Spanning Tree Protocol - N10-008 CompTIA Network+ : 2.3

Summary

The video script discusses the absence of a time-to-live mechanism in layer 2 Ethernet, which can lead to network loops and congestion. It introduces the IEEE 802.1D Spanning Tree Protocol (STP), developed by Radia Perlman, to prevent loops by blocking certain ports. The script explains STP's port states, including blocking, listening, and forwarding, and how it identifies root and designated ports to maintain a loop-free network. It also touches on the Rapid Spanning Tree Protocol (RSTP), which improves convergence time to handle network changes more swiftly.

Takeaways

• 🌐 IP version 4 has a time to live (TTL) field to prevent looping packets, but Ethernet (layer 2) lacks this mechanism.
• 🔁 Without a TTL equivalent, Ethernet frames can loop indefinitely in a network until the loop is physically removed.
• 🛠 Loop protection is crucial in Ethernet networks to prevent frames from endlessly circulating and consuming resources.
• 🔌 A simple mistake like connecting two cables between switches can create a loop in a network.
• 🚫 Loops can lead to network congestion and ultimately a complete halt of communication on the affected network segment.
• 🔄 Unplugging the problematic cable is a straightforward but crude solution to break a loop and restore network functionality.
• 📚 The IEEE 802.1D standard, developed by Radia Perlman, introduced the Spanning Tree Protocol (STP) to prevent loops in bridged or switched networks.
• 🌳 STP operates by placing interfaces in different states (like blocking, listening, and learning) to understand and prevent network loops.
• 🚦 STP can block traffic on certain ports to prevent loops, ensuring a loop-free network topology.
• 🔄 In case of network changes, STP can reconfigure the network to maintain a loop-free path, even if some connections are lost.
• ⏱ Traditional STP has a convergence time of 30-50 seconds, which is improved to about 6 seconds with the Rapid Spanning Tree Protocol (RSTP).

Q & A

What is the primary function of the Time to Live (TTL) field in IP version 4?

The primary function of the TTL field in IP version 4 is to identify when a packet has been looping through separate routers and to eventually drop that packet from the network to prevent it from circulating indefinitely.

Why is there no Time to Live mechanism in layer 2 Ethernet?

Layer 2 Ethernet does not have a Time to Live mechanism because it operates at the data link layer and does not have the capability to track packet loops over a network. It is designed to handle frame transmission between directly connected devices without the need for such a mechanism.

What happens if a loop is created in a network with Ethernet switches?

If a loop is created in a network with Ethernet switches, frames introduced into that loop will circulate indefinitely, consuming all available bandwidth and network resources, eventually leading to a complete communication breakdown for all devices connected to the switches involved in the loop.

How can a loop in an Ethernet network be prevented?

A loop in an Ethernet network can be prevented by using loop protection mechanisms, such as the Spanning Tree Protocol (STP), which disables certain ports to avoid loops and ensures a loop-free network topology.

Who created the Spanning Tree Protocol and for what purpose?

The Spanning Tree Protocol was created by Radia Perlman in 1990 as an IEEE standard 802.1D to prevent loops in bridged or switched networks by creating a loop-free logical topology.

What are the different port states that an interface can be placed in by the Spanning Tree Protocol?

The different port states that an interface can be placed in by the Spanning Tree Protocol include blocking, listening, learning, forwarding, and disabled states.

What is the purpose of the blocking port state in the Spanning Tree Protocol?

The purpose of the blocking port state in the Spanning Tree Protocol is to administratively block all traffic from going in or out of that interface to prevent the creation of loops in the network.

How does the Spanning Tree Protocol handle network outages or changes?

The Spanning Tree Protocol handles network outages or changes by reconverging and recalculating the network topology to adapt to the new conditions, enabling alternative paths for traffic flow and ensuring connectivity is maintained without loops.

What is the Rapid Spanning Tree Protocol (RSTP) and how does it improve upon the traditional Spanning Tree Protocol?

