Overview
MSTP (Multiple Spanning Tree Protocol, IEEE 802.1s) is a spanning tree variant that groups VLANs into multiple spanning tree instances (MSTIs), each with its own Root Bridge election and forwarding topology. Unlike STP and RSTP — which run a single tree for all VLANs — MSTP enables per-VLAN load balancing across redundant links, reducing wasted bandwidth on blocked ports. This guide covers the complete MSTP configuration procedure for ORing 9000 Series managed switches ([PRODUCT_MODELS]).
Prerequisites
- Applicable models: [PRODUCT_MODELS]
- Minimum firmware: [FIRMWARE_VERSION]
- Login level required: Administrator
- All switches in the MST region must be configured with identical Configuration Name and Configuration Revision values (Step 2)
- VLANs to be mapped to MSTIs must already be created on the switch before Step 3
MSTP vs STP vs RSTP — quick reference
| Protocol | Standard | Convergence time | Multiple trees | VLAN load balancing |
|---|---|---|---|---|
| STP | IEEE 802.1D | 30–50 seconds | No (single tree) | ❌ No |
| RSTP | IEEE 802.1w | < 1 second | No (single tree) | ❌ No |
| MSTP | IEEE 802.1s | < 1 second | Yes (up to 64 MSTIs) | ✅ Yes |
Step-by-step MSTP configuration
Complete all six steps on every switch that will participate in the same MST region. Steps 1–3 and Step 5 must be performed on each switch individually; Step 4 must be applied to every port that connects to another MSTP-enabled device.
Step 1 — Select Spanning Tree type: MSTP
Navigate to [MENU_PATH_STP]. In the Spanning Tree Protocol field, change the type from STP or RSTP to MSTP. Click Apply to save the change. Switching to MSTP mode activates the MST-specific configuration fields (Configuration Name, Revision, and MSTI settings) that are hidden when STP or RSTP is selected.
Step 2 — Set the same Configuration Name and Revision on every switch
Navigate to [MENU_PATH_STP] and set the Configuration Name and Configuration Revision fields. Every switch in the same MST region must use identical values for both fields. The Configuration Name can be any alphanumeric string (up to 32 characters); the Configuration Revision is a number between 0 and 65535.
If any switch has a mismatched Configuration Name or Revision, it will be treated as a separate MST region boundary. In this case, VLAN-to-MSTI mappings will not be shared across the boundary, and inter-region traffic will fall back to CIST (Common and Internal Spanning Tree) behaviour. Use a consistent naming convention such as the site name or network segment (e.g., FACTORY_FLOOR_A, revision 1) to avoid accidental mismatches.
Step 3 — Map VLANs to MSTI instances
Navigate to [MENU_PATH_MSTI]. For each MSTI instance, enter the VLAN IDs that should follow that instance's spanning tree topology. Each VLAN can be assigned to only one MSTI; VLANs not explicitly assigned default to MSTI 0 (CIST).
Example mapping:
- MSTI 1 → VLAN 10, 20 (e.g., production floor devices)
- MSTI 2 → VLAN 30, 40 (e.g., office and management devices)
With this mapping, you can configure a different Root Bridge for MSTI 1 and MSTI 2 in Step 5, distributing traffic across both uplinks instead of blocking one entirely.
Step 4 — Enable MSTP on each port
Navigate to [MENU_PATH_PORT]. For each port that connects to another MSTP-enabled switch or endpoint device, set the port's STP mode to MSTP and click Apply. Ports left in STP or RSTP mode will not participate in MSTP instance topology and may cause loop detection issues on VLAN-tagged trunks.
Typically, enable MSTP on all uplink (trunk) ports that carry tagged VLAN traffic between switches. Access ports connecting to end devices (PCs, PLCs, cameras) can remain in their default state or be set to Edge Port mode to speed up link-up convergence.
Step 5 — Set MSTI bridge priority
Navigate to [MENU_PATH_MSTI] and set the Bridge Priority for each MSTI on each switch. The switch with the lowest priority value becomes the Root Bridge for that MSTI. Priority values must be multiples of 4096 (valid range: 0–61440; default: 32768).
