1. CCNA 2 v3.1 Module 9 Basic Router Troubleshooting

2. Objectives
One of the primary functions of a router is to determine the best path to a given destination.
A router learns paths, also called routes, from an administrator's configuration or from other routers by way of routing protocols
This module will describe methods for examining and interpreting the contents of the routing table
Network testing and troubleshooting are perhaps the most time consuming components of every network administrator’s job.
Efficient testing and troubleshooting must be done in a logical, orderly, and well-documented fashion
This module will introduce several of the most important of these tools and provide practice in their use.

3. The show ip route Command
The show ip route command displays the contents of the IP routing table.

4. Examining the Routing Table
A routing table contains a list of the best available routes.
Routers use routing tables to make packet forwarding decisions.
Dublin#
show ip route
Codes, how the routes were learnt
Networks being advertised
Administrative Distance
Metric
Next Hop
Time since last update
Out interface

5. Static Routing

6. Dynamic Routing

7. Default Route
Default routes are used when there isn’t a more specific entry in routing table.
The router uses the default route to reach the gateway of last resort. 

8. Configuring a Default Route
Alternatively use the default network command
Router(config)# ip default-network

9. Determining Route Source and Destination

10. Routing – Role of Layer 2 and Layer 3 Addressing
Hop
Source MAC
Destination MAC
Source IP
Destination IP 1 2 3
00:ac:2d:34:56:11
00:01:46:26:23:a4
00:01:46:26:23:a4
00:04:24:98:55:44
00:04:24:98:55:44
00:2a:3b:3c:a4:33

11. Role of Packets and Frames in Path Traversal

12. Determining L2 and L3 Addresses

13. Determining Administrative Distance
AD measures the trustworthiness of the source of the route information.
The lower the AD, the more trustworthy the source.
Different routing protocols have different default ADs.
When there are two paths available to a destination the path with the lowest AD is used in the routing table.

14. Determining the Route Metric
Routing algorithms generate a number, called the metric value, for each path through the network .
The metric value is used to determine the best route to a destination.
The various protocols base their metrics on different factors: 

15. Routing Metrics Hop Count number of hops Bandwidth speed of the link
Delay
the time taken to traverse the path
Reliability
the error rate on a link, from 1(bad) to 255 (good).
Load
the amount of traffic on the link from 1(good) to 255(bad)
MTU
Largest packet allowed on link
Cost
Arbitrary meaning – assigned by administrator – lower is better

16. Routing Metric Calculations
RIP
Hop Count
IGRP
Metric = [K1 * Bandwidth + (K2 * Bandwidth)/(256-load) + K3*Delay] * [K5/(reliability + K4)]
Defaults:
K1 = K3 = 1 and K2 = K4 = K5 = 0.
If K5 = 0, then [K5/(reliability + K4)] =0.
Metric = Bandwidth + Delay.

17. Viewing the Routing Table
Use the following commands to find the last routing update:
show ip route
show ip route network
show ip route connected
show ip route static
show ip protocols
show ip rip database

18. Show ip route Command

19. Observing Multiple Paths to a Destination
Some routing protocols support multiple paths to the same destination.
Unlike single path algorithms, these multi-path algorithms permit traffic over multiple lines, provide better throughput, and are more reliable.

20. Introduction to Network Testing

21. Structured Approach to Troubleshooting

22. Typical Layer 1 Errors Broken cables Disconnected cables
Cables connected to the wrong ports
Intermittent cable connection
Wrong cables used for the task at hand
Transceiver problems
DCE cable problems
DTE cable problems
Devices turned off

23. Troubleshooting Using show interfaces
Useful for troubleshooting Layer1, Layer 2 in addition to Layer 3 addressing

24. Layer 1 Troubleshooting – using show interfaces
BHM#
show interface s0
Serial0 is up, line protocol is up
< output omitted >
Last clearing of "show interface" counters never
Received 73 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
135 packets output, 7361 bytes, 0 underruns
0 output errors, 0 collisions, 37 interface resets
0 output buffer failures, 0 output buffers swapped out
54 carrier transitions
Physical layer (line status)
Up: has Layer 1 connectivity
Down: L1 problem
Administratively down: disabled
Faulty hardware, cable or noise
Bad line,
bad hardware
To reset: BHM# clear counters
Line interruptions, faulty hardware

25. Layer 1 Troubleshooting – using show interfaces
The show interfaces serial command

26. Layer 1 Troubleshooting – show controllers
show controllers serial 0
HD unit 0, idb = 0x1086D4, driver structure at 0x10E568
buffer size HD unit 0, V.35 DCE cable, clockrate 56000
Router#

27. Troubleshooting Using show controllers serial
The show controllers serial Command

28. Typical Layer 2 Errors Improperly configured serial interfaces
Improperly configured Ethernet interfaces
Improper encapsulation set
Improper clock rate settings on serial interfaces
Network interface card (NIC) problems

29. Troubleshooting Using show cdp neighbors

30. Troubleshooting Using show cdp neighbors detail

31. Layer 2 Troubleshooting – show interfaces
Data-Link layer (up or down)
Are keepalives being received?
BHM#
show interface serial 0
Serial0 is up, line protocol is up
Hardware is HD64570
Internet address is /16
MTU 1500 bytes, BW 1544 Kbit, DLY usec, rely 255/255, load 1/255
Encapsulation HDLC, loopback not set, keepalive set (10 sec)
Last input 00:00:01, output 00:00:00, output hang never
Last clearing of "show interface" counters never
< output omitted >
Encapsulation must match at both ends of connection
If the interface is up and the line protocol is down, a Layer 2 problem exists.
Among the possible causes are:
No keepalives ,No clock rate ,Mismatch in encapsulation type

32. Layer 2 Troubleshooting
If the interface is up and the line protocol is down, a Layer 2 problem exists.
Indicates whether the IOS processes consider the interface usable.
Keepalives are defined as messages sent by one network device to inform another network device that the virtual circuit between the two is still active
If the interface misses three consecutive keepalives, line protocol is down
When the line is down, the protocol is always down    

33. show interfaces Command

34. Typical Layer 3 Errors Routing protocol not enabled
Wrong routing protocol enabled
Incorrect IP addresses
Incorrect subnet masks

35. Layer 3 Troubleshooting Using Ping

36. Ping Extended ping provides extra facilities for ping
ping SanJose1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to , timeout is 2 seconds:
..!!!
Success rate is 60 percent (3/5),
round-trip min/avg/max = 32/33/36 ms
‘!’ indicate a successful echo
‘.’ indicate the application timed out waiting for an echo
vista#
Extended ping
provides extra facilities for ping

37. Troubleshooting Using traceroute Command
Test on a hop-by-hop basis – Each time is the RTT

38. Troubleshooting Routing Issues
The show ip route Command

39. Troubleshooting Routing Issues
The show ip protocols Command

40. Introduction to debug
Debug syntax

41. Debug
Used to display dynamic data and events as they occur.
Displays current events of the router, example:
traffic on an interface
error messages generated by nodes on the network
protocol-specific diagnostic packets
Debug commands effect router performance due to high processor overhead.
Debug should be used sparingly to avoid disrupting normal router operation.   

42. Debug output and system messages are sent to the console.
Debug all
Router# debug all
But don’t use as it can crash the system
Debug output and system messages are sent to the console.
To redirected to a Telnet client:
Router# terminal monitor
To add timestamps to debug messages:
Router(config)# service timestamps debug uptime
To Turn off debugging
Router# no debug all
Router# undebug all

43. Layer 7 Troubleshooting Using Telnet

44. Summary