Troubleshooting Flashcards
Troubleshooting Steps
- ID Problem
- Est. Theory
- Test Theory
- Est. Plan of Action
- Implement
- Verify Success
- Document Findings
ID Problem
What Changed
Duplicate if possible
Break into smaller pieces, address each
Establish Theory
Start with Obvious
Top-down, Bottom-up along OSI layers
Break to small pieces, remove pieces that don’t apply
Test Theory
Implement change, examine result
Plan of Action
Minimize impact on ops
Plan B
Plan C
Have backup to go back/undo changes
Implement Plan
Try the fix
Escalate if necessary
Verify Functionality
Verify full system functionality
Document Findings
Help Desk Database/Knowledge Base
Signal errors on Fiber network
Check that right fiber type is used
Single/multimode fiber, different fiber diameters
Writing on outside of fiber cable
Cable analyzer shows interference measuring NEXT or low SNR
Near End CrossTalk
Signal to Noise Ratio
Measure at both ends of cable
Maybe need shielded cable
Check crimp
Maintain max twists in pairs
Tool to monitor status of network devices and connections
With SNMP track metrics in a MIB
Management Information Base
What to monitor with SNMP
Link Status
Utilization, throughput over time chart
Error Rate
- CRC errors, runts/giants, drops
CRC Error
Cyclic Redundancy Check
FCS at end of frame does not match result from CRC
Frame Check Sequence - checksum at end of frame
Problem with signal, check cable, connections
Runts
Received Frame lass than 64 bytes
May be result of collision if half-duplex network
Giants
Received Frame greater than 1518 bytes (or max configured frame size)
Drops
Frame not transmitted or received
How to prevent Switching Loops
Enable STP
Root Bridge Selection - STP
All bridges/switches are assigned a bridge ID
Lowest ID is root
If tie, lowest MAC is root
Port Roles - STP
Root Port - port traffic takes to get to the root bridge
Designated Port - active port to other devices on net
Blocked Port - port deactivated to prevent loops
Port States - STP
Blocking/Discarding
Listening - not forwarding, cleaning the MAC table
Learning - not forwarding, adding to the MAC table
Forwarding - actively forwarding messages
Disabled - admin manually turned off port
Everything appears to be correctly configured but packets are still getting dropped
Check if an ACL is configured somewhere on the network blocking traffic
Access Control List
ICMP Host Unreachable Message
Tells you that router couldn’t find a route to destination
Check for missing entry in routing tables
A client is assigned an APIPA address despite having DHCP configured
DHCP address pool may be exhausted
With APIPA, can only access resources on local subnet
Monitor for this using IPAM
Mitigate by reducing lease time
IPAM
IP Address Management
Tool to monitor and report IP address shortages
