Lab ID: PTL-0037
Title: DNS Resolution Failure Troubleshooting
Status: TESTED_PASS (Verified live in Cisco Packet Tracer 9.0.1 via MCP loopback bridge automation)
1. Packet Tracer Topology Construction Manifest#
To construct or manually verify this topology in Cisco Packet Tracer:
Device Inventory#
| Device Role | PT Device Model | Display Name | Modules / Slots |
|---|---|---|---|
| Default Gateway Router | Cisco ISR 4321 | R1 | Default GigabitEthernet0/0/0 & GigabitEthernet0/0/1 |
| Client Access Switch | Cisco Catalyst 2960-24TT | S1 | 24 FastEthernet + 2 GigabitEthernet ports |
| Server Access Switch | Cisco Catalyst 2960-24TT | S2 | 24 FastEthernet + 2 GigabitEthernet ports |
| Client Workstation 1 | PC-PT | PC-1 | FastEthernet0 |
| Client Workstation 2 | PC-PT | PC-2 | FastEthernet0 |
| Corporate DNS Server | Server-PT | DNS-Server | FastEthernet0 |
| Corporate Web Portal | Server-PT | Web-Server | FastEthernet0 |
Physical Cabling Matrix#
| Link ID | Source Device | Source Port | Destination Device | Destination Port | Cable Type |
|---|---|---|---|---|---|
| L-01 | R1 | GigabitEthernet0/0/0 | S1 | FastEthernet0/1 | Copper Straight-Through |
| L-02 | R1 | GigabitEthernet0/0/1 | S2 | FastEthernet0/1 | Copper Straight-Through |
| L-03 | S1 | FastEthernet0/2 | PC-1 | FastEthernet0 | Copper Straight-Through |
| L-04 | S1 | FastEthernet0/3 | PC-2 | FastEthernet0 | Copper Straight-Through |
| L-05 | S2 | FastEthernet0/2 | DNS-Server | FastEthernet0 | Copper Straight-Through |
| L-06 | S2 | FastEthernet0/3 | Web-Server | FastEthernet0 | Copper Straight-Through |
2. Positive Verification Test Matrix#
| Test ID | Objective | Command / Procedure | Expected Result | Pass Criteria |
|---|---|---|---|---|
| POS-01 | Router Interface Status | R1# show ip interface brief |
GigabitEthernet0/0/0 (192.168.1.1) and GigabitEthernet0/0/1 (192.168.2.1) status up, protocol up |
Both routed gateway interfaces up/up |
| POS-02 | Direct IP Subnet Reachability | PC-1> ping 192.168.2.20 |
4/4 echo replies from 192.168.2.20 with TTL=127 across router R1 |
Layer 1-3 network transport verified |
| POS-03 | DNS Service & Database Verification | DNS-Server > Config > Services > DNS |
DNS Service is On, resource record database contains 3 records |
Service enabled with required A & CNAME records |
| POS-04 | Primary Domain Name Resolution | PC-1> nslookup corpweb.local |
Query against 192.168.2.10 returns Name: corpweb.local, Address: 192.168.2.20 |
Primary FQDN resolves to target server IP |
| POS-05 | Hostname ICMP Reachability | PC-1> ping corpweb.local |
Hostname resolves to 192.168.2.20, 4/4 echo replies received (TTL=127) |
Hostname resolution integrated into ping utility |
| POS-06 | Secondary Portal Name Resolution | PC-2> nslookup portal.corpweb.local |
Query against 192.168.2.10 returns Name: portal.corpweb.local, Address: 192.168.2.20 |
Secondary FQDN resolves to target server IP |
| POS-07 | Canonical Name Alias Resolution | PC-2> nslookup www.corpweb.local |
Query against 192.168.2.10 returns Aliases: corpweb.local, Address: 192.168.2.20 |
CNAME alias resolves correctly to canonical host IP |
3. Negative & Fault Tolerance Test Matrix#
| Test ID | Fault Scenario | Injection Procedure | Observed Symptom | Recovery Procedure |
|---|---|---|---|---|
| NEG-01 | Invalid Client DNS Server IP | On PC-1, set DNS Server IP to non-existent 192.168.2.99. Run nslookup corpweb.local. |
Query fails with Server: [192.168.2.99] followed by DNS request timed out. timeout was 15000 milli seconds. |
Execute ipconfig /all on client, note invalid DNS address, and update client DNS Server IP to 192.168.2.10. |
| NEG-02 | Missing / Non-Existent Domain Record | On PC-2, execute nslookup unknown.corpweb.local. |
DNS server responds with *** UnKnown can't find unknown.corpweb.local: Non-existent domain. |
Verify DNS server database; if record should exist, add an authoritative A record mapping domain to target IP. |
4. Save and Persistence Verification Check#
- On R1, S1, and S2, execute
copy running-config startup-config. - Save the completed topology file (
completed.pkt). - Reload
completed.pktin Cisco Packet Tracer. - Execute
ping corpweb.localonPC-1and confirm DNS service and database records persist across reloads. - Reload
starter.pktand confirm that direct IP pingping 192.168.2.20succeeds whilenslookup corpweb.localfails with timeout to192.168.2.99, confirming clear differentiation between starter and solution files.