The Rapid Spanning Tree Protocol (RSTP) is an updated version of the Spanning Tree Protocol that reduces convergence time from 30 to 50 seconds down to six seconds, making it more suitable for modern networks that require faster recovery from topology changes.

How does the Spanning Tree Protocol ensure that only one bridge is the root bridge?

The Spanning Tree Protocol ensures that only one bridge is the root bridge by using a set of algorithms that determine the bridge with the highest priority or the lowest bridge ID to be the root bridge, thus preventing loops by designating a single logical path for network traffic.

Outlines

00:00

🔁 Understanding Ethernet Loops and Spanning Tree Protocol

This paragraph discusses the absence of a time-to-live mechanism in layer 2 Ethernet, which can lead to network loops and continuous frame circulation without a mechanism to drop frames. The paragraph emphasizes the importance of preventing loops through loop protection and introduces the IEEE 802.1D standard, known as the Spanning Tree Protocol (STP), created by Radia Perlman. STP is designed to prevent loops in bridged or switched networks by placing interfaces in different states, such as blocking, listening, and forwarding, to ensure a loop-free topology. The paragraph also explains the concept of root ports, designated ports, and blocked ports, which are crucial in STP's operation to avoid loops.

05:03

🚀 Rapid Spanning Tree Protocol: Enhancing Network Resiliency

The second paragraph delves into the challenges of traditional Spanning Tree Protocol, particularly its slow convergence time which can range from 30 to 50 seconds, impacting network reliability. To address this, the Rapid Spanning Tree Protocol (RSTP) was introduced as an updated version, compliant with the 802.1w standard. RSTP significantly reduces convergence time to six seconds, enhancing network responsiveness. It maintains backward compatibility, allowing for a mix of old and new equipment in a network setup. The paragraph outlines the similarities between RSTP and traditional STP, indicating that understanding one protocol facilitates comprehension of the other.

Keywords

💡Time to Live (TTL)

Time to Live (TTL) is a field in IP version 4 that limits the lifespan of a data packet by counting the number of router hops it makes. Once it reaches a certain threshold, the packet is dropped from the network to avoid looping indefinitely. In the context of the video, TTL is contrasted with Ethernet at Layer 2, where no such mechanism exists, making loops problematic.

💡Layer 2 Ethernet

Layer 2 Ethernet refers to the data link layer of the OSI model, which handles frame transmission between devices. Unlike Layer 3 (IP layer), Ethernet lacks a mechanism like TTL, making it vulnerable to endless loops if a physical loop is created in the network. The video highlights the absence of loop prevention at Layer 2, necessitating protocols like Spanning Tree Protocol.

💡Loop Protection

Loop protection refers to the mechanisms used to prevent network loops, which occur when data frames are continuously passed between switches without being dropped. This can overwhelm network resources and stop communication. The video emphasizes the importance of implementing loop protection in Ethernet networks, as loops can be accidentally created by connecting two cables between switches.

💡Spanning Tree Protocol (STP)

The Spanning Tree Protocol (STP) is a network protocol defined by IEEE standard 802.1D that prevents loops in bridged or switched networks. It works by identifying potential loops and blocking the ports that could cause them. The video explains how STP automatically disables certain interfaces to ensure a loop-free topology, making it essential for network stability.

💡Root Port

The root port is the port on a bridge or switch that is closest to the root bridge in a Spanning Tree Protocol-enabled network. Only one bridge is designated as the root bridge, and the root port allows other bridges to communicate with it. In the video, the root port is illustrated as a critical part of the STP process in preventing loops and managing network traffic.

💡Designated Port

A designated port is an operational port on a switch or bridge that forwards traffic to its connected network segment. It is responsible for forwarding frames to the segment where it is located. The video describes how designated ports work alongside root ports and blocked ports to manage network traffic efficiently while avoiding loops.

