Courseiva
Spanning TreeGlobal Config

spanning-tree pathcost method long

Configures the path cost calculation method for Spanning Tree Protocol (STP) to use 32-bit values (long) instead of the default 16-bit values (short), allowing support for higher-speed interfaces (10 Gbps and above).

Definition: spanning-tree pathcost method long is a Cisco IOS global config command. Configures the path cost calculation method for Spanning Tree Protocol (STP) to use 32-bit values (long) instead of the default 16-bit values (short), allowing support for higher-speed interfaces (10 Gbps and above).

Overview

The `spanning-tree pathcost method long` command, configured in global configuration mode, changes the Spanning Tree Protocol (STP) path cost calculation from the default 16-bit (short) method to a 32-bit (long) method. This is critical in modern networks where high-speed interfaces (10 Gbps, 40 Gbps, 100 Gbps) are common. The original IEEE 802.1D standard defined path costs as 16-bit values ranging from 1 to 65535, with a default cost of 4 for 1 Gbps and 19 for 100 Mbps.

As link speeds increased beyond 1 Gbps, the 16-bit method could not represent costs accurately—for example, 10 Gbps would have a cost of 2, and 100 Gbps would be 1, leaving no room for differentiation. The long method uses 32-bit values (1 to 200,000,000), allowing finer granularity: 10 Gbps = 2000, 40 Gbps = 500, 100 Gbps = 200. This command is essential when deploying high-speed links in a network running STP (802.1D), Rapid PVST+, or MST.

Without it, the root bridge selection and port roles may be suboptimal, potentially causing traffic to traverse slower paths. The command is applied globally and affects all VLANs on the switch. It does not require a reload; the change takes effect immediately for new STP calculations, but existing STP states may need a topology change to recalculate.

The command is available in IOS 12.2(25)SEC and later. When configuring, ensure all switches in the Layer 2 domain use the same path cost method to avoid inconsistencies. The default method is 'short' on most IOS platforms.

This command is a prerequisite for proper STP operation in high-speed environments and is often used alongside other STP tuning commands like `spanning-tree vlan <vlan> priority` or `spanning-tree portfast`. In troubleshooting, mismatched path cost methods can cause unexpected blocking or forwarding decisions. The command is saved to the running configuration and can be verified with `show spanning-tree` or `show spanning-tree detail`.

Syntax·Global Config
spanning-tree pathcost method long

When to Use This Command

  • When deploying 10 Gigabit Ethernet interfaces that require path costs greater than the 16-bit maximum of 65535.
  • When mixing high-speed and low-speed links in a network to ensure proper STP convergence and root bridge selection.
  • When migrating from legacy 100 Mbps/Fast Ethernet to Gigabit or 10 Gigabit Ethernet and need consistent path cost calculations.
  • When using Rapid PVST+ or MST and want to leverage the extended range of path costs for better load balancing.

Parameters

ParameterSyntaxDescription
longlongSelects the 32-bit path cost calculation method. This is the only option for this command; there is no 'short' parameter because the command enables the long method. The default method is short. Once configured, all STP instances on the switch use 32-bit costs.

Command Examples

Enable long path cost method globally

spanning-tree pathcost method long

No output is displayed upon successful configuration. The command is entered in global configuration mode and takes effect immediately for all interfaces.

Verify the path cost method

show spanning-tree
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     0011.2233.4455
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0011.2233.4455
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1          Desg FWD 20000     128.1    P2p
Gi1/0/2          Desg FWD 20000     128.2    P2p
Te2/0/1          Desg FWD 2000      128.3    P2p

The 'Cost' column shows path costs. With long method, GigabitEthernet (1 Gbps) cost is 20000, and 10 GigabitEthernet cost is 2000. Without long method, GigabitEthernet cost would be 4 (short) and 10 GigabitEthernet would be 2, but long method allows higher granularity.

Understanding the Output

The 'show spanning-tree' command displays the STP state per VLAN. The 'Cost' column under each interface shows the path cost to the root bridge. With the long method, costs are 32-bit values (range 1 to 200,000,000).

Typical values: 10 Mbps = 2,000,000; 100 Mbps = 200,000; 1 Gbps = 20,000; 10 Gbps = 2,000; 100 Gbps = 200. Lower cost indicates a more preferred path. If you see costs like 4 for GigabitEthernet, the long method is not enabled.

Ensure all switches in the network use the same method to avoid loops or suboptimal paths.

Configuration Scenarios

Enable Long Path Cost Method on a Switch with 10 Gbps Uplinks

A distribution switch (SW1) connects to two core switches (Core1, Core2) via 10 Gbps links. The default short method assigns cost 2 to these links, making them equal to 1 Gbps links (cost 4) in terms of cost difference. To properly differentiate and utilize the higher speed, the long method is needed.

