Is spending tree important in switched networks.
What happens when you disable spending creep.
Do you actually need spending tree in a layer to Ethernet to network.
Okay so let's see what happens after the moment on both of these switches.
A default configuration is being used so show spending tree shows us that spending tree is enabled on
the VLAN one
on switch one all ports of forwarding switch one is the root of the spending tree spending tree is also
running on switch to on VLAN one the switch is not the root switch interface gigabit.
1 0 2 is blocking on this switch so lets a disabled spending tree confetti no spending tree VLAN 1 on
the side no spending tree V Line 1 so on such one shows spending tree shows us that spending tree is
disabled on switch to show spending tree shows us of that spending tree is disabled notice all ports
are now showing green no ports are being blocked please note too that I'm running in simulation mode
in packet tracer and what I'm going to do now is send a ping from P.S. 1 2 P.S. to P.S. 2s IP address
is 10 dot 1 dot 1 to 2 the MAC address of P.S. 2 Is this
on P.S. 1
IP address is 10 1 1 1 MAC address is this.
So what happens if we paying P.C. to we sending an ICMP message but the PCI doesn't know the MAC address
of PCI to so it's going to send an OP into the network which is a broadcast and it's going to try and
find out of the MAC address of P.C. to I'm going to click capture forward the a message is sent to the
switch
notice what happens.
It's sent to switch to switch to however duplicates of the packet and floods it out of all ports so
it goes back to switch one on gigabit wonder 0 1 and it's received by P.C. to CPC to he's receiving
this broadcast.
And now P.S. One is receiving the broadcast or that it's sent notice of the source MAC addresses.
P.S. One destination is broadcast it's looking for the MAC address of P.C. to CPC one will drop that
packet.
But notice we now have multiple packets being flooded through the network.
P.S. One has received the message once again so as P.C. to CPC to is receiving multiple OP requests
from the network the switches are also duplicating packets when we look after the MAC address table
of switch to
we can see that.
P.S. To is found on gigabyte 1 0 3 and P.S. 1 is found on gigabit 1 0 2
capture forward
notice now the switch thinks that P.S. 2 is connected to gigabit 1 02 whereas in actual fact P.S. 2
is connected to gigabit 1 0 3.
Once again this is the MAC address of P.C. to so the switch is receiving conflicting information.
Previously it thought that P.S. 2 is connected to this port.
Now it thinks that P.S. 2 is connected to this port capture forward again.
Now it thinks that P.S. To is connected to gigabit 1 0 1 so the switch previously thought that P.S.
2 is connected to 1 0 3 which is correct then it's photo that P.S. 2 is connected to 1 0 2 and now it
thinks that P.S. 2 is connected to 1 0 1 previously it's thought that P.S. 1 is connected to 1 0 2 then
2 1 0 1 and now 2 1 0 2.
So the MAC address table is constantly being updated.
This is how Broadcom storms happen in life networks.
Which can bring down an entire network.
We have duplication of packets.
We have mac address table instability we have hosts receiving the packets that they sent out into the
network such as here P.S. 1 receiving its own op request message.
Generally we don't talk to ourselves and in the same way a P.C. doesn't send a broadcast to itself like
we see in this network.
And if I continued doing that notice we constantly we constantly have these op messages being duplicated
and flooded through the network packet tracer isn't showing all the duplication here.
But notice that this just continues on and on and on and can cause a broadcast storm and network meltdown
in a real network.
The same original packet is being duplicated multiple times.
Notice this is stolen off message.
Looking for the mac address of P.S. 10 1 1 2.
And if I continue to capture forward we just see those messages being sent continuously by the switches
throughout the network.
So is spending three important.
Definitely should you disable spending tree in a layer to network in most cases no late to switch networks
or a single broadcast domain.
A broadcast gets flooded throughout the Layer 2 domain.
If you had a more complex network like this your broadcast form would be even worse.
Here's a very simple example of what happens when spending tree is disabled so get back on to switch
one and all enable spending tree.
I'll do something similar on switch to enable spanning tree.
Notice.
Now we see spending tree messages being sent between the switches.
That's how the switches learn about one another.
They are sending out BP to use
to inspect trace so we can see the actual BP EU messages.
I'll turn this back to real time and what should happen now is the ports should transition to green.
Once the switches have learnt about one another
and decided to the root bridge is notice the ports are currently in the learning state show spanning
tree on switch to we see something similar.
These ports are now forwarding this port is blocking on switch 1 all ports have transitioned to the
forwarding state so we can now see that this port is blocking in Packet Tracer.
So let's do that ping again.
We've got an op message being sent into the network that now gets sent to switch to a cross port gigabit
wonder 0 1
it's sent to P.S. 2 but notice of this packet is going to be dropped.
The packet will not be forwarded out of port 1 0 2 spanning trees blocking that broadcast the packet
is only forwarded to P.S. To
we now have the OP or reply from P.S. 2 back to P.S. 1 in the inbound PD you we can see Target MAC address
target IP address.
Source MAC address is P.C. 2.
This is an op a reply message gets sent across the top link to switch 1 and gets sent to P.S. 1 and
now P.S. 1 can send the ICMP message
using the top link to P.S. 2 and the reply can be sent back.
2 P.S. 1.
That's what we want to see in an ether net network.
We want to have the devices communicating with each other so hopefully this package trace a demonstration
has shown you how important spanning tree is in a layer to switched network even into this small topology.
The network breaks when spending tree is disabled on both such as make sure that you have a spending
tree enabled on layer to switch networks unless you have a very good reason to disable spanning tree
on your switches.