https://networkhaven.org/tags/spanning-tree/ Recent content in Spanning-Tree on Network Haven Hugo en-US Fri, 02 Oct 2026 00:00:00 +0000 https://networkhaven.org/posts/rpvst-behavior-of-rpvst-and-vlans-that-are-not-allowed-via-trunk-links/ Fri, 02 Oct 2026 00:00:00 +0000 https://networkhaven.org/posts/rpvst-behavior-of-rpvst-and-vlans-that-are-not-allowed-via-trunk-links/ <p>Today I encountered a scenario in which an HPE Aruba ProCurve switch (AOS-S), used as an access switch in a classic three-tier network topology, declares certain VLANs (in this case, VLAN 16) as root bridges. However, on the core switch, all VLANs are configured with a spanning tree priority of 4096 — see VLANs 12 and 53.</p> <pre tabindex="0"><code> VLAN Root Mac Root Root Root Hello ID Address Priority Path-Cost Port Time(sec) ----- --------------- ---------- ---------- -------------------- --------- 1 f8f8f8-c3c3c3 32,768 0 This switch is root 2 998 f8f8f8-c3c3c3 32,768 0 This switch is root 2 999 f8f8f8-c3c3c3 32,768 0 This switch is root 2 53 1bfdec-1bfdec 4096 2250 Trk1 2 12 1bfdec-1bfdec 4096 2250 Trk1 2 16 f8f8f8-c3c3c3 32,768 0 This switch is root 2 </code></pre><p>If you think about it, the reason is relatively clear. All VLANs that have been created on the switch participate in Rapid Per-VLAN Spanning Tree formation and also send <strong>different</strong> BPDUs per VLAN over a trunk link. If the VLAN is not permitted on the trunk, the BPDUs for that VLAN will never be sent over the trunk and therefore cannot negotiate with the neighboring switch to determine who will be the root bridge.</p>