💡Blocked Port

A blocked port is a port that has been disabled by the Spanning Tree Protocol to prevent loops from occurring. Blocked ports do not forward traffic but remain in standby in case the network topology changes and they are needed to forward traffic. The video shows how blocked ports play a critical role in maintaining a loop-free network.

💡Topology

Topology refers to the arrangement and structure of a network, including the devices and the connections between them. In the video, the topology is described as a map that the Spanning Tree Protocol learns and adapts to in order to prevent loops and manage traffic flow between switches and networks.

💡Convergence

Convergence is the process by which the Spanning Tree Protocol recalculates and adjusts the network topology after a change, such as a link failure. The video explains that traditional STP can take 30-50 seconds to converge, during which time network communication may be disrupted. This is why faster alternatives like Rapid Spanning Tree Protocol are preferred.

💡Rapid Spanning Tree Protocol (RSTP)

The Rapid Spanning Tree Protocol (RSTP) is an enhanced version of STP, defined by IEEE standard 802.1w, that reduces the convergence time from 30-50 seconds to around 6 seconds. The video explains how RSTP provides faster recovery from network topology changes while remaining backward compatible with traditional STP.

Highlights

IP version 4 has a time to live field to prevent packet looping, but layer 2 Ethernet lacks this mechanism.

Ethernet networks can suffer from loops that cause frames to circulate indefinitely without a mechanism to stop them.

Loops in Ethernet can be resolved by physically disconnecting cables to break the loop.

Loop protection in Ethernet is crucial and can be accidentally created by connecting two cables between switches.

Frames in a loop can quickly consume all network bandwidth and resources, halting communication.

IEEE standard 802.1D, created by Radia Perlman, introduces the Spanning Tree Protocol to prevent loops in bridged networks.

Spanning Tree Protocol operates by placing interfaces in different states to prevent loops, such as blocking state.

The learning port state allows the protocol to understand the network topology and determine if a loop might occur.

Once a loop risk is assessed, interfaces can transition to a forwarding state to allow data passage.

Administrative actions, like disabling a port, can influence how the Spanning Tree Protocol operates.

The Spanning Tree Protocol can dynamically adjust to network changes, such as outages, to reconfigure loops and maintain connectivity.

The protocol identifies root ports, designated ports, and blocked ports to manage network traffic and prevent loops.

Rapid Spanning Tree Protocol (RSTP) offers faster convergence times, reducing the impact of network outages.

RSTP, standardized as 802.1w, decreases convergence time from 30-50 seconds to just six seconds.

RSTP is backward compatible, allowing for a mix of old and new equipment in the network.

Understanding traditional Spanning Tree Protocol facilitates comprehension of the Rapid Spanning Tree Protocol.

Transcripts

In an earlier video, we described

how IP version 4 has a time to live field, where

it will identify when a packet has been looping

through separate routers and eventually drop

that packet from the network.

Unfortunately, with layer 2 ethernet,

there is not a time to live mechanism.

If you've created a loop in the network

and a frame is introduced into that loop,

there's no mechanism to drop or remove

that frame from the network.

The only way that you would stop from occurring

is to physically unplug the cable so

that the loop no longer exists.

The key with ethernet and switching

is to make sure that a loop doesn't

occur in the first place.

And we do that by using loop protection.

Unfortunately, this is very easy to do on a switched network.

You only have to accidentally plug

2 cables in between two switches and you've created a loop.

Because there's no counting mechanism at the MAC address

layer, that frame will go back and forth

between those switches indefinitely.

It doesn't take long for more frames

to be added to the loop, and more and more frames,

using up all of the bandwidth and all

of the resources on the network.

And eventually, there is no communication

at all for anything connected to either of those switches.

This is relatively easy to resolve.

You simply unplug one of the cables, remove the loop,

and everything will go back to normal.

