CCNA 2 v7.0 Curriculum: Module 16 – Troubleshoot Static and Default Routes

16.0 Introduction

16.0.1 Why should I take this module?

Welcome to Troubleshoot Static and Default Routes!

Well done! You have come to the final module in the Switching, Routing, and Wireless Essentials v7.0 (SRWE) course. This course gave you the in-depth knowledge and skills you need to set up switches and routers (including wireless devices) on your growing network. You really are good at network administration!

But what makes a good network administrator into a great one? The ability to effectively troubleshoot. The best way to gain network troubleshooting skills is simple: always be troubleshooting. In this module, you will troubleshoot both static and default routes. There is a Syntax Checker, a Packet Tracer, and a hands-on Lab where you can hone your troubleshooting skills. Let’s get to it!

16.0.2 What will I learn to do in this module?

Module Title: Troubleshoot Static and Default Routes

Module Objective: Troubleshoot static and default route configurations.

Topic Title Topic Objective
Packet Processing with Static Routes Explain how a router processes packets when a static route is configured.
Troubleshoot IPv4 Static and Default Route Configuration Troubleshoot common static and default route configuration issues.

16.1 Packet Processing with Static Routes

16.1.1 Static Routes and Packet Forwarding

Before diving into the troubleshooting portion of this module, this topic provides a brief review of how packets are forwarded in static routes. In the figure, click Play to see the animation where PC1 is sending a packet to PC3.

CCNA 2 v7.0 Curriculum: Module 16 - Troubleshoot Static and Default Routes 5

The following describes the packet forwarding process with static routes, as demonstrated in the animation:

  1. The packet arrives on the GigabitEthernet 0/0/0 interface of R1.
  2. R1 does not have a specific route to the destination network, 192.168.2.0/24. Therefore, R1 uses the default static route.
  3. R1 encapsulates the packet in a new frame. Because the link to R2 is a point-to-point link, R1 adds an “all 1s” address for the Layer 2 destination address.
  4. The frame is forwarded out of the Serial 0/1/0 interface. The packet arrives on the Serial 0/1/0 interface on R2.
  5. R2 de-encapsulates the frame and looks for a route to the destination. R2 has a static route to 192.168.2.0/24 out of the Serial 0/1/1 interface.
  6. R2 encapsulates the packet in a new frame. Because the link to R3 is a point-to-point link, R2 adds an “all 1s” address for the Layer 2 destination address.
  7. The frame is forwarded out of the Serial 0/1/1 interface. The packet arrives on the Serial 0/1/1 interface on R3.
  8. R3 de-encapsulates the frame and looks for a route to the destination. R3 has a connected route to 192.168.2.0/24 out of the GigabitEthernet 0/0/0 interface.
  9. R3 looks up the ARP table entry for 192.168.2.10 to find the Layer 2 Media Access Control (MAC) address for PC3. If no entry exists, R3 sends an Address Resolution Protocol (ARP) request out of the GigabitEthernet 0/0/0 interface, and PC3 responds with an ARP reply, which includes the PC3 MAC address.
  10. R3 encapsulates the packet in a new frame with the MAC address of the GigabitEthernet 0/0/0 interface as the source Layer 2 address, and the MAC address of PC3 as the destination MAC address.
  11. The frame is forwarded out of GigabitEthernet 0/0/0 interface. The packet arrives on the network interface card (NIC) interface of PC3.

16.2 Troubleshoot IPv4 Static and Default Route Configuration

16.2.1 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.

16.2.2 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.

CCNA 2 v7.0 Curriculum: Module 16 - Troubleshoot Static and Default Routes 6

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

ping

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#

traceroute

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#

show ip route

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#

show ip interface brief

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#

show cdp neighbors

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#

16.2.3 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.

CCNA 2 v7.0 Curriculum: Module 16 - Troubleshoot Static and Default Routes 7

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

16.2.4 Syntax Checker – Troubleshoot IPv4 Static and Default Routes

Troubleshoot IPv4 Static and Default Routes based on the specified requirements

CCNA 2 v7.0 Curriculum: Module 16 - Troubleshoot Static and Default Routes 8

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.

16.3 Module Practice and Quiz

16.3.1 Packet Tracer – Troubleshoot Static and Default Routes

In this activity you will troubleshoot and static and default routes and repair any errors that you find.

  • Troubleshoot IPv4 static routes.
  • Troubleshoot IPv6 static routes.
  • Configure IPv4 static routes.
  • Configure IPv4 default routes.
  • Configure IPv6 static routes.

16.3.1 Packet Tracer – Troubleshoot Static and Default Routes

16.3.2 Lab – Troubleshoot Static and Default Routes

In this lab, you will complete the following objectives:

  • Evaluate Network Operation
  • Gather information, create an action plan, and implement corrections

16.3.2 Lab – Troubleshoot IPv4 and IPv6 Static and Default Routes

16.3.3 What did I learn in this module?

Packet Processing with Static Routes

  1. The packet arrives on the interface of R1.
  2. R1 does not have a specific route to the destination network; therefore, R1 uses the default static route.
  3. R1 encapsulates the packet in a new frame. Because the link to R2 is a point-to-point link, R1 adds an “all 1s” address for the Layer 2 destination address.
  4. The frame is forwarded out of the appropriate interface. The packet arrives on the interface on R2.
  5. R2 de-encapsulates the frame and looks for a route to the destination. R2 has a static route to the destination network out of one of its interfaces.
  6. R2 encapsulates the packet in a new frame. Because the link to R3 is a point-to-point link, R2 adds an “all 1s” address for the Layer 2 destination address.
  7. The frame is forwarded out of the appropriate interface. The packet arrives on the interface on R3.
  8. R3 de-encapsulates the frame and looks for a route to the destination. R3 has a connected route to the destination network out of one of its interfaces.
  9. R3 looks up the ARP table entry for the destination network to find the Layer 2 MAC address for PC3. If no entry exists, R3 sends an ARP request out of one of its interfaces, and PC3 responds with an ARP reply, which includes the PC3 MAC address.
  10. R3 encapsulates the packet in a new frame with the MAC address of the appropriate interface as the source Layer 2 address and the MAC address of PC3 as the destination MAC address.
  11. The frame is forwarded out of the appropriate interface. The packet arrives on the network interface card (NIC) interface of PC3.

Troubleshoot IPv4 Static and Default Route Configuration

Networks are frequently subject to events that can cause their status to change. An interface can fail, or a service provider drops a connection. Links can become oversaturated, or an administrator may enter a wrong configuration. Common IOS troubleshooting commands include the following:

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

16.3.4 Module Quiz – Troubleshoot Static and Default Routes

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