Lab ts-na-01 - Troubleshooting Switching and VLAN Tickets

Layer 2 faults are the ones that make engineers doubt their own eyes. Two devices on the same subnet, both interfaces up, both pinging their own gateway, and yet they cannot reach each other. The IP configuration is flawless. The problem is one layer down, in how the switch decides which port belongs to which broadcast domain. This lab gives you three switching tickets on the five-node base topology. It is part of the PingLabz CCNA Troubleshooting Labs, fits Cisco Modeling Labs Free, and every output was captured on a Cisco IOS XE managed switch.

The topology and healthy state

SW1 is the managed switch in the middle of the LAN. Its ports: Et0/0 to R1, Et0/1 to R2, Et0/2 to HOST1. In the healthy state all three are access ports in VLAN 10 (USERS), VLAN 20 (SERVERS) also exists in the database, and R1 (10.20.0.1) pings R2 (10.20.0.2) cleanly:

R1# ping 10.20.0.2
!!!!!
Success rate is 100 percent (5/5)

Lab setup: this topology boots with all three faults already in place, one per ticket. Faults can mask each other, so work bottom-up and re-test after each fix. The downloadable topology and the full ticket walkthroughs are part of PingLabz Pro.

What you will learn

• Why same-subnet hosts that cannot ping each other is almost always a VLAN problem, not an IP problem.
• How to read show vlan brief, show interfaces status, and show mac address-table together to place each device in its broadcast domain.
• The silent trap of a mistyped VLAN ID that IOS "helpfully" auto-creates.
• What happens when an access port to a router gets set to trunk mode, and the native-VLAN behavior that explains it.

Ticket 1: "Two devices on the same subnet can't ping each other"

Reported symptom: "R1 and R2 are both 10.20.0.x/24, both interfaces are up, but they can't ping. I've checked the IPs four times."
Success criterion: R1 can ping 10.20.0.2.

R1# ping 10.20.0.2
.....
Success rate is 0 percent (0/5)

The IPs really are correct, so stop checking them. When same-subnet hosts cannot reach each other, drop to Layer 2 and ask the switch where each device lives:

SW1# show vlan brief
VLAN Name      Status    Ports
10   USERS     active    Et0/0, Et0/2
20   SERVERS   active    Et0/1            <-- R2 is stranded here