To achieve load balancing across two uplink switches:
- Set Switch A priority to
4096for MSTI 1 and8192for MSTI 2 → Switch A becomes Root Bridge for MSTI 1 - Set Switch B priority to
8192for MSTI 1 and4096for MSTI 2 → Switch B becomes Root Bridge for MSTI 2
This configuration directs VLAN 10/20 traffic through Switch A and VLAN 30/40 traffic through Switch B, utilising both uplinks simultaneously.
Step 6 — Verify the MSTP configuration
Navigate to [VERIFY_PATH] to confirm the MSTP configuration is operating correctly. Check the following on each switch in the region:
- MST Region: Configuration Name and Revision match across all switches
- Root Bridge per MSTI: The MAC address shown as Root Bridge matches the switch you intended in Step 5
- Port roles: Uplink ports show the expected role (Root, Designated, or Alternate) for each MSTI
- Port states: No ports should remain in Discarding state on links that should be active for a given MSTI
If the Root Bridge is not as expected, re-check priority values (must be multiples of 4096) and confirm MSTP is enabled on all relevant ports (Step 4). If port roles appear incorrect across the region boundary, verify that Configuration Name and Revision are identical on all switches (Step 2).
Troubleshooting common MSTP issues
Configuration name or revision mismatch
Symptom: switches appear in separate MST regions; VLAN-to-MSTI mappings do not propagate. Fix: navigate to [MENU_PATH_STP] on each switch and verify the Configuration Name (case-sensitive) and Revision number are byte-for-byte identical. A common mistake is a trailing space in the Configuration Name field.
Root Bridge not switching to the intended switch
Symptom: after setting a lower priority in Step 5, the Root Bridge does not change. Fix: (1) confirm the priority value is a valid multiple of 4096; (2) verify MSTP is enabled on the connecting ports (Step 4); (3) check that the switch's MSTP is active — some models require a save-and-reboot for spanning tree type changes to take effect.
VLAN traffic not reaching devices after MSTP configuration
Symptom: devices on specific VLANs lose connectivity after MSTP is configured. Fix: (1) confirm the affected VLANs are mapped to an MSTI in Step 3 (unmapped VLANs fall to CIST/MSTI 0); (2) check that trunk ports carry the relevant VLANs; (3) verify port states in [VERIFY_PATH] — if a port is in Discarding state for the relevant MSTI, check for duplicate Root Bridge priority values on multiple switches.
Frequently asked questions
What happens if two switches have different MSTP configuration names?
Switches with different MST Configuration Name or Revision values are treated as separate MST region boundaries. VLAN-to-MSTI mappings are not shared across the boundary, and inter-region traffic falls back to CIST behaviour. Always verify all switches in the same region have identical Configuration Name (case-sensitive) and Revision values.
Why is the Root Bridge not switching to the expected switch after setting MSTI priority?
Root Bridge election uses the lowest Bridge Priority value, with the lowest MAC address as a tiebreaker. If the Root Bridge is not switching: (1) confirm the priority is a valid multiple of 4096; (2) verify MSTP is enabled on the relevant ports; (3) check Configuration Name and Revision match on all switches. Some models require a save-and-reboot for spanning tree type changes to take full effect.
Can a VLAN belong to more than one MSTI?
No. Each VLAN can be mapped to exactly one MSTI. Assigning a VLAN to a second MSTI automatically removes it from the first. VLANs not explicitly assigned to any MSTI default to MSTI 0 (CIST).
What is the difference between MSTP, RSTP, and STP?
STP (IEEE 802.1D) uses a single spanning tree for all VLANs with 30–50 second convergence. RSTP (IEEE 802.1w) improves convergence to under 1 second but still uses one tree. MSTP (IEEE 802.1s) extends RSTP with multiple instances (MSTIs), each carrying a group of VLANs and electing its own Root Bridge, enabling load balancing across redundant uplinks. See What is STP, RSTP, and MSTP? for a full comparison.
