Spanning Tree Protocol - N10-008 CompTIA Network+ : 2.3
Summary
TLDRThe 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
🔁 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.
🚀 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.
Mindmap
Keywords
💡Time to Live (TTL)
💡Layer 2 Ethernet
💡Loop Protection
💡Spanning Tree Protocol (STP)
💡Root Port
💡Designated Port
💡Blocked Port
💡Topology
💡Convergence
💡Rapid Spanning Tree Protocol (RSTP)
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.
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