0
L        172.16.2.2/32 is directly connected, Serial0/1/0
S        172.16.3.0/24 \[1/0\] via 172.16.2.1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, Serial0/1/1
L        192.168.1.2/32 is directly connected, Serial0/1/1
S     192.168.2.0/24 \[1/0\] via 192.168.1.1
  • Send a ping from R1 to the G0/0/0 interface on R3.
R1#ping 192.168.2.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/4 ms

You have successfully performed troubleshooting on IPv4 static and default routes.

Glossary: If you have doubts about any special term, you can consult this computer network dictionary.

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