Summary : Troubleshooting and resolving LAN/WAN performance, connectivity, and related network problems

 As a Wireless NOC L1 Engineer or CCNA candidate, troubleshooting LAN/WAN issues means finding the root cause of problems affecting connectivity, performance, or availability. A structured approach helps you resolve issues quickly.

1. LAN (Local Area Network) Troubleshooting

LAN issues occur within a building, office, or campus.

Common Problems

ProblemPossible CauseTroubleshooting
PC cannot ping default gatewayWrong IP, subnet mask, gatewayCheck ipconfig, verify IP settings
Switch port downCable unplugged, interface shutdownCheck LEDs,
show interfaces status
Slow LANDuplex mismatch, congestionCheck speed/duplex settings
VLAN communication failureWrong VLAN assignmentshow vlan brief
STP blocking trafficIncorrect topologyshow spanning-tree
DHCP not assigning IPDHCP server unreachableipconfig /renew, check DHCP server
MAC address learning issueCAM table problemsshow mac address-table

2. WAN Troubleshooting

WAN connects branch offices through leased lines, MPLS, VPNs, or the Internet.

Common Problems

ProblemPossible CauseTroubleshooting
Branch cannot reach HQWAN link downCheck interface status
VPN tunnel downAuthentication mismatchVerify IPsec/IKE configuration
High latencyISP congestionUse ping and traceroute
Packet lossInterface errorsCheck interface counters
Routing failureMissing routesshow ip route
Internet inaccessibleDefault route missingVerify show ip route

3. Connectivity Problems

These are the most common issues encountered by network engineers.

Example 1: PC Cannot Reach Internet

PC
 |
Switch
 |
Router
 |
Internet

Step-by-step troubleshooting

  1. Check physical cable.
  2. Verify IP address.
  3. Verify subnet mask.
  4. Verify default gateway.
  5. Ping localhost (127.0.0.1).
  6. Ping own IP.
  7. Ping default gateway.
  8. Ping ISP router.
  9. Ping 8.8.8.8.
  10. Ping a domain name (for example, google.com).

If IP pings work but domain names do not, the problem is likely DNS.

Example 2: Interface Down

R1# show ip interface brief

GigabitEthernet0/0
Status: administratively down

Solution:

R1(config)# interface g0/0
R1(config-if)# no shutdown

Example 3: Wrong VLAN

PC1 ---- SW1 ---- PC2

PC1 is in VLAN 10.

PC2 is in VLAN 20.

They cannot communicate without inter-VLAN routing.

Check:

show vlan brief

4. Performance Problems

Connectivity exists, but the network is slow.

Common causes:

  • High bandwidth utilization
  • Duplex mismatch
  • Broadcast storms
  • Packet drops
  • High CPU utilization
  • Memory exhaustion
  • Congested WAN links

Useful commands:

show interfaces

Look for:

  • Input errors
  • CRC errors
  • Output drops
  • Collisions

5. Routing Problems

Symptoms:

  • Remote networks unreachable
  • Ping fails after the first router

Commands:

show ip route
show ip protocols
traceroute

Verify:

  • Static routes
  • Default route
  • Dynamic routing protocol neighbors (OSPF, EIGRP, RIP)

6. DNS Problems

Symptoms:

Ping 8.8.8.8  ✓

Ping google.com ✗


nslookup google.com
Example of a DNS failure:
*** Request timed out

Cause:

DNS server unreachable or incorrectly configured.

Check the Configured DNS Server

ipconfig /all

DNS Servers . . . . . : 192.168.1.1


Check that:
  • A DNS server is configured.

    • 

  • The IP address is correct.

7. DHCP Problems

Symptoms:

169.254.x.x

This indicates the host assigned itself an Automatic Private IP Address (APIPA) because it could not obtain an IP lease from a DHCP server.

Check:

  • DHCP server availability
  • VLAN configuration
  • Relay agent (ip helper-address) if the DHCP server is on a different subnet

Commands:

ipconfig /release

ipconfig /renew

8. VPN Problems

Symptoms:

  • Tunnel down
  • Users cannot access remote resources

Check:

  • Internet connectivity
  • Peer IP address
  • Pre-shared key
  • Encryption settings
  • ACLs (traffic selectors)
  • Tunnel interface status

Useful troubleshooting commands:

show crypto isakmp sa
show crypto ipsec sa

9. Wireless Problems

Symptoms:

  • Cannot connect to Wi-Fi
  • Slow speeds
  • Frequent disconnections

Check:

  • SSID
  • WPA/WPA2/WPA3 password
  • Signal strength
  • Channel interference
  • Authentication server (such as RADIUS in enterprise Wi-Fi)

10. Basic Troubleshooting Flow

User reports problem
        │
        ▼
Check Physical Layer
(Cables, LEDs, Interfaces)
        │
        ▼
Check IP Configuration
(IP, Mask, Gateway)
        │
        ▼
Check VLAN/Switch Port
        │
        ▼
Check Routing
        │
        ▼
Check DNS/DHCP
        │
        ▼
Check WAN/VPN
        │
        ▼
Verify Resolution

Common Cisco Commands for Troubleshooting

CommandPurpose
show ip interface briefCheck interface IPs and status
show interfacesView errors, bandwidth, duplex, speed
show ip routeDisplay routing table
show arpView IP-to-MAC mappings
show mac address-tableCheck learned MAC addresses
show vlan briefVerify VLAN assignments
show spanning-treeInspect STP state
show cdp neighborsDiscover directly connected Cisco devices
show running-configReview current configuration
pingTest basic reachability
tracerouteIdentify where packets stop along the path
show loggingReview system log messages

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