So on switch 1 the only port that’s forwarding is gigabit 0/0 as shown here.
Other ports on the switch are blocking.
So taking a picture of the topology
this port is blocking, this port is blocking and this port is blocking.
The only port that is forwarding is gigabit 0/0 and we can see that in the output here.
So gigabit 0/0 is a root port.
So we could say this is the root port of that switch.
Other ports are blocking.
Now what about switch 2?
So on switch 2 for VLAN 10 all ports are forwarding
because this is the root bridge.
On a root bridge, all ports are in the forwarding state
you can't get closer to the root bridge than the root bridge itself.
So all of this ports on the switch are forwarding
and all of these ports are designated ports
so each port here is a designated port
and the state of the port is forwarding. What about switch 3?
so on switch 3 enable sh spanning-tree vlan 10
we can see in the output that gigabit 0/0 is a designated port, it's forwarding
so this is a designated port and the state is forwarding.
We can see that gigabit 0/1 is a root port
and its forwarding, so this is a root port and the state is forwarding
and we can also see that gigabit 0/2 is a designated port and its forwarding.
So this is a designated port and the state is forwarding.
What about switch 4? so on switch 4 sh spanning-tree vlan 10
on this switch, we’ve got 2 ports that are in VLAN 10.
The reason why this port is not showing up is that this port is in VLAN 20
and gigabit 0/0 is the root port and is forwarding and 0/1 is the designated port
and is forwarding, so this is the root port and status forwarding.
This is a designated port and state is forwarding
This port is in different VLAN, so it's not shown for VLAN 10.
So I’m hoping you can see a problem here.
I’ll take another screen shot of this topology.
What I want you to see is that this port
this port and this port have been taken out of the topology
because of the ports being blocked by Spanning Tree.
User data is not gonna be sent through the blocking ports.
So logically from a user point of view.
These ports are not connected to the topology
even though they're there while those ports on the blocking state
user traffic doesn’t go through those ports.
Other traffics such as CDP and LLDP will go through a blocking port
but user traffic doesn’t. So what is that mean?
If this user is sending large files as an example, to this server
that traffic will flow this way to get to the server which is inefficient.
Traffic from this host will go this path to get to this server.
So if a large number of hosts here were sending traffic to the server
that traffic is having to go across this link between the 2 core switches
and it could be over run because we only have 1 link that’s enabled.
So if I clear this diagram up
we only have 1 forwarding link here.
The other link is broken and the path used is inefficient.
one of the things that you must remember with the Spanning Tree
is you must set the Spanning Tree root
you don’t wanna allow Spanning Tree to determine
where the root is without your input
because the situation could be a lot worst than it is currently.
If this switch became the root bridge of the topology
it could be a lot worst
because this switch would use this as it's root port
this switch would use this as it's root port
and let’s assume that the MAC address of this switch
is lower than the switch or has a lower priority
that would mean that this would be the root port of this edge switch.
This port and this port would be designated port
this would be a designated port
and in this topology assuming that this has a lower bridge ID
these ports would be designated ports.
Let’s assume that this has a lower bridge ID than this
this would be a designated port in that scenario
which means that this port is blocked
this port gets blocked and this port gets blocked.
So now if this host has to send a large number of files to this server
the traffic will flow like this to get to the server.
Assume once again that you have many, many host devices here
they are sending all their traffic via this access switch to get to the server.
If this was a lower end switch let’s say it’s very old 2950 access switch
and it became the root of your topology
you would be forcing lots of lots of user data through this little switch
and the same would be true if you have many, many access switches
so imagine you have 20 access switches here
all the user traffic on those 20 access switches would now be pushed through
the access switch to get to the server
that’s definitely not what we want.
We wanna make sure that the core switches of the root switches
and to improve this we want to make sure
that one of the switches is the root for some VLANs
so in this topology, we gonna make switch 1, the root for VLAN 1 and VLAN 10
and we're gonna make this switch the root for other VLANs
now in this topology, we would split it as follows
because VLAN 10 hosts are connected to this switch
and it’s gonna be more efficient if they send traffic directly to that core switch
and this host VLAN 20 send their traffic directly to switch 2.
So in our topology, we're going to make this switch the root for VLAN 1
and VLAN 10 and this switch will be the root for VLAN 20
that means once again that VLAN 10 host will send their traffic there
and VLAN 20 hosts will send their traffic there.
So let’s set that up.