Section 13: The Cisco Troubleshooting Methodology

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71: The Cisco Troubleshooting Methodology  – Introduction

https://www.udemy.com/cisco-icnd1/learn/lecture/8605212#overview

72: The Cisco Troubleshooting Methodology

https://www.udemy.com/cisco-icnd1/learn/lecture/8605222#overview

  • Define the Problem
  • Gather Information
  • Analyze Information
  • Eliminate Potential Causes
  • Propose Hypothesis
  • Test Hypothesis
  • Solve Problem and Document Solution

Troubleshooting Methods

  • Top Down
    • Start at Layer 7 and work your way down to Layer 1
  • Bottom Up
    • Start at Layer 1 and work your way up to Layer 7
  • Divide and Conquer
    • Go straight to the most obvious layer and work your way either up or down.

Troubleshooting methods

  • Compare configurations
  • Trace the path
    • Very common
    • Start at the source and work your way to the destination
  • Swap out components

Common Commands

  • Ping
    • Uses ICMP
    • Verifies 2-way connectivity
    • Source sends a PING request
    • Destination sends a PING reply
  • Traceroute
    • Similar to PING but measures hop by hop based on a ‘TTL’ type value
    • Each TTL is a hop.  First one hop, then two, then 3…
    • This way you can tell if something breaks in between
    • Many routers are configured NOT to return PING replies, so you may not get values ‘mid stream’ but will get values farther down the path
  • Telnet
    • Generally used to access command line on a device
      • Very similar to SSH, but insecure.  All data is sent clear text.
    • Telnet can be used to check specific ports.  Default is 23, but can be easily set to something else to see if:
      • A port is open (not blocked by a firewall)
      • A service is ‘listening’ on a specific host, such as:
        • 53 for DNS
        • 80 for HTTP

73: Cisco Troubleshooting Methodology – Lab

https://www.udemy.com/cisco-icnd1/learn/lecture/8605228#overview

The Issue

  • User on PC behind R1 claims DNS isn’t working
  • DNS is hosted on R3

Troubleshooting

Ping R3 from R1 using the hostname

R1#ping R3
Translating "R3"...domain server (10.10.30.1)
% Unrecognized host or address, or protocol not running

DNS not working confirmed.

Ping the IP to see if connectivity exists

R1#ping 10.10.30.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.30.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

Cannot access DNS router confirmed.

Run a traceroute to see where the breakdown occurs

R1#traceroute 10.10.30.1
Type escape sequence to abort.
Tracing the route to 10.10.30.1
VRF info: (vrf in name/id out name/id)
  1 10.10.10.2 4 msec 4 msec 0 msec
  2 *  in  *

R2 is OK, but no response from R3.

Check if R2 can ping R3

R2#ping 10.10.30.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.30.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

R2 cannot access R3 confirmed.

Check if there is a route set from R2 to 10.10.30.1

R2#show route
.. lots of stuff, but no route to 10.10.30.1 ...

Route does not exist.

Fix the route

R2(config)#ip route 10.10.30.0 255.255.255.0 10.10.20.1

Try Ping gain

R2#ping 10.10.30.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.30.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round trip min/avg/max = 1/3/0 ms

Ping re-established

Verify the issue has been corrected

Attempt Ping from R1 to R3 again

R1#ping 10.10.30.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.30.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round trip min/avg/max = 1/1/4 ms

Full connection exists between R1 and R3.

Check if DNS is working now

R1#ping R3
Translating "R3"...domain server (10.10.30.1)
% Unrecognized host or address, or protocol not running

DNS still not working.

Check if DNS is running on the router

R1#telnet 10.10.30.1 53
% Connection refused by remote host

DNS Not running on R3 confirmed.

Enable DNS Services on R3

R3(config)#ip dns server
R3(config)#exit
R3(config)#do telnet 10.10.30.1 53
Trying 10.10.30.1, 53 ... Open

[Connection to 10.10.30.1 closed by foregn host]

DNS Service has been re-enabled

Confirm DNS is working on R1

R1#ping R3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.30.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round trip min/avg/max = 1/2/4 ms

Problem Solved!

74: The Cisco Troubleshooting Methodology – Lab Exercises

https://www.udemy.com/cisco-icnd1/learn/lecture/8605232#content

13 The Cisco Troubleshooting Methodology Lab Exercise

13 The Cisco Troubleshooting Methodology Answer Key

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