Module 16: Troubleshoot Static and Default Routes
Switching, Routing and Wireless Essentials v7.0 (SRWE)
Module Objectives
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.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2
16.1 Packet Processing with Static Routes
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 3
Packet Processing with Static Routes
Static Routes and Packet Forwarding
• PC1 addresses a packet to PC3 and sends it to the default gateway address.
• When the packet arrives on the R1 G0/0/0 interface, R1 decapsulates the packet and searches the routing table for a matching destination network entry.
If the destination IP address:
• Matchesastaticrouteentry,R1willusethestaticroutetoidentifythenext-hopIPaddressorexit interface.
• Doesnotmatchaspecificroutetothedestinationnetwork,thenR1willusethedefaultstaticroute (if configured).
• Doesnotmatcharoutetableentry,thenR1willdropthepacketandsendanICMPmessageback to the source (i.e., PC1).
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 4
Packet Processing with Static Routes
Static Routes and Packet Forwarding (Cont.)
Assuming R1 matched a routing table entry, it encapsulates the packet in a new frame and forwards it out of interface S0/1/0 to R2.
• R2 receives the packet on its S0/1/0 interface. • It decapsulates and processes the packet the
same way R1 did.
• When R2 finds a match in the routing table, it uses the identified next-hop IP address or exit interface and sends the packet out of its interface S0/1/1 towards R3.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 5
Packet Processing with Static Routes
Static Routes and Packet Forwarding (Cont.)
• R3 receives the packet, decapsulates it, and searches the routing table for a match.
• The destination IP address of PC3 matches the directly connected G0/0/0 interface. Therefore, R3 searches the ARP table for the Layer 2 MAC address of PC3.
• If no ARP entry exists, then R3 sends an ARP request out of the G0/0/0 interface.
• PC3 responds with an ARP reply containing its MAC address.
• R3 encapsulates the packet in a new frame and uses the PC3 MAC address as the
destination MAC address and the G0/0/0 MAC address as the source MAC address.
• The frame is forwarded out of interface G0/0/0 and PC3 receives and processes it accordingly.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 6
16.2 Troubleshoot IPv4 Static and Default Route Configuration
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 7
Troubleshoot IPv4 Static and Default Route Configuration Network Changes
Networks fail for a number of reasons:
• An interface can fail
• A service provider drops a connection
• Links can become oversaturated
• An administrator may enter a wrong configuration.
Network administrators are responsible for pinpointing and solving the problem.
To efficiently find and solve these issues, it is advantageous to be intimately familiar with tools to help isolate routing problems quickly.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 8
Troubleshoot IPv4 Static and Default Route Configuration Common Troubleshooting Commands
Command
Description
ping
• VerifyLayer3connectivitytodestination.
• Extendedpingsprovideadditionaloptions.
traceroute
• Verifypathtodestinationnetwork.
• ItusesICMPechoreplymessagestodeterminethehopsto
the destination.
show ip route
• Displaystheroutingtable.
• UsedtoverifyrouteentriesfordestinationIPaddresses.
show ip interface brief
• Displaysthestatusofdeviceinterfaces.
• UsedtoverifytheoperationalstatusandIPaddressofan
interface.
show cdp neighbors
• DisplaysalistofdirectlyconnectedCiscodevices. • AlsousedtovalidateLayer1and2connectivity.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 9
Troubleshoot IPv4 Static and Default Route Configuration Solve a Connectivity Problem
Connectivity from PC1 to PC3 fails.
• •
•
• •
•
Extended pings from the R1 G0/0/0 interface to PC3 fail.
Pings from R1 (i.e., S0/1/0 interface) to R2 are successful.
Pings from R1 (i.e., S0/1/0 interface) to R3 are successful.
R2 routing table reveals the problem and the incorrect static route is removed.
A new static route solves the problem.
ip route 172.16.3.0 255.255.255.0 172.16.2.1
R2# show ip route | begin Gateway Gateway of last resort is not set
C L C L S
C
L
S R2#
172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks 172.16.1.0/24 is directly connected, GigabitEthernet0/0/0 172.16.1.1/32 is directly connected, GigabitEthernet0/0/0
172.16.2.0/24 is directly connected, Serial0/l/0 172.16.2.2/32 is directly connected, Serial0/l/0 172.16.3.0/24 [1/0] via 192.168.1.1
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks 192.168.1.0/24 is directly connected, Serial0/1/1 192.168.1.2/32 is directly connected, Serial0/1/1
192.168.2.0/24 [1/0] via 192.168.1.1
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 10
16.3 Module Practice and Quiz
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 11
Structured Design
Packet Tracer – Troubleshoot Static and Default Routes – Physical Mode
Lab – Troubleshoot Static and Default Routes
In this Packet Tracer Physical Mode activity and in the Lab, you will complete the following objectives:
• Evaluate Network Operation
• Gather Information, Create an Action Plan, and Implement Corrections
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 12
Module Practice and Quiz
What did I learn in this module?
• A host sends a packet to another host and sends it to the default gateway address.
• When the packet arrives on a router interface, it decapsulates the packet and searches the
routing table for a matching destination network entry.
• If the destination IP address:
• Matchesastaticrouteentry,therouterwillusethestaticroutetoidentifythenexthopIPaddressorexit interface.
• Doesnotmatchaspecificroutetothedestinationnetwork,thentherouterwillusethedefaultstatic route (if configured).
• Doesnotmatcharoutetableentry,thentherouterwilldropthepacketandsendanICMPmessage back to the source.
• If the router matched a routing table entry, then the router encapsulates the packet and forwards it out of the appropriate interface.
• The packet is forwarded from router to router until it reaches its destination network.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 13
Module Practice and Quiz
What did I learn in this module? (Cont.)
• When the packet reaches the destination network, that router will search the routing table for a match.
• When the destination IP address matches a directly connected Ethernet interface, the router searches the ARP table for the Layer 2 MAC address of the destination IP address.
• If no ARP entry exists, the router sends an ARP request out of the Ethernet interface
• The destination host responds with an ARP reply containing its MAC address.
• The router then encapsulates the packet in a new frame. It uses the MAC address of the destination host as the frame destination MAC address, and the MAC address of the router Ethernet interface as the source MAC address in the frame.
• The frame is forwarded out of the appropriate interface.
• The packet arrives on the network interface card (NIC) interface of destination host and is processed accordingly.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 14
Module Practice and Quiz
What did I learn in this module? (Cont.)
Common IOS troubleshooting commands to troubleshoot IPv4 Static and default routes include:
• ping
• traceroute
• show ip route
• show ip interface brief
• show cdp neighbors detail
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 15