A Spanning Tree is going to be a Logical, Loop Free Topology and,
i say it’s a “Logical Loop Free Topology”,
because physically like the Topology, we see on Picture.
Physically it’s looks like we have a Loop,
it’s looks like we got the path between Switches A and B,
where packets could Circulate Endlessly but, with
Spanning Tree Protocol we going to logically
make one of the ports, on one of those Switches Block Traffic.
We gonna prevent traffic from flowing in and out of
one of those ports, that’s what Spanning Tree Protocol can do for us. It can
give us a Logical Loop Free Topology, which is
gonna give us Redundancy. Well avoiding the ugly
side of facts that come with Layer 2 Topological Loop like
we discussed, a Spanning Tree has a Root Bridge, and we want to be able to administrative influences, which Bridge, which Switch
becomes the Root Bridge.
If we just plug several
Switches together without configuring Spanning Tree Protocol.
Spanning Tree Protocol will automatically elect the Root bridge, however The Bridge, The Switch that’s the elected is the Root Bridge, it might not be the Optimal Switch, and order to influence the Root Bridge, we need too able to look at the topology
like this, and on paper to determine.
Ø Who
is the Root Bridge in this Topology?
Ø Which
Ports Are Forwarding?
Ø Which
Ports Are Blocking?
The first question that we ask, who is the Root Bridge. In a Spanning Tree Topology, we have One Root Bridge, and Switch that has the Lowest Bridge
ID.
The Bridge ID is made up of a Switch Priority, which is something that we can
administratively set as well as a Mac-Address,
and on Cisco Catalyst Switches the default Switch Priority, that we have is 32768, we could make that value lower to influence that
Switch to become the Root Bridge.
What
if we interconnect bunch of Switches, and we do not set the Switch Priority to something other than the default.
Well the Mac-addresses, is the different Switches acts the Tie Breaker, the Switch with the Lowest Mac-Address become the Root
Bridge, if we do not go in, and influence the Switch Priority.
Let’s
take a look at the Topology on Picture.
We got a couple of switches,
and the Priority Value of these switches are the default values 32768, that means we gonna use the Switches Mac-Addresses as the Tie-Breaker.
The Question we need to ask is
Ø Who
is the Root Bridge?
They have an Equal Priorities that means we look to see who has the
lowest mac-address. Let’s look at the first 3 Hexadecimal digits in each Mac-Address. On Switch
A, the First 3 digits are 001, and Switch B the
first 3 digits are 000 “Oh! That’s less than 001” that’s tell us, that
Switch B, because the Priority is equal. The Switch B is going to be Elected
as the Root Bridge.
We now know, which Switch is going to be the Base of our Spanning Tree. However, we do not yet know
Ø Which port
are going to be Blocking
Ø And Which Port are going to be Forwarding
To
determined that, we need to understand the different ports states that Spanning
tree has.
Well
we know that, Switch B is not going to have any Root Port
because it’s the Root Bridge, that means Switch A going to have Root
Port. It’s only gonna have One (1).
It’s
the Port, that is Closest in Terms of Cost, and
when we talk about Cost, we can look at the Interface Speeds, even though both interfaces that we
see on Switch A are Fastethernet Interfaces. It doesn’t
necessarily mean that they configured to run at Fastethernet
Speeds.
Notice that Fastethernet 1/0/1,
up of the top of Switch A. It is running 100Mbps, however, Fastethernet
1/0/2 at the bottom of Switch A. It’s configured to run at 10Mbps, in you see from table on Picture.
That an “Interface Speed or Ports Speed” of 10Mbps, has an associated Spanning
Tree Protocol Port Cost of 100. The Cost
for a 100Mbps Port is 19.
In order for Switch A, to get back to the Root Bridge, if it goes out of fastethernet 1/0/1,
it’s gonna have a Cost of 19 to get there. If it goes out of Fastetherent 1/0/2, that’s gonna be Cost of 100 because that Port
Speed is 10Mbps.
The Cost is lower, if we go out of Fastetherent
1/0/1. Therefore, we can label our Root Port on
Switch A as Fastethernet
1/0/1.
The
next Port State is a “Designated
Port” and we have a Designated Port on
every Network Segment. We got 2 segments on
Picture “Top One and Bottom One”, and on each Segment,
we going to have One Designated Port, and “It’s the Port, that is Closest to the Root in Terms of Cost”
and since both of these Segments touch the Root Bridge.
This is going to be easy in this Topology because we are not gonna get Closer
to the Root, that actually being on the Root. This means that both Ports on our
Root Bridge are “Designated Ports Gigabit 0/9 and Gigabit
0/10”.
The Next Type of Port, we have is
a “Non-Designated Port or a Blocking Port”.
By the way we could have a Port, that was “Administratively
Shutdown”. It would “be Disabled” and if
it were “Administratively Shutdown”, Spanning Tree Protocol would not Administratively
bring at Backup, if it were needed it’s just
going to be Shutdown. We assuming that in this Topology,
we truly do have Backup Path. We were assuming
that, none of these Ports are Shutdown, that
means that any Remaining Ports, that we not
already identified as a “Root Port or as a
Designated Port”, any another Remaining Ports,
we gonna be “Non-Designated Ports”,
another words that’s gonna be “Blocking Ports”,
and that means the Bottom Port on “Switch A Fastethernet 1/0/2”, that’s a “Blocking Port”.
Conclusion: - This gives
us our “Loop Free Topology”. We not going to
have a “Layer2 Topological Loop” because Production Traffic is not going to be flowing “Into or Out” of a Fastethernet
1/0/2 on Switch A.
If You Like the Post. Don’t forget
to “Subscribe/Share/Comment”. Thank You.
0 comments:
Post a Comment