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Aisalkyn Aidarova
Aisalkyn Aidarova

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Runbook: Application Down (Packet Tracer / Network-Based App)

📌 Scenario

User reports: “Application is down (app.local not working)”

🎯 Goal

Quickly identify whether the issue is:

  • DNS
  • Network (IP / routing)
  • VLAN / switching
  • Server / application

🔍 Step-by-Step Troubleshooting

1. Check DNS Resolution 

ping app.local
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Result:

  • ❌ Fails → possible DNS issue
  • ✅ Works → DNS is OK → go to Step 2

2. Check IP Connectivity 

ping 192.168.50.10
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Result:

  • ❌ Fails → network/routing problem
  • ✅ Works → network OK → go to Step 3

3. Verify IP Configuration (Client) 

ipconfig
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Check:

  • IP address assigned?
  • Correct subnet?
  • Gateway present?
  • DNS server present?

Common issues:

  • No IP → DHCP failure
  • Wrong subnet → VLAN issue
  • Missing DNS → DHCP misconfig

4. Check Default Gateway 

ping 192.168.X.1
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(X = client VLAN)

Result:

  • ❌ Fails → VLAN / switch / trunk issue
  • ✅ Works → gateway OK → continue

5. Check Server Reachability 

ping 192.168.50.10
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If fails:

  • Server down
  • VLAN 50 issue
  • Routing issue

6. Check Server Services

On Server:

  • DNS → ON
  • HTTP → ON

Test:

Browser:

http://192.168.50.10
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7. Check VLAN Configuration

On Switch:

show vlan brief
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Verify:

  • PC ports in correct VLAN
  • Server in VLAN 50

8. Check Trunk Links 

show interfaces trunk
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Verify:

  • VLANs 10,20,30,50 allowed
  • trunk is active

9. Check Routing (Router) 

show ip interface brief
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Verify:

  • g0/0.10 → 192.168.10.1
  • g0/0.20 → 192.168.20.1
  • g0/0.30 → 192.168.30.1
  • g0/1.50 → 192.168.50.1

10. Check DHCP Relay 

show run | include helper
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Verify:

ip helper-address 192.168.50.10
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🧠 Root Cause Mapping

Symptom Likely Issue
ping app.local fails, IP works DNS issue
no IP assigned DHCP issue
gateway unreachable VLAN/trunk issue
server unreachable routing or VLAN 50 issue
IP works, app fails application/service issue

⚡ Quick Diagnosis Flow

  1. DNS?
  2. IP?
  3. Gateway?
  4. Server?
  5. VLAN?
  6. Routing?

💡 SRE Insight

Always troubleshoot layer by layer:

  1. DNS
  2. Network
  3. Routing
  4. Application

Never jump randomly — follow the chain.

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