Topology

Core1(Gi1/0/1)---10.0.1.0/30---(Gi1/0/1)SW1(Gi1/0/2)---10.0.2.0/30---(Gi1/0/2)Core2

Steps

  1. 1.Step 1: Enter global configuration mode: SW1> enable
  2. 2.Step 2: Enter global configuration mode: SW1# configure terminal
  3. 3.Step 3: Enable long path cost method: SW1(config)# spanning-tree pathcost method long
  4. 4.Step 4: Exit configuration mode: SW1(config)# end
  5. 5.Step 5: Verify the change: SW1# show spanning-tree | include Pathcost
Configuration
! Enable long path cost method
SW1(config)# spanning-tree pathcost method long
SW1(config)# end

Verify: Use 'show spanning-tree' and look for 'Pathcost method: long' in the output. Example output: SW1# show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0011.2233.4455 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0011.2233.4455 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Gi1/0/1 Desg FWD 2000 128.1 P2p Gi1/0/2 Desg FWD 2000 128.2 P2p Note the cost of 2000 for 10 Gbps links.

Watch out: If the command is not applied on all switches in the Layer 2 domain, the path cost values will be inconsistent. For example, one switch may advertise cost 2 (short) while another advertises cost 2000 (long) for the same link speed, leading to incorrect root bridge selection or port roles. Always ensure consistency across the network.

Migrating from Short to Long Path Cost Method in a Live Network

A campus network currently uses short path costs but is upgrading core links from 1 Gbps to 10 Gbps. To maintain proper STP topology, the network engineer must change the path cost method to long on all switches. This scenario covers the migration steps and verification.

Topology

Access1(Gi0/1)---1G---(Gi0/1)Dist1(Gi0/2)---10G---(Gi0/2)Core1 Access2(Gi0/1)---1G---(Gi0/1)Dist2(Gi0/2)---10G---(Gi0/2)Core2

Steps

  1. 1.Step 1: On each switch, enter global configuration mode and apply the command: Dist1(config)# spanning-tree pathcost method long
  2. 2.Step 2: After applying on all switches, allow STP to reconverge (typically 30-50 seconds).
  3. 3.Step 3: Verify the path cost method on each switch: Dist1# show spanning-tree | include Pathcost
  4. 4.Step 4: Check that the root bridge and port roles are as expected: Dist1# show spanning-tree root
  5. 5.Step 5: Optionally, manually trigger a topology change to speed up convergence: Dist1# clear spanning-tree detected-protocols
Configuration
! On Dist1
Dist1(config)# spanning-tree pathcost method long
Dist1(config)# end
! Repeat on all switches

Verify: Use 'show spanning-tree' on each switch. Expected output shows 'Pathcost method: long' and costs like 2000 for 10G links, 20000 for 1G links. Example: Dist1# show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0011.2233.4455 This bridge is the root ... Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Gi0/1 Desg FWD 20000 128.1 P2p Gi0/2 Root FWD 2000 128.2 P2p

Watch out: During migration, STP may temporarily create loops or cause traffic disruption if the method is changed on only some switches. Plan a maintenance window and apply the command as quickly as possible across all switches. Also, note that the 'spanning-tree pathcost method long' command is not supported on all IOS versions; verify compatibility before migration.

Troubleshooting with This Command

When troubleshooting STP issues related to path cost, the first step is to verify the path cost method in use. Use `show spanning-tree` and look for the line 'Pathcost method: long' or 'Pathcost method: short'. If the method is short but the network has high-speed links (10 Gbps+), the costs may be too low (e.g., cost 2 for 10G) and may not differentiate properly, leading to suboptimal forwarding.

For example, a 10 Gbps link and a 1 Gbps link might both have cost 2 and 4 respectively, but with short method, the difference is minimal. With long method, 10G = 2000, 1G = 20000, making the 10G link clearly preferred. Common symptoms include: a root bridge not being the expected core switch, a high-speed link being blocked while a slower link is forwarding, or frequent topology changes.

To diagnose, first check the path cost method: `show spanning-tree | include Pathcost`. If short, consider changing to long. Next, examine the cost of each interface: `show spanning-tree interface <interface>`.

Look for the 'Cost' field. If costs seem too low or too high relative to link speed, recalculate using the appropriate formula (short: 1000 Mbps / speed; long: 200000000 / speed). For example, a 10 Gbps link should have cost 2000 (long) or 2 (short).

If the cost is incorrect, it may be manually overridden with `spanning-tree cost <cost>` on the interface. Also, verify that all switches in the STP domain use the same path cost method. Mismatched methods cause inconsistent cost values: a switch using short method will advertise cost 2 for a 10G link, while a switch using long method will advertise cost 2000.

