NET4009_Module 1 - Tshooting basics

7 Troubleshooting Steps

1. Defining the problem
2. Gathering information
3. Analyzing the information
4. Eliminating potential causes
5. Proposing a hypothesis (likely cause of the problem)
6. Testing and verifying validity of the proposed hypothesis
7. Solving the problem and documenting the work

Unstructured Troubleshooting Approach

Shoot-from-the-hip

Structured Troubleshooting Approach

• Top-down
• Bottom-up
• Divide-and-conquer
• Follow-the-path
• Spot-the-differences
• Move-the-problem

Top down approach

Starting from the top OSI layer and moving down to the physical layer to find the issue

Bottom-Up approach

Starting from the bottom of the OSI layer and working your way up to application layer.
This approach can troubleshoot spanning tree loops

Divide-and-Conquer Troubleshooting Approach

If it is not clear which of the top-down or bottom-up approach is appropriate the alternative is start at the middle and “divide and conquer”

Follow-the-Path Troubleshooting Approach

Following the network path from source to destination.
This approach can troubleshoot routing loops

Compare-Configurations Troubleshooting Approach

This approach is suited for configuration issues

Swap-Components Troubleshooting Approach

This approach is suited for hardware issues

Fundamental Tools and Applications in Network Maintenance Toolkit

• Console; SSH/Telnet

• HTTP/HTTPS

• NTP

• Logging

• Backups

Logging severity levels

0. Emergency

1. Alert

2. Critical

3. Error

4. Warning

5. Notification

6. Informational

7. Debugging

What are the three functional components of network devices that manage data flow and processing?

1. Data Plane: Handles the actual data forwarding and packet switching.

2. Control Plane: Manages routing protocols and network topology, making decisions on where traffic should be sent.

3. Management Plane: Provides network management functions, including monitoring and configuration of devices.

What command is used to troubleshoot cef for data plane?

show cef not-cef-switched

Cisco Express Forwarding

This technique allows for quick decision-making on packet forwarding by using a combination of a forwarding information base (FIB) and the CEF adjacency table.

How to display the CEF FIB table?

• show ip cef ip-address

• show ip cef network mask

Why should a ACL be used when enabling “debug ip packet”?

to limit traffic to:
- a particular host or between hosts
- protocol
- port
- network range

What are the most common reasons for High CPU utilization on routers/switches? How do you fix them?

1. Packet Fragmentation/Reassmblies = adjust MTU size

2. Software Encryption = Install hardware encryption module

3. Inappropriate switching path = check weather or not the router is running CEF; "show ip cef summary” and “show cef not-cef-switched”

4. High traffic load on node

5. Spanning Tree loop on switch = root switch election is not done properly. make sure to manually choose a root switch

How to see CPU utilization on cisco router?

“show processes cpu”

What are the DHCP messages?

DORA = Discover, Offer, Request, Ack

DHCP Relay Agent Command

ip helper-address ip_address interface configuration mode command to configure a router to relay DHCP messages to a DHCP server

What are some DHCP Issues?

• router not forwarding broadcasts

• DHCP pool out of IP addresses

• duplicate IP addresses

• interface not configured with IP address in DHCP pool

DHCP Troubleshooting command to see IP conflict

show ip dhcp conflict

DHCP Troubleshooting command to see debug information

debug ip dhcp server packet

What is SLACC?

stateless address autoconfiguration for IPv6. Only works for /64 prefix

How to see stateful DHCPv6?

ipv6 nd managed-config-flag interface configuration command

How to see stateless DHCPv6?

ipv6 nd other-config-flag interface configuration command

How to configure DHCPv6 Relay Agent

ipv6 dhcp relay destination interface command