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 Troubleshooting Common Issues with Spanning Tree Protocol

Spanning Tree Protocol (STP) plays a crucial role in LAN Switching. There are multiple devices are connected with their uplink or downlink devices as a part of Distribution and access layer till Core layer.It is&nbsp; always recommend having redundancy in the network in the form of Link, hardware or power.In this article we will discuss about Spanning Tree Convergence and advance Features. So, let’s start.Spanning Tree Protocol (STP) is a protocol that used in Switches to avoid Loops. Its plays a very important role in LAN network to ensuring network stability, preventing loops, and maintaining redundancy in Ethernet networks.Spanning Tree Protocol (STP)used in Switches to avoid Loops and PortFast, UplinkFast, BackboneFast is important feature to converge STP<div dir="ltr">
Table of Contents
</div><h2>Redundant Link Convergence </h2>As we discussed and know that STP will take 30 to 52 sec to make a block port into forwarding, but in some organization where every second is important and production. Every second is matter for such industries where we cannot wait for 50 sec.So, there are some features in STP ( PVST+)which converge the redundant link and will put direct into forwarding state. It allows faster STP convergence in the event of a link failure include the following:■ PortFast—It allow connectivity to be established on access layer switch ports to workstations that are booting up■ UplinkFast—Enables uplink failover on an access layer switch when dual uplinks are connected into the distribution layer■ BackboneFast—provides fast convergence in the network backbone (core) after a Spanning Tree topology change occurs<h3>STP PortFast</h3>Suppose, we have an end device (Workstation, production machine etc) connected with Layer-2 switch. That End device is very critical to send or receive data. If it goes shut down or port goes down and came up then STP will run their process to put it in forwarding state and it can take from 30 to 50 sec. and till the time that end device cannot send any traffic on that port.To minimize this delay Cisco introduced a STP Feature called PortFast.Port Fast allows the interface to move from a blocking state to a forwarding state immediately. But PortFast should not be enable on any uplink switch ports. This feature is typically used on access ports, which are the ports that connect to end devices, rather than on uplink or trunk ports that connect switches together.By bypassing the STP listening and learning states, PortFast reduces the time it takes for an end device to gain network connectivity after being connected to the switch.It is important to note that PortFast should only be enabled on access ports where you are confident that a loop-free topology exists. If a switch port with PortFast enabled inadvertently becomes part of a loop in the network, it can lead to broadcast storms and network instability. Therefore, caution should be exercised when configuring PortFast and proper network design and planning should be undertaken.<img src="https://i0.wp.com/techblog.kbrosistechnologies.com/wp-content/uploads/2023/06/stpadv-1.png?resize=669%2C350&amp;ssl=1" alt="" width="669" height="350" srcset="https://i0.wp.com/techblog.kbrosistechnologies.com/wp-content/uploads/2023/06/stpadv-1.png?resize=300%2C157&amp;ssl=1 300w, https://i0.wp.com/techblog.kbrosistechnologies.com/wp-content/uploads/2023/06/stpadv-1.png?w=516&amp;ssl=1 516w"><h4>STP UplinkFast</h4>Same like Port Fast, STP have another feature called STP UplinkFast. As its name says, whenever we have uplink port from one switch to another switch (should not be Ether Channel), then if forwarding port goes down then STP start re-calculating as per default timer from 30-50 sec. To avoid this delay, we can configure STP UplinkFast on the uplink ports, so port will be come in forwarding state immediately. The Root Bridge cannot have UplinkFast enabled.UplinkFast is a Cisco proprietary feature that improves the convergence time in STP. It includes:<UL><LI>If any uplink port goes down then UplinkFast feature detect it designates one of its redundant ports as an alternate or backup uplink port.</LI></UL><UL><LI>Its send a message to all neighbouring switches for that alternate port that it is now the new uplink port for the affected VLANs.</LI></UL><UL><LI>The neighbouring switches update their forwarding tables to direct traffic through the newly designated uplink port</LI></UL>Also, we need to keep in mind, this will not provide the complete Spanning Tree loop protection, so be careful before implementing STP UplinkFast.UplinkFast is configured globally for all VLANs on the switch:Switch(config)# spanning-tree uplinkfast<h5>STP BackboneFast</h5>UplinkFast allows faster convergence if a directly-connected interface fails, but what happens if indirectly-connected link goes down.BackboneFast is another Cisco proprietary feature. BackboneFast provides the same benefit is an indirectly-connected interface fails.For example, if the Root Bridge fails, another switch will be elected the Root. A switch learning about the new Root Bridge must wait its Max Age timer 20 seconds to flush out the old information.BackboneFast allows a switch to bypass the Max Age timer if it detects an indirect failure on the network. It will update itself with the new Root info immediately. BackboneFast is configured globally, and should be implemented on all switches in the network when used.BackboneFast helps expedite the convergence process by detecting certain types of link failures faster. Here’s how it works:<UL><LI>When a switch detects a failure on its uplink port, it immediately begins a timer called the BackboneFast timer.</LI><LI>The switch then sends a special BPDU, called the Proposal BPDU, toward the root bridge. This BPDU indicates that the switch has alternate paths available and can take over the root bridge role if necessary.</LI><LI>If the root bridge receives the Proposal BPDU, it knows that there is an alternate path, and it responds with a Confirmation BPDU to acknowledge the switch’s capability to take over as the root bridge.</LI><LI>Upon receiving the Confirmation BPDU, the switch triggers the Topology Change (TC) flag, which prompts the network to undergo STP convergence.</LI></UL>Switch(config)# spanning-tree backbonefastFor understand STP, Please read this also: <a href="https://techblog.kbrosistechnologies.com/spanning-tree-protocol-stp-basic/" dir="ltr">https://techblog.kbrosistechnologies.com/spanning-tree-protocol-stp-basic/</a><h6>Conclusion</h6>In STP there are convergence time which switch take to put any port in forwarding state from blocking state, and it can take from 30 – 50 sec to complete this process, but we can eliminate this process and make the port up immediately by PortFast, UplinkFast and BackboneFast. But at the same time also we need to take care about STP loop creation. Due to these features STP might be can not work properly and can create loop, so we need to configure BPDU Guard, Root Guard and UDLD feature as well as protect the switch to create loop.Visit to our site :&nbsp;<a href="https://www.kbrosistechnologies.com/" dir="ltr">https://www.kbrosistechnologies.com/</a>Watch more Video&nbsp;&nbsp;<a href="https://www.youtube.com/channel/UCpcd6IshE1caAbf9EdJd3gw" rel="noopener" dir="ltr">https://www.youtube.com/channel/UCpcd6IshE1caAbf9EdJd3gw&nbsp;</a>

