In which configuration mode is revision [number] used?

The revision [number] command is entered in MST Config mode on Cisco IOS devices.

Spanning TreeMST Config

revision [number]

Sets the revision number for the MST configuration, which is used to identify the MST region and must match on all switches in the same region.

Definition: revision [number] is a Cisco IOS mst config command. Sets the revision number for the MST configuration, which is used to identify the MST region and must match on all switches in the same region.

Overview

The 'revision' command in MST (Multiple Spanning Tree) configuration mode sets the revision number for the MST region. This number, along with the region name and VLAN-to-instance mapping, uniquely identifies an MST region. All switches in the same MST region must have identical revision numbers, region names, and VLAN mappings; otherwise, they will consider each other as belonging to different regions, breaking MST interoperability.

The revision number is a user-defined integer from 0 to 65535, typically incremented when changes are made to the MST configuration to ensure consistency. This command is crucial when deploying MST in a network to prevent loops and ensure efficient load balancing across multiple spanning tree instances. Without a matching revision number, switches will treat each other as separate regions, falling back to CST (Common Spanning Tree) behavior, which may lead to suboptimal path selection or even loops.

The command is used during initial MST setup or when modifying an existing MST configuration. It is entered in MST configuration mode, accessed via 'spanning-tree mst configuration'. The revision number is stored in the running configuration and must be saved to startup configuration to persist.

A common mistake is forgetting to update the revision number after changing VLAN mappings, which can cause region mismatches. The command does not require any special privilege level beyond privileged EXEC (enable mode). It has no immediate impact on traffic; changes take effect after exiting MST configuration mode with 'exit' or 'end'.

Syntax·MST Config

revision [number]

When to Use This Command

Configuring a consistent revision number across all switches in an MST region to ensure they recognize each other as part of the same region.
Changing the revision number when modifying the MST configuration to force a new region identifier.
Setting the revision number to a non-zero value to avoid conflicts with the default revision of 0.
Troubleshooting MST region mismatches by verifying and aligning revision numbers.

Parameters

Parameter	Syntax	Required	Description
number	<0-65535>	Required	Sets the revision number for the MST region. Valid values are from 0 to 65535. The default is 0. Common mistakes include using a number that does not match other switches in the region, or forgetting to increment the revision number after making changes to the MST configuration.

Command Examples

Setting MST revision number to 1

revision 1

No output is generated for this command. It simply sets the revision number to 1 in the MST configuration context.

Verifying MST configuration after setting revision

show current

Current MST configuration:
Name      []
Revision  1
Instance  Vlans mapped
--------  -----------------------------------------------------------------
0         1-4094

The output shows the current MST configuration. 'Revision 1' confirms the revision number was set to 1. The instance mapping shows VLANs 1-4094 are mapped to instance 0 by default.

Understanding the Output

The 'revision' command itself produces no output. To verify the revision number, use 'show current' or 'show pending' in MST configuration mode. The revision number is displayed as 'Revision <number>'.

It must match on all switches in the same MST region. A mismatch will prevent switches from forming a single region, causing them to treat each other as separate regions (CST). The default revision is 0, which is commonly used but can be changed to any integer.

Ensure consistency across all switches in the region.

Configuration Scenarios

Configure MST region with revision number on two switches

Two switches, SW1 and SW2, need to be part of the same MST region to support multiple spanning tree instances for VLAN load balancing. They must have identical region name and revision number.

Topology

SW1(Gi0/1)---trunk---(Gi0/1)SW2

Steps

1.Step 1: Enter global configuration mode on SW1: configure terminal
2.Step 2: Enter MST configuration mode: spanning-tree mst configuration
3.Step 3: Set the region name: name COURSEIVA
4.Step 4: Set the revision number: revision 1
5.Step 5: Exit MST configuration mode: exit
6.Step 6: Repeat steps 1-5 on SW2 with identical name and revision number.

Configuration

! SW1 configuration
configure terminal
spanning-tree mst configuration
name COURSEIVA
revision 1
exit
end

Verify: show spanning-tree mst configuration Expected output: Name [COURSEIVA], Revision 1

Watch out: If the revision number does not match on both switches, they will not form a common MST region. Always verify with 'show spanning-tree mst configuration' on all switches.

Update revision number after modifying VLAN-to-instance mapping

After adding a new VLAN mapping to an MST instance, the revision number should be incremented to ensure all switches in the region have the same configuration. This prevents region mismatches.

Topology

SW1(Gi0/1)---trunk---(Gi0/1)SW2

Steps

1.Step 1: Enter MST configuration mode on SW1: spanning-tree mst configuration
2.Step 2: Modify VLAN mapping: instance 1 vlan 10, 20
3.Step 3: Increment revision number: revision 2
4.Step 4: Exit and save: exit, write memory
5.Step 5: Repeat on SW2 with same revision number and VLAN mapping.

