Let’s see, what we learned about Spanning
Tree Protocol so far, and do a paper exercise. Let’s take a look at this
topology and Determined Theoretically.
Ø Who
would be the Root Bridge?
Ø What
would be the Root Port’s?
Ø Which
Ports would be the Designated Ports?
Ø Which
Ports would be Blocking?
The First Question we ask is “Who is the
Root Bridge”
Remember that
the Mac-Address is the Tie-Breaker, if the
Priority is equal. Let’s take a look at Priorities “do
they equal”.
I am noticing that, the Priorities for Switches
A and B, they are lower, they 16384. Those
are lower than the Priorities on Switches C and D,
that means that either, Switches A or Switch B is
going to be our Root Bridge, but which one.
Here what the Mac-Addresses come as a
Tie-Breaker, which switch as the “Lowest
Mac-Address”, if we take a look at first 3 Hexadecimal digits in the
Mac-Addresses of these Switches. Switch A has the first
3 Hex Digits as “000”. On Switch B, its “001”. “000”
that’s lower than “001”, that tells us
that, Switch A is going to be our Root Bridge.
Now that we determined the Root Bridge.
The next thing we ask is
Ø What
are the Root Ports?
Remember
the definition of the Root Port. We said that “the Root Port is a port on a Non-Root Bridge, the Root Bridge,
Switch A is not going to have any of these, but it’s the port on a Non-Root
Bridge, that is closest to Root Bridge in terms of Cost”, and we
going to assume that, all over those interfaces are running out their maximum
speed, another words this “Te interface”
that’s a “10 Gigabit interface”,
we assuming, it running out “10 Gigabit”
and if does that means, it’s gonna have “Cost of
2”, and a “Gigabit interface”,
which is what all over the other interfaces are, there gonna have a Cost of 4.
And
let’s take at Switch B and ask: -
What
is the Root Port on Switch B? What would be the Cost
to get back to Switch A.
Well if I went out of the 10 Gigabit Port, it would be a Cost
of 2 to cross the link to get to Switch A. I got other ways of getting the
Switch A though.
I could go
down to “Switch C with Cost of 4” and then go,
from Switch C, up to Switch
A, with another Cost of 4, that the Total
of “8”, 8 is much more than 2.
Ø Same thing,
if I went from Switch B to D, Switch D to Switch A, that would be a Cost of 4+4=8,
much greater than 2. This tells me that my
“Root Port” on Switch B is my “Te 1/0/1(10 Gigabit)”.
What
about Switch D, Well Switch D could
go directly to the “Root Bridge over a Gigabit1/0/1
Port”, that would be a “Cost of 4”
or It could go over to Switch B, which would be a “Cost of 4”, than over to Switch A that an additional “Cost of 2”, 4+2=6,
6 is greater than 4, that tells me
that, Gigabit 1/0/1, that is my Root Port on
Switch D.
Next Switch C, on Switch C if, I went from
to Switch C to B to A, that’s the “Total Cost of 6”, but Notice Switch C has a couple of Ports, that would get
me up to Switch A. Now let’s start to get a bit tricky.
Ø Which
one of these is going to be Root Port, which one is considered closet to the
Root in terms of Cost.
It looks like, we have an equal
Cost, either path is a Cost of 4,
what do, we do now. Well this is a common point
of confusion for student learning Spanning Tree Protocol. We
going to go with the “Lowest Port ID”,
but it’s not the “Lowest Port ID on Switch C”. It’s the Lowest Port
ID, that is sending traffic into Switch C. It’s the “Sender’s Port ID”, that’s we gonna be comparing. In
this case “Gigabit 1/0/3 and Gigabit 1/0/4”.
With
default settings “Gigabit 1/0/3”
is going to have the “Lowest Port ID”,
because the “Port Number is Lower”.
You see the way the Port ID works is,
there is a Port Priority, that defaults to 128
and then there is a Dot (.), after the Dot (.), there is a Port Identifier, and typically on many Cisco Catalyst Switches. This Port Identifier is
going to be a number, that’s too greater than this number.