Spanning Tree Protocol (STP)used in Switches to avoid Loops and PortFast, UplinkFast, BackboneFast is important feature to converge STP

Spanning Tree Protocol (STP) plays a crucial role in LAN Switching. There are multiple devices are connected with their uplink or downlink devices as a part of Distribution and access layer till Core layer.

It is  always recommend having redundancy in the network in the form of Link, hardware or power.

In this article we will discuss about Spanning Tree Convergence and advance Features. So, let’s start.

Spanning Tree Protocol (STP) is a protocol that used in Switches to avoid Loops. Its plays a very important role in LAN network to ensuring network stability, preventing loops, and maintaining redundancy in Ethernet networks.

Spanning Tree Protocol (STP)used in Switches to avoid Loops and PortFast, UplinkFast, BackboneFast is important feature to converge STP

As we discussed and know that STP will take 30 to 52 sec to make a block port into forwarding, but in some organization where every second is important and production. Every second is matter for such industries where we cannot wait for 50 sec.

So, there are some features in STP ( PVST+)which converge the redundant link and will put direct into forwarding state. It allows faster STP convergence in the event of a link failure include the following:

■ PortFast—It allow connectivity to be established on access layer switch ports to workstations that are booting up

■ UplinkFast—Enables uplink failover on an access layer switch when dual uplinks are connected into the distribution layer

■ BackboneFast—provides fast convergence in the network backbone (core) after a Spanning Tree topology change occurs

Suppose, we have an end device (Workstation, production machine etc) connected with Layer-2 switch. That End device is very critical to send or receive data. If it goes shut down or port goes down and came up then STP will run their process to put it in forwarding state and it can take from 30 to 50 sec. and till the time that end device cannot send any traffic on that port.