Fortunately, we introduced a standard in 1990 that allows us

to prevent any loops from occurring on a bridged

or switched network this is an IEEE standard 802.1D,

and it was created by Radia Perlman to prevent these loops

on these bridged networks.

This is the spanning tree protocol,

and it's used on many switches to provide

a loop-free environment.

When an interface is connected to a network,

spanning tree begins the process of identifying

whether a loop would be created with that interface or not.

And there are a number of modes that interface

will be placed in.

One of those port states is a blocking port state.

If the spanning tree protocol identifies

that a loop would be created by turning on this interface,

it will administratively block all traffic

from going in or out of that interface

to prevent a loop from occurring.

To be able to make that determination of whether it

should block or not block the traffic,

it needs to listen for a certain amount of time

to be able to know what devices and switches may already

be on the network.

The process of building its own internal topology

so that it understands whether a loop may be occurring or not

is called the learning port state.

Once it is comfortable that no loop would be created,

it can begin forwarding traffic.

Data will pass through that interface

and the interface will be fully operational on the network.

Of course, you as the administrator

could administratively disable that port.

That's not necessarily part of Spanning Tree Protocol,

but it does have an effect on how STP operates.

Here's a network that we'll look at to see how spanning tree

can prevent loops from occurring.

You can see that we have five bridges on this network

and they are connecting many different networks together.

If we didn't have spanning tree, you

could easily see that you could create on this network

where traffic would constantly be

going back and forth between all of these different bridges.

But thanks to spanning tree, a number of these interfaces

have been disabled so that a loop doesn't occur.

There are three separate modes we're

going to look at for every interface on these bridges.

There is a root port--

the root port designates the interface

that is closest to what we call the root of the network.

And only one bridge on the network

is the root bridge or root switch.

There's also a designated port, which

is all of the other operational ports on every other bridge.

And then there are blocked ports.

Spanning tree protocol will identify potential loops

and it will disable or block individual ports

so that a loop will not occur.

You can see on this network, for example, on network C,

if network C wanted to communicate to network Y,

it would not be able to pass through bridge 11

because that would create a loop.

Instead, one of those interfaces on bridge 11 has been blocked.

And if network C wants to communicate to Network Y,

it has to go through bridge 21, bridge 1, bridge 6, bridge 5,

finally down to network Y.

Let's look at another communication

on this network between network A and network

B. You can see that this bridge has all three interfaces

enabled.

One of them is the root port closest to bridge 1,

or the root of the network, and the other two

are designated ports, so traffic can traverse all three

of those interfaces.

If network A wanted to talk to network B,

it would simply communicate through bridge 6.

But of course on many networks there could be an outage.

Maybe someone cuts a cable or accidentally unplugs

a particular interface, and suddenly

the connection between network A and bridge 6 is severed.

Spanning tree protocol will recognize

that there's been a change to the network

and it will converge and recreate

the design of the network based around this change.

Spanning tree will recognize that there's

no communication available between network A and bridge

6, which means the root port on bridge 5

is no longer able to communicate to the root

bridge of the network.

Spanning tree will now change the root port

to be the other side of bridge 5 so that network A can now

communicate out to network B by using

the other direction of the network

and eventually make its way all the way down to network B.

One of the challenges with the traditional spanning tree

protocol is that convergence process can take anywhere

from 30 to 50 seconds.

And on today's networks, that is a very long time

to be without any type of data connectivity.

To be able to resolve that, there's an updated

version of spanning tree protocol

called the Rapid Spanning Tree Protocol, or RSTP.

This is also 802.1w as the standard.

This updated rapid version of spanning tree

will decrease the convergence time

from 30 to 50 seconds down to six seconds.

This is also backwards compatible with older spanning

tree devices, so you can mix old equipment and new equipment

in your network and implement the rapid spanning tree

protocol as needed.

This also follows a lot of the same processes and procedures

as the traditional spanning tree protocol.

So if you know spanning tree protocol,

you'll have no problem understanding

the process used for rapid spanning tree protocol.