The receiving switch interprets the cost based on its own method, leading to incorrect STP calculations. To detect this, compare the 'Cost' field on the same link from both ends. If they differ significantly (e.g., 2 vs 2000), the methods are mismatched.

Additionally, use `debug spanning-tree events` to see STP state changes and cost updates. However, use debug sparingly in production. Another useful command is `show spanning-tree detail` which shows per-interface cost and port role.

When troubleshooting, also check the root bridge: `show spanning-tree root`. If the root is not the expected switch, examine the path cost to the root from each switch. The switch with the lowest bridge ID becomes root, but if path costs are misconfigured, a different switch may become root.

Finally, correlate with `show interfaces` to confirm link speed and duplex. If a link is operating at a lower speed than expected (e.g., 1 Gbps instead of 10 Gbps), the STP cost will reflect that, potentially causing unexpected blocking. In summary, the key to troubleshooting path cost issues is to ensure consistent method, correct cost values, and proper link speeds.

CCNA Exam Tips

1.

CCNA exam may ask: Which command allows STP to support 10 Gbps interfaces? Answer: 'spanning-tree pathcost method long'.

2.

Remember that the long method is required for speeds above 10 Gbps (e.g., 40 Gbps, 100 Gbps) because short method maxes out at 65535.

3.

The command is global; it affects all VLANs and all STP instances (PVST+, Rapid PVST+, MST).

4.

Be aware that changing the path cost method can cause temporary STP reconvergence; the exam might test the impact on network stability.

Common Mistakes

Applying the command in interface configuration mode instead of global configuration mode — it is a global command only.

Assuming the long method is default on newer switches — it is not; you must explicitly configure it.

Forgetting to configure the same method on all switches in the network, leading to inconsistent path cost calculations and potential loops.

spanning-tree pathcost method long vs spanning-tree port-priority [value]

Both 'spanning-tree pathcost method long' and 'spanning-tree port-priority' influence Spanning Tree Protocol (STP) topology, but they operate at different levels: one adjusts the global cost calculation method, the other sets per-interface priority. They are often confused because both can affect which ports become root ports, though via distinct mechanisms.

Aspectspanning-tree pathcost method longspanning-tree port-priority [value]
ScopeGlobal (affects all interfaces)Interface-specific
Configuration modeGlobal configurationInterface configuration
Effect on path costUses 32-bit cost values (long) instead of 16-bit (short)Does not change cost; sets port priority (0–240, increments of 16)
Impact on root port selectionIndirectly via cost (lower cost preferred)Directly via priority (lower priority preferred when costs equal)
Typical use caseNetworks with 10 Gbps or faster linksManual tuning of root port on specific interfaces

Use spanning-tree pathcost method long when you have high-speed interfaces (≥10 Gbps) on any switch in the network to avoid cost resolution issues.

Use spanning-tree port-priority [value] when you need to influence which specific interface on a switch becomes the root port, typically to force traffic through a preferred link.

Platform Notes

On Cisco IOS (classic), the command is `spanning-tree pathcost method long` in global configuration mode. On IOS-XE (e.g., Catalyst 3650, 3850, 9300), the syntax is identical. However, some IOS-XE versions may also support the `spanning-tree pathcost method long` command under the VLAN configuration mode?

No, it remains global. On NX-OS (e.g., Nexus 9000), the equivalent command is `spanning-tree pathcost method long` as well, but NX-OS uses a different default: the long method is the default on many Nexus platforms. Verify with `show spanning-tree` output.

On NX-OS, the command is also global. On Cisco ASA, STP is not typically used; ASA does not support this command. On IOS-XR (e.g., ASR 9000), STP is not supported in the same way; instead, Ethernet OAM or other mechanisms are used.

Therefore, this command does not exist in IOS-XR. In older IOS versions (12.x), the command may not be available; it was introduced in 12.2(25)SEC. In IOS 15.x and 16.x, it is fully supported.

Some platforms may require the 'long' keyword to be spelled out; no abbreviation is allowed. The output format of `show spanning-tree` differs slightly between IOS versions: newer versions show 'Pathcost method' line clearly, while older versions may not display it unless you use `show spanning-tree detail`. On IOS-XE, the output is similar to IOS.

On NX-OS, `show spanning-tree` includes 'Pathcost method' as well. Always check the documentation for your specific IOS version.

Related Commands

show spanning-tree

Displays the Spanning Tree Protocol (STP) state and configuration for all VLANs or a specific VLAN, used to verify root bridge, port roles, and STP topology.

spanning-tree port-priority [value]

Sets the port priority value for a specific interface to influence which port becomes the root port in a Spanning Tree topology.

Practice for the CCNA 200-301

Test your knowledge with practice questions covering all CCNA 200-301 exam domains.

Practice CCNA 200-301 Questions