To minimize this delay Cisco introduced a STP Feature called PortFast.

Port Fast allows the interface to move from a blocking state to a forwarding state immediately. But PortFast should not be enable on any uplink switch ports. This feature is typically used on access ports, which are the ports that connect to end devices, rather than on uplink or trunk ports that connect switches together.

By bypassing the STP listening and learning states, PortFast reduces the time it takes for an end device to gain network connectivity after being connected to the switch.

It is important to note that PortFast should only be enabled on access ports where you are confident that a loop-free topology exists. If a switch port with PortFast enabled inadvertently becomes part of a loop in the network, it can lead to broadcast storms and network instability. Therefore, caution should be exercised when configuring PortFast and proper network design and planning should be undertaken.

Same like Port Fast, STP have another feature called STP UplinkFast. As its name says, whenever we have uplink port from one switch to another switch (should not be Ether Channel), then if forwarding port goes down then STP start re-calculating as per default timer from 30-50 sec. To avoid this delay, we can configure STP UplinkFast on the uplink ports, so port will be come in forwarding state immediately. The Root Bridge cannot have UplinkFast enabled.

UplinkFast is a Cisco proprietary feature that improves the convergence time in STP. It includes:

<ul><li>If any uplink port goes down then UplinkFast feature detect it designates one of its redundant ports as an alternate or backup uplink port. </li> </ul>

<ul><li>Its send a message to all neighbouring switches for that alternate port that it is now the new uplink port for the affected VLANs. </li> </ul>

<ul><li>The neighbouring switches update their forwarding tables to direct traffic through the newly designated uplink port </li> </ul>

Also, we need to keep in mind, this will not provide the complete Spanning Tree loop protection, so be careful before implementing STP UplinkFast.

UplinkFast is configured globally for all VLANs on the switch:

Switch(config)# spanning-tree uplinkfast

UplinkFast allows faster convergence if a directly-connected interface fails, but what happens if indirectly-connected link goes down.

BackboneFast is another Cisco proprietary feature. BackboneFast provides the same benefit is an indirectly-connected interface fails.

For example, if the Root Bridge fails, another switch will be elected the Root. A switch learning about the new Root Bridge must wait its Max Age timer 20 seconds to flush out the old information.

BackboneFast allows a switch to bypass the Max Age timer if it detects an indirect failure on the network. It will update itself with the new Root info immediately. BackboneFast is configured globally, and should be implemented on all switches in the network when used.

BackboneFast helps expedite the convergence process by detecting certain types of link failures faster. Here’s how it works:

<ul><li>When a switch detects a failure on its uplink port, it immediately begins a timer called the BackboneFast timer. </li><li>The switch then sends a special BPDU, called the Proposal BPDU, toward the root bridge. This BPDU indicates that the switch has alternate paths available and can take over the root bridge role if necessary. </li><li>If the root bridge receives the Proposal BPDU, it knows that there is an alternate path, and it responds with a Confirmation BPDU to acknowledge the switch’s capability to take over as the root bridge. </li><li>Upon receiving the Confirmation BPDU, the switch triggers the Topology Change (TC) flag, which prompts the network to undergo STP convergence. </li> </ul>

Switch(config)# spanning-tree backbonefast

For understand STP, Please read this also: <a href="https://techblog.kbrosistechnologies.com/spanning-tree-protocol-stp-basic/">https://techblog.kbrosistechnologies.com/spanning-tree-protocol-stp-basic/ </a>

In STP there are convergence time which switch take to put any port in forwarding state from blocking state, and it can take from 30 – 50 sec to complete this process, but we can eliminate this process and make the port up immediately by PortFast, UplinkFast and BackboneFast. But at the same time also we need to take care about STP loop creation. Due to these features STP might be can not work properly and can create loop, so we need to configure BPDU Guard, Root Guard and UDLD feature as well as protect the switch to create loop.

Visit to our site : <a href="https://www.kbrosistechnologies.com/">https://www.kbrosistechnologies.com/ </a>

Watch more Video  <a href="https://www.youtube.com/channel/UCpcd6IshE1caAbf9EdJd3gw">https://www.youtube.com/channel/UCpcd6IshE1caAbf9EdJd3gw  </a>

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Troubleshooting Common Issues with Spanning Tree Protocol

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