Lab ID: PTL-0019
Topic: Layer 3 Switch SVIs and Routed Ports
Simulator: Cisco Packet Tracer 9.0.1 (Localhost IPC Bridge)
Execution Timestamp: 2026-10-31 02:46:00
Status: tested_pass
1. Test Summary Matrix#
| Test ID | Test Description | Command / Action | Expected Result | Simulator Result | Status |
|---|---|---|---|---|---|
| POS-01 | Verify VLAN Existence | show vlan brief on MLS1 & S1 |
VLAN 10 (Staff) and VLAN 20 (Management) active | Both VLANs active on MLS1 and S1 | PASS |
| POS-02 | Verify SVI Operational States | show ip interface brief on MLS1 |
Vlan10 and Vlan20 report Status: up, Protocol: up |
Vlan10 (192.168.10.1) & Vlan20 (192.168.20.1) up/up | PASS |
| POS-03 | Verify Routing & Connected Subnets | show ip route on MLS1 |
Connected routes for 10.0.0.0/30, 192.168.10.0/24, 192.168.20.0/24, and default route via 10.0.0.2 | Routing table populated with 3 connected and 1 static candidate default route | PASS |
| POS-04 | Local Gateway Ping (Staff) | ping 192.168.10.1 from PC-Staff |
100% ICMP ping success to local SVI gateway | 4/4 replies received (0% loss, <1ms) |
PASS |
| POS-05 | Local Gateway Ping (Management) | ping 192.168.20.1 from PC-Management |
100% ICMP ping success to local SVI gateway | 4/4 replies received (0% loss, <1ms) |
PASS |
| POS-06 | Inter-VLAN Routing Verification | ping 192.168.20.10 from PC-Staff |
Packets routed across SVIs through MLS1 backplane; TTL=127 | 4/4 replies received (0% loss, <1ms, TTL=127) |
PASS |
| POS-07 | Layer 3 Routed Port Operational Mode | show interfaces Gi0/1 switchport |
Switchport disabled; operates purely as routed interface | Switchport: Disabled confirmed on MLS1 Gi0/1 |
PASS |
| POS-08 | Multilayer Switch Upstream Reachability | ping 10.0.0.2 from MLS1 |
100% ICMP success to R1 point-to-point interface | 5/5 ICMP echos succeeded (100%, 0/0/0 ms) | PASS |
| POS-09 | End-to-End External Loopback Reachability | ping 203.0.113.1 from PC-Staff & PC-Management |
Both VLAN hosts reach simulated external destination through R1; TTL=254 | 4/4 replies received on both hosts (0% loss, TTL=254) | PASS |
| POS-10 | Save and Reopen Persistence | fileOpen completed.pkt |
Reopened project maintains full routing table and ping reachability | All devices boot cleanly, inter-VLAN and external pings pass | PASS |
| NEG-01 | Disable Global IP Routing | no ip routing on MLS1 |
Inter-VLAN forwarding disabled (pings time out); local gateway remains reachable | Staff->Mgmt timed out (0/4); Staff->Gateway succeeded (4/4); restored | PASS |
| NEG-02 | SVI Shutdown Fault Injection | shutdown on interface Vlan10 |
SVI goes down; PC-Staff loses gateway and inter-VLAN connectivity | Gateway and inter-VLAN pings timed out (0/4); restored with no shutdown |
PASS |
| NEG-03 | Missing Upstream Default Route | no ip route 0.0.0.0 0.0.0.0 10.0.0.2 on MLS1 |
MLS1 drops packets to external loopback; returns Destination Host Unreachable | Reply from 192.168.10.1: Destination host unreachable; inter-VLAN unaffected |
PASS |
| NEG-04 | Asymmetric Return Route Blackhole | no ip route 192.168.10.0 ... on R1 |
Forward packets reach R1, but return packets dropped; PC-Mgmt still succeeds | PC-Staff external ping timed out (0/4); PC-Mgmt ping succeeded (4/4); restored | PASS |
| NEG-05 | Final Health Confirmation Pass | Comprehensive ping audit across all devices | 100% reachability confirmed across local gateways, inter-VLAN, and external routes | All positive checks verified passing | PASS |
2. Detailed Verification Scenarios#
POS-03: Multilayer Switch Routing Table#
MLS1# show ip route
Gateway of last resort is 10.0.0.2 to network 0.0.0.0
10.0.0.0/30 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, GigabitEthernet0/1
C 192.168.10.0/24 is directly connected, Vlan10
C 192.168.20.0/24 is directly connected, Vlan20
S* 0.0.0.0/0 [1/0] via 10.0.0.2
Analysis: The presence of C entries for both Vlan10 and Vlan20 demonstrates active SVIs participating in Layer 3 routing. The static candidate default route S* 0.0.0.0/0 forwards non-local traffic toward the edge router R1.
POS-06: Inter-VLAN Forwarding (PC-Staff to PC-Management)#
PC> ping 192.168.20.10
Pinging 192.168.20.10 with 32 bytes of data:
Reply from 192.168.20.10: bytes=32 time<1ms TTL=127
Reply from 192.168.20.10: bytes=32 time<1ms TTL=127
Reply from 192.168.20.10: bytes=32 time<1ms TTL=127
Reply from 192.168.20.10: bytes=32 time<1ms TTL=127
Analysis: Workstations in separate VLANs communicate directly across the multilayer switch backplane. The TTL of 127 indicates exactly one Layer 3 routing hop (255 - 1 = 254 or 128 - 1 = 127 depending on host OS stack).
NEG-01: Global IP Routing Disabled Impact#
PC> ping 192.168.20.10
Pinging 192.168.20.10 with 32 bytes of data:
Request timed out.
Request timed out.
PC> ping 192.168.10.1
Pinging 192.168.10.1 with 32 bytes of data:
Reply from 192.168.10.1: bytes=32 time<1ms TTL=255
Analysis: When no ip routing is issued, the multilayer switch reverts to pure Layer 2 bridge behavior. It continues responding to ICMP echos addressed directly to its own local SVI IP (192.168.10.1), but refuses to route packets across subnet boundaries.
NEG-03: Missing Default Static Route Impact#
PC> ping 203.0.113.1
Pinging 203.0.113.1 with 32 bytes of data:
Reply from 192.168.10.1: Destination host unreachable.
Reply from 192.168.10.1: Destination host unreachable.
Analysis: Because MLS1 has no routing table entry matching 203.0.113.1 and no gateway of last resort, the local SVI gateway (192.168.10.1) immediately generates an ICMP Destination Network/Host Unreachable message back to the originating client.
NEG-04: Asymmetric Return Routing Fault#
PC-Staff (192.168.10.10)> ping 203.0.113.1
Request timed out. (100% loss)
PC-Management (192.168.20.10)> ping 203.0.113.1
Reply from 203.0.113.1: bytes=32 time<1ms TTL=254 (100% success)
Analysis: When ip route 192.168.10.0 ... is removed from R1, packets from PC-Staff reach R1, but R1 has no path to return echo replies, resulting in silent timeouts. Concurrently, PC-Management experiences zero packet loss because its return route on R1 remains active.
3. Simulator Evidence Reference#
The complete unabridged Cisco Packet Tracer CLI session capture is preserved in: AI_AGENT_GUIDE/state/PTL-0019-SIMULATOR-RESULTS.md