Successful inter-VLAN routing has been operating on a network with multiple VLANs across multiple switches for some time. When an inter-switch trunk link fails and Spanning Tree Protocol brings up a backup trunk link, it is reported that hosts on two VLANs can access some, but not all the network resources that could be accessed previously. Hosts on all other VLANS do not have this problem. What is the most likely cause of this problem?
- The protected edge port function on the backup trunk interfaces has been disabled.
- The allowed VLANs on the backup link were not configured correctly.
- Dynamic Trunking Protocol on the link has failed.
- Inter-VLAN routing also failed when the trunk link failed.
Explanation: When a backup trunk link is activated by STP and only specific VLANs experience connectivity issues while others remain functional, the most likely cause is that the backup interface's allowed VLAN list was not configured to include all the necessary VLAN IDs. Because trunks only propagate traffic for VLANs explicitly permitted in their configuration, an incomplete allowed list on the backup link will block traffic for the missing VLANs once the primary link fails, even if inter-VLAN routing is otherwise operational.
Related exam: Switching, Routing and Wireless Essentials (Version 7.00) – SRWEv7 Final Exam
