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.