So in apology let's determine why certain ports are set to forwarding and why certain ports are blocking.
So we'll work through the spending tree process.
The first decision that needs to be made is election of a root bridge.
So one of the switches in the typology needs to become the root of the spending tree.
So on switch one as we saw previously show spending tree shows us that the switch or bridge is the root
of the spending tree.
Switch to the
is not the root of the spending tree.
So the outputs here we can see that it has a path cost to get to the root switch.
It seems that the route bridge or route switch has a route ID with a party of this and MAC address of
this which is different to the local switch MAC address.
So first decision how is the route determined.
It's based on lowest bridge ID which consists of the property and MAC address so which one has the same
priority as switch 2 3 2 7 6 9.
So that cant be used to determine the spanning tree route.
So the tie breaker is based on the MAC address so lowest MAC address will one switch.
One has a lower MAC address when compared to switch to.
Once again 0 0 1 1 is the same on both switches.
But notice C-6 e a is greater than C-Six a C in hexadecimal.
So switch one becomes the root of the spanning tree.
So that's the first decision.
Determine the route bridges.
Once again Protea 6:57 6:08 is the default.
So we've now determined to route bridges or route switches.
The next decision is that every non routes switch needs to determine its route port the route port is
its best port to get to the root bridge.
The route to Port is chosen based on lowest Poth cost.
If there's a tie breaker on that then it's based on lowest neighbor bridge ID.
If Poth costs are the same if that can be used to determine the route port then the lowest port priority
is used.
The poor priority is 128 by default.
And if that can't be used then the lowest port IDs used as a tiebreaker.
So first decision is based on lowest possed cost.
Here's a table showing you the path costs of Siska switches.
They are either based on a 1998 triple E cost or 2004 I triple E cost in the 1998 cost values a 10 meg
link has a cost of 100 hundred make 19 1 gig 4 and 10 gig to in the triple E cost in 2004 and later
the costs change to the following.
So in our typology we have gig interfaces on the switches and if we look at the path cost of various
ports notice the value associated is full.
So these gigabit links have a pot cost a value of four which means that the switches are using the old
Poth cost method to determine the best path to a destination.
The first decision is to determine the route port based on the path cost in this typology we have gigabit
to 00 connected directly to switch 1 gigabit.
0 1 is also directly connected to switch 1 gigabit.
0 3 is connected to a hub which in turn is connected to switch one.
So the path cost of gigabit in 0 3 would be 8 if there was a switch connected to here.
But at the moment the power cost is four because we have a hub instead of a switch.
So we have three ports with the same path cost to get to switch one on switch two we can top show spending
tree root as an example and we can see that gigabit 00 was chosen as the root port to get to switch
one but that couldn't have been determined based on the path cost it would need to be determined based
on something else.
So once again shows spending tree so on switched to its chosen gigabit 00 as its root port can path
cost be used to determine the best path to the route bridge based on its port numbers.
And the answer is no the path of this link is for the path cost of the Slinky's for the path cost of
this link is for.
But that cant be used as the determining factor.
So the next choice is neighbor bridge ID now and this example switch to is connected to switch one on
two ports that are directly connected to switch one.
So the neighboring bridge ID on both these ports is the same.
So that cannot be used as the tiebreaker.
The next decision criteria is based on priority but the priority of the ports are the same.
So that can be used as a tiebreaker.
So the port number is used as the tiebreaker.
One is a lower number than 2.
So hence kickabout is 0 0 is chosen as the route port in this topology.
Now once the route ports are chosen on a per segment basis a designated port needs to be chosen.
The easiest way to work this out is imagine that you have a PC in the middle of the cable and it needs
to get to the root bridge using either the port on the left or the port on the right.
So if I had a PC in this apology which port would use to get to the root bridge and hopefully it's fairly
obvious that this port is closer to the root bridge than this port and hence on the segment gigabit
00 to Gigabit 00 this port Port is 00 on switch one is the designated port a designated port is the
best port to use on a per segment basis to get to the root bridge.
So this port is the base port to use on the top segment to get to the root bridge.
What about the segment on this segment.
Imagine once again that you had a PC here.
What's its best port to use to get to the root bridge.
Well it would be this port here on switch one.
And once again on switch one we can see that by typing show spanning tree nodes gigabit zero one on
switch one is the designated port for this segment.
The same is true for the segment which is the base port to use to get to the root bridge.
It's going to be gigabit 0 3 on switch 1 and on the segment looking at least two switches running spending
three.
This port is the base port to use to get to the root bridge.
So if not chosen a designated port for this top link.
The second link these links through the hub as well as the link the last remaining link is this link.
And the best port to use to get to the root bridge is this port on switch to any other ports on the
network will go blocking.
So this port gigabit 0 1 is put in the blocking state and so is gigabit.
0 3.
Also put in the blocking state and Reppert spending tree or Reppert Peavey's T.
These are known as alternate ports.
In other words on this hub as an example if we had a PC connected to it if this link went down PCs could
send traffic into the network using this alternate port because it would not transition to the forwarding
state when this link goes down.