Ethernet Switching and Spanning Tree
What the Ethernet Switching and Spanning Tree domain tests
This domain tests VLAN membership, trunking, RSTP/MSTP/VSTP, root election, and loop protection on Junos switches. You must predict which port blocks and why.
Core rules
- Access ports carry one untagged VLAN; trunk ports carry tagged VLANs and may carry a native untagged VLAN.
- RSTP uses root, designated, alternate, and backup roles; the root bridge is the lowest bridge ID.
- MSTP maps VLANs to instances; all switches in a region must share the region name, revision, and VLAN-to-instance mapping.
- Junos VSTP can run per-VLAN spanning tree; edge ports should use BPDU guard and loop protection.
- A blocked alternate port is normal; a root port election follows path cost and bridge ID.
Worked example
Two switches in an MSTP region fail to converge as expected. Switch A maps VLAN 10 to instance 1, while Switch B maps VLAN 10 to instance 2. The region configuration does not match, so the switches treat each other as boundary bridges. One link blocks unexpectedly. Aligning the region name, revision, and VLAN-to-instance mapping makes both switches agree on the instance topology and restores the intended forwarding path.
Traps
A native VLAN mismatch on a trunk does not necessarily block STP, but it can leak untagged traffic between VLANs. Do not enable BPDU guard on uplinks. Do not assume MSTP instance 0 carries all VLANs unless mapped. Check both interface mode and VLAN membership.