Configuration

! SW1 configuration
configure terminal
spanning-tree mst configuration
instance 1 vlan 10, 20
revision 2
exit
end

Verify: show spanning-tree mst configuration Expected output: Instance 1 Vlans mapped: 10,20; Revision 2

Watch out: Forgetting to update the revision number on all switches after changing VLAN mappings will cause MST region inconsistency. Always increment the revision number on every switch in the region.

Troubleshooting with This Command

When troubleshooting MST region issues, the 'revision' command is often a key suspect. Healthy output from 'show spanning-tree mst configuration' should show identical region name, revision number, and VLAN mappings on all switches in the same region. If the revision number differs, switches will not form a common region, and you may see messages like 'MST region mismatch' in logs.

Focus on the revision number field; a mismatch indicates that at least one switch has a different revision. Common symptoms include unexpected blocking of ports, suboptimal path selection, or loops. To diagnose, start by checking the MST configuration on each switch with 'show spanning-tree mst configuration'.

Compare the revision numbers. If they differ, determine which switch has the correct revision and update the others. Also check the VLAN mapping consistency; even if revision numbers match, different VLAN mappings will cause a mismatch.

Use 'debug spanning-tree mst events' to see region negotiation messages. Correlate with 'show spanning-tree mst' to see which instances are active. If a switch is isolated, it will show 'IST master' as itself.

A step-by-step flow: 1) Verify MST is enabled globally. 2) Check MST configuration on all switches. 3) Ensure region name, revision, and VLAN mappings match exactly. 4) If mismatched, correct the configuration and reload or renegotiate by toggling the MST configuration. 5) Verify with 'show spanning-tree mst' that all switches agree on the IST master. Remember that the revision number is just a number; it does not affect MST operation beyond region identification. However, it must be incremented after any change to ensure all switches have the same configuration.

A common pitfall is using revision 0 (default) and not changing it, which is fine as long as all switches have revision 0 and identical mappings. But if any switch has a non-zero revision, all must match.

CCNA Exam Tips

Remember that the revision number must match on all switches in the same MST region; otherwise, they will not form a single region.

The default revision number is 0; changing it to a non-zero value can help avoid accidental region mismatches.

CCNA exam may test that MST region configuration requires matching name, revision, and VLAN-to-instance mapping.

Be aware that 'revision' is configured in MST configuration mode, not global config.

Common Mistakes

Setting different revision numbers on switches that should be in the same MST region, causing region mismatch.

Forgetting to apply the configuration with 'exit' or 'apply' after setting the revision number.

Using the 'revision' command in global configuration mode instead of MST configuration mode.

revision [number] vs instance [id] vlan [range]

Both commands are configured in MST configuration submode and are critical for MSTP operation, but they serve distinct purposes: one defines the region revision number to ensure consistency across switches, while the other maps VLANs to MST instances for per-instance spanning tree behavior.

Aspect	revision [number]	instance [id] vlan [range]
Scope	MST region level	MST instance level
Effect	Sets revision number for region identification	Maps VLANs to an MST instance
Impact on MST topology	Must match across all switches in the region to avoid region mismatch	Determines VLAN membership per instance, affecting per-instance bridge priority and topology
Required for MSTP operation?	Yes, a revision number must be set for a valid MST region	Optional; default mapping assigns all VLANs to instance 0
Typical use	Ensure consistency when adding or removing switches from MST region	Customize VLAN load balancing or separate traffic domains

Use revision [number] when you need to ensure all switches in an MST region have the same revision number to form a consistent region.

Use instance [id] vlan [range] when you want to assign specific VLANs to a particular MST instance for optimized spanning tree behavior or load balancing.

Platform Notes

In IOS-XE, the 'revision' command syntax is identical to classic IOS. The output of 'show spanning-tree mst configuration' is similar. In NX-OS, the equivalent command is 'spanning-tree mst configuration' followed by 'revision <number>', but NX-OS uses a different configuration hierarchy; you must enter 'configure terminal' then 'spanning-tree mst <instance>' for instance-specific settings, but the region configuration is done under 'spanning-tree mst configuration' as well.

For ASA, MST is not supported; ASA uses PVST+ or Rapid PVST+. In IOS-XR, MST is not supported; instead, use MSTP via 'spanning-tree mode mst' but the configuration is different; XR uses a hierarchical XML-like CLI. For IOS versions, the command has been consistent from 12.2(33)SXH onward.

In older IOS (12.1), MST was introduced but the command set was similar. There are no significant differences between 15.x and 16.x. Always ensure the switch supports MST; some older switches may only support PVST+.

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