Ø So, I would
guess that the Port ID of Gigabit 1/0/3 would be “128.5”
Ø And the Port ID of Gigabit 1/0/4 would be “128.6” and Switch C can
see that information.
It knows the “Port ID of the
Sending Switch of the Far End Switch”, and the Root Port that Switch see is going to choose, is going
to be on the link, that connects to the Lowest
Port ID on the Far End. Which is Gigabit
1/0/3 with the Port ID of 128.5
What
if we wanted to manipulate that, though in the Real world?
What
if I prefer that the link connecting into Gigabit 1/0/4 be used?
Well we not able to
change this (.6) component of the
Port ID, that’s the Physical Location within the Switch. That’s Not Configurable.
However,
we can change the Port Priority (128), in this example, I could go to “Gigabit 1/0/4” and I could change the Port Priority from 128 to 64, making the Port ID 64.6 that
would make it Lower than 128.5,
that would make this Gigabit 1/0/4 Port,
the link that was forwarding traffic.
But
not in this case. IN this case, we have a Default
Settings and since, Sender’s Port ID is less for 1/0/3 this link,
that means that “Gigabit 1/0/10 is going to be my
Root Port”, “that’s gonna be closet
to the Root Bridge in terms of Cost”. The actual Cost was equal, so the Tie-Breaker
became the link, which connected to the Lowest Port ID.
The Port ID on far end Switch, Switch A in our
case.
We now identified the Root Ports. Next up we have Designated
Ports?
Ø Who
are the Designated Ports in this Topology?\
Remember the definition of a Designated
Port, we said that every link,
every network segment has a Designated Port, and
“it’s the Port on that segment that’s closet to
Root in terms of Cost”.
And we
said that we not going to get any closer to the
Root, then actually being on the Root,
which means that, all of the Ports on the Root
Bridge are Designated Ports, and
that count for 4 our links.
We don’t have that many lefts. We do have link between Switch D and B, and
we going to ask “Which end of that is closet to
the Root in terms of Cost”
Well
if I am looking from the prospective of, this Gigabit
1/0/5 end. If I am on Switch B, I got the Cost of 2, to get back to the Root, if I am on
this Gigabit 1/0/2 end of the link, that would have a Cost of 4 to get back. So, this Gigabit 1/0/5 has the Lowest Cost. This means
that, this Gigabit 1/0/5 is going to be my
Designated Port on Switch B.
And we have one other link, and it’s this Gigabit
1/0/2 – 1/0/7 between Switch C and Switch B
Ø Who
is the Designated Port Here?
Well by the same logic, if I am this Gigabit1/0/7 end of link, we gonna have the Cost of 2 going
over that Te 1/0/x link, to get back to the Root Bridge, if we are in this Gigabit 1/0/2 (Switch C) end of link then I would have a Cost of 4, that means that this Gig1/0/7 going to be My Designated Port on Switch B.
We have 6 Links or Wire connected between
Switches, So we have 6 Designated Port
And
we assuming that all of the interfaces are administratively up. We don’t have
any to these interfaces that are administratively shut down, and if that’s the
Case, any port that we don’t yet Labeled, is
going to be a Blocking Port or rest of the Ports are Blocking Port.
Ø This
Gigabit 1/0/2 on Switch D is going to be a Blocking Port
Ø This
Gigabit 1/0/11 on Switch C is going to be a Blocking Port.
Ø And
this Gigabit 1/0/2 on Switch C is going to be a Blocking Port.
And
what we able to do in this Practice Exercise is determine
Who is the Root Bridge: - Bridge with a lowest Bridge ID, we look for
the Switches with Lowest Priorities and if we have Tie, Mac-Addresses of those switches become the Tie-Breaker.
Who were the Root Ports: - We said
that “Root Ports was a Port on Non-Root Bridge,
that was closest to Root in terms of Cost”, and we use the
default Port Costs that were associated with different
Interface Speeds:-
Ø if we had a
“10Mpbs Port that would be a Port Cost of 100”.
Ø “A 100 Mbps Port that would be a Cost of 19”.
Ø A
Gigabit Port is a Cost of 4
Ø And
10Gigaport is a Cost of 2
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