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So in apology let's determine why certain ports are set to forwarding and why certain ports are blocking.

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So we'll work through the spending tree process.

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The first decision that needs to be made is election of a root bridge.

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So one of the switches in the typology needs to become the root of the spending tree.

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So on switch one as we saw previously show spending tree shows us that the switch or bridge is the root

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of the spending tree.

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Switch to the

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is not the root of the spending tree.

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So the outputs here we can see that it has a path cost to get to the root switch.

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It seems that the route bridge or route switch has a route ID with a party of this and MAC address of

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this which is different to the local switch MAC address.

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So first decision how is the route determined.

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It's based on lowest bridge ID which consists of the property and MAC address so which one has the same

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priority as switch 2 3 2 7 6 9.

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So that cant be used to determine the spanning tree route.

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So the tie breaker is based on the MAC address so lowest MAC address will one switch.

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One has a lower MAC address when compared to switch to.

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Once again 0 0 1 1 is the same on both switches.

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But notice C-6 e a is greater than C-Six a C in hexadecimal.

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So switch one becomes the root of the spanning tree.

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So that's the first decision.

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Determine the route bridges.

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Once again Protea 6:57 6:08 is the default.

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So we've now determined to route bridges or route switches.

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The next decision is that every non routes switch needs to determine its route port the route port is

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its best port to get to the root bridge.

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The route to Port is chosen based on lowest Poth cost.

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If there's a tie breaker on that then it's based on lowest neighbor bridge ID.

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If Poth costs are the same if that can be used to determine the route port then the lowest port priority

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is used.

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The poor priority is 128 by default.

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And if that can't be used then the lowest port IDs used as a tiebreaker.

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So first decision is based on lowest possed cost.

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Here's a table showing you the path costs of Siska switches.

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They are either based on a 1998 triple E cost or 2004 I triple E cost in the 1998 cost values a 10 meg

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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

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the costs change to the following.

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So in our typology we have gig interfaces on the switches and if we look at the path cost of various

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ports notice the value associated is full.

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So these gigabit links have a pot cost a value of four which means that the switches are using the old

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Poth cost method to determine the best path to a destination.

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The first decision is to determine the route port based on the path cost in this typology we have gigabit

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to 00 connected directly to switch 1 gigabit.

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0 1 is also directly connected to switch 1 gigabit.

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0 3 is connected to a hub which in turn is connected to switch one.

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So the path cost of gigabit in 0 3 would be 8 if there was a switch connected to here.

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But at the moment the power cost is four because we have a hub instead of a switch.

48
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So we have three ports with the same path cost to get to switch one on switch two we can top show spending

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tree root as an example and we can see that gigabit 00 was chosen as the root port to get to switch

50
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one but that couldn't have been determined based on the path cost it would need to be determined based

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on something else.

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So once again shows spending tree so on switched to its chosen gigabit 00 as its root port can path

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cost be used to determine the best path to the route bridge based on its port numbers.

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And the answer is no the path of this link is for the path cost of the Slinky's for the path cost of

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this link is for.

56
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But that cant be used as the determining factor.

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So the next choice is neighbor bridge ID now and this example switch to is connected to switch one on

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two ports that are directly connected to switch one.

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So the neighboring bridge ID on both these ports is the same.

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So that cannot be used as the tiebreaker.

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The next decision criteria is based on priority but the priority of the ports are the same.

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So that can be used as a tiebreaker.

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So the port number is used as the tiebreaker.

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One is a lower number than 2.

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So hence kickabout is 0 0 is chosen as the route port in this topology.

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00:05:44,450 --> 00:05:51,290
Now once the route ports are chosen on a per segment basis a designated port needs to be chosen.

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The easiest way to work this out is imagine that you have a PC in the middle of the cable and it needs

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to get to the root bridge using either the port on the left or the port on the right.

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00:06:06,440 --> 00:06:12,680
So if I had a PC in this apology which port would use to get to the root bridge and hopefully it's fairly

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00:06:12,680 --> 00:06:20,240
obvious that this port is closer to the root bridge than this port and hence on the segment gigabit

71
00:06:20,330 --> 00:06:30,680
00 to Gigabit 00 this port Port is 00 on switch one is the designated port a designated port is the

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00:06:30,680 --> 00:06:36,540
best port to use on a per segment basis to get to the root bridge.

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00:06:36,560 --> 00:06:42,620
So this port is the base port to use on the top segment to get to the root bridge.

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00:06:42,620 --> 00:06:45,500
What about the segment on this segment.

75
00:06:45,530 --> 00:06:48,470
Imagine once again that you had a PC here.

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00:06:48,470 --> 00:06:52,090
What's its best port to use to get to the root bridge.

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Well it would be this port here on switch one.

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And once again on switch one we can see that by typing show spanning tree nodes gigabit zero one on

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switch one is the designated port for this segment.

80
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The same is true for the segment which is the base port to use to get to the root bridge.

81
00:07:13,720 --> 00:07:23,940
It's going to be gigabit 0 3 on switch 1 and on the segment looking at least two switches running spending

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three.

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00:07:24,740 --> 00:07:28,930
This port is the base port to use to get to the root bridge.

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00:07:29,000 --> 00:07:32,190
So if not chosen a designated port for this top link.

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The second link these links through the hub as well as the link the last remaining link is this link.

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00:07:41,120 --> 00:07:48,320
And the best port to use to get to the root bridge is this port on switch to any other ports on the

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00:07:48,320 --> 00:07:50,010
network will go blocking.

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So this port gigabit 0 1 is put in the blocking state and so is gigabit.

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00:07:57,380 --> 00:07:58,440
0 3.

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00:07:58,470 --> 00:08:03,980
Also put in the blocking state and Reppert spending tree or Reppert Peavey's T.

91
00:08:04,070 --> 00:08:07,220
These are known as alternate ports.

92
00:08:07,220 --> 00:08:14,210
In other words on this hub as an example if we had a PC connected to it if this link went down PCs could

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send traffic into the network using this alternate port because it would not transition to the forwarding

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state when this link goes down.