Original subtitles

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So in this topology, let’s determine why certain ports are set to forwarding

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and why certain points are blocking

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so we'll work through the Spanning Tree process.

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

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So 1 of the switches in the topology

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needs to become the root of the Spanning Tree.

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So on switch 1 as we saw previously

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sh spanning-tree

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shows us that this switch or bridge is the root of the Spanning Tree.

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Switch 2 is not the root of this Spanning Tree.

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So in the output here we can see

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that it has a path cost to get to the root switch

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it sees that the root bridge or root switch

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has the root ID with priority of this

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and MAC address of this which is different to the local switch MAC address.

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So first decision, how is the root determined?

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It's based on lowest bridge ID

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which consist of the priority and MAC address.

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Switch 1 has the same priority as switch 2, 32769

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so that can't be used to determine the Spanning Tree root.

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So the tie breaker is based on the MAