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After a switch replacement over the weekend, users on the third floor can reach each other but cannot reach the file server on the second floor. Both VLANs are configured on both switches. Both trunks are up. The problem started the moment the new switch was put in.
Classic trunk mismatch. The most likely cause: the new switch's trunk has a different allowed VLAN list, a mismatched native VLAN, or the wrong encapsulation. When a trunk partially works — some VLANs pass, others don't — the issue is almost always in the allowed VLAN list.
VLAN 20 is on the allowed list, but not in the active or forwarding list. That means VLAN 20 doesn't exist on this switch — it's allowed on the trunk but the switch doesn't have the VLAN in its database. Users in VLAN 20 have no path.
! Verify VLAN 20 existsSW1# show vlan brief | include 20! If it's missing:SW1(config)# vlan 20SW1(config-vlan)# name ServersSW1(config-vlan)# exit! Verify it's now active on the trunkSW1# show interfaces trunk
Users in VLAN 10 can reach the core switch. Users in VLAN 40 cannot, even though VLAN 40 exists on both access and core switches. `show interfaces trunk` on the access switch shows VLAN 40 is NOT in the allowed list. What's the fix?
Select the safest next action.
VLAN/trunk failures come down to four things: VLAN doesn't exist, port not in VLAN, trunk not up, allowed list incomplete. The `show interfaces trunk` output tells you which one. Section 19 continues with STP and EtherChannel troubleshooting.
Finish the lesson once you have worked through the material. This awards ★ 40 XP.
Select each vulnerable line, then classify the finding.
Findings
Review target: 13 lines · 2 expected findings