FS S3900 SERIES SWITCHES

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The S3900 series switches consist of three models: S3900-24T4S, S3900-24F4S, S3900-48T4S, which are fully managed stackable Gigabit switches, equipped with Broadcom chips, supporting 10G SFP+ uplinks, designed for SMB, campus and enterprise networks.

S3900-24T4S S3900-24F4S S3900-48T4S

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STP Configuration on FS S3900 Series Switches

Posted on By FS.COM

STP (Spanning Tree Protocol) is a Layer 2 protocol that runs on network switches. The main purpose of STP is to prevent the loop caused by redundant paths, avoiding the broadcast storm and MAC address table unstable. FS S3900 series high-performance Gigabit Ethernet L2/L3 Lite managed switches are suitable for home use, SMB, enterprise and campus networks. They support STP to provide link redundancy and backups while simultaneously preventing undesirable loops.

STP Application on S3900 Series Switches

Figure 1 is the typical STP application on S3900 series switches. Switch A, Switch B and Switch C are all running STP. STP can eliminate the loop that may exist in the network by blocking redundant links, ensuring a loop-free network. In figure 1, Switch B is selected as the root of the spanning tree. The packets will go by PC 1 - Switch B - Switch C - PC 2, which is link 1. The STP algorithm can calculate the best loop-free path for transmission, so the port on Switch C that connected with Switch A is blocked. Link 2 that stands for PC 1 - Switch B -Switch A - Switch C - PC 2 is unavailable. If link 1 breaks down, STP can activate the blocked port on Switch C. Then the packets will go through by link 2, at last, the packet will successfully arrive at PC 2.

STP application.jpg

Note that, there are STP, RSTP (Rapid Spanning Tree Protocol) and MSTP (Multiple Spanning Tree Protocol).

RSTP is the improvement of STP. Because the STP network convergence speed is slow, IEEE introduced RSTP to provide significant recovery in response to network changes or failures. RSTP is backward compatible with standard STP. MSTP enables multiple VLAN instances to be mapped to the same spanning-tree without creating loops. It can generate multiple spanning trees that are used independently to deliver traffic in different VLANs.

FS S3900 series switches consist of S3900-24F4S, S3900-24T4S, and S3900-48T4S. All the switches support STP, RSTP and MSTP. They can be used with switches from vendors like Cisco and Juniper for configuring STP. Note that, RSTP is the default option of STP, while MSTP is usually used in a large enterprise environment.

STP Configuration Roadmap of S3900 Series Switches

Successful configuration of running STP, RSTP and MSTP on S3900 series switches can be achieved via CLI commands. There are four main steps to configure STP and RSTP on S3900 series switches.


  • Enable STP.

  • Configure the STP mode.

  • Configure the root bridge.

  • Verify the configuration.

S3900 Series Switches STP/RSTP Configuration via CLI


Figure 2 is the networking diagram of STP/RSTP configuration on S3900 series switches.

STP connection.jpg

STP Configuration Commands

1. Configure S3900-24F4S-A first. Enable STP in a global schema, and set STP mode and priority, configuring it as the root bridge.

S3900-24F4S-A-2(config)#spanning-tree
S3900-24F4S-A-2(config)#spanning-tree mode stp
S3900-24F4S-A-2(config)#spanning-tree priority 0

2. Configure S3900-24F4S-B. Enable STP in a global schema, and set STP mode.

S3900-24F4S-B(config)#spanning-tree
S3900-24F4S(config)#interface vlan 1

3. Configure S3900-48T4S. Enable STP in a global schema, and set STP mode.

S3900-48T4S-3(config)#spanning-tree
S3900-48T4S-3(config)#spanning-tree mode stp

4. Check information about STP state on each switch and verify whether STP configuration is successful.

Check S3900-24F4S-A state.

S3900-24F4S-A-2#show spanning-tree brief
Spanning Tree Mode: STP
Spanning Tree Enabled/Disabled: Enabled
Designated Root: 0.ECD68A369C78
Current Root Port: 0
Current Root Cost: 0

stp 1.png

Check S3900-24F4S-B state.

S3900-24F4S-B#show spanning-tree brief
Spanning Tree Mode: STP
Spanning Tree Enabled/Disabled: Enabled
Designated Root: 0.ECD68A369C78
Current Root Port (Eth): 1/21
Current Root Cost: 20000

stp 2.png

Check S3900-48T4S state.

S3900-48T4S-A-3#show spanning-tree brief
Spanning Tree Mode: STP
Spanning Tree Enabled/Disabled: Enabled
Designated Root: 0.ECD68A369C78
Current Root Port (Eth): 1/1
Current Root Cost: 20000

stp 3.png


Result: S3900-24F4S-A is set as the root bridge because of its highest priority. Eth 1/22 port of S3900-24F4S-B switch is blocked. Thus, a loop-free network is achieved.

RSTP Configuration Commands

1. Configure S3900-24F4S-A first. Enable STP in a global schema, and set STP mode and priority, configuring it as the root bridge.

S3900-24F4S-A-2(config)#spanning-tree
S3900-24F4S-A-2(config)#spanning-tree priority 0
S3900-24F4S-A-2(config)#spanning-tree mode rstp

2. Configure S3900-24F4S-B. Enable STP in a global schema, and set STP mode.

S3900-24F4S-B(config)#spanning-tree
S3900-24F4S-B(config)#spanning-tree mode rstp

3. Configure S3900-48T4S. Enable STP in a global schema, and set STP mode.

S3900-48T4S-1(config)#spanning-tree
S3900-48T4S-1(config)#spanning-tree mode rstp

4. Check information about RSTP state on each switch and verify whether RSTP configuration is successful.

Check S3900-24F4S-A state.

S3900-24F4S-A-2#show spanning-tree brief
Spanning Tree Mode: RSTP
Spanning Tree Enabled/Disabled: Enabled
Designated Root: 0.ECD68A369C78
Current Root Port: 0
Current Root Cost: 0

rstp 1.png

Check S3900-24F4S-B state.

S3900-24F4S-B#show spanning-tree brief
Spanning Tree Mode: RSTP
Spanning Tree Enabled/Disabled: Enabled
Designated Root: 0.ECD68A369C78
Current Root Port (Eth): 1/21
Current Root Cost: 20000

rstp 2.png

Check S3900-48T4S state.

S3900-48T4S-1#show spanning-tree brief
Spanning Tree Mode: RSTP
Spanning Tree Enabled/Disabled: Enabled
Designated Root: 0.ECD68A369C78
Current Root Port (Eth): 1/1
Current Root Cost: 20000

rstp 3.png


Result: S3900-24F4S-A is set as the root bridge because of its highest priority. Eth 1/22 port of S3900-24F4S-B switch is blocked. Thus, a loop-free network is achieved.

Related Articles:

Interface-based VLAN Configuration on FS S3900 Series Switches
Voice VLAN Configuration on FS S3900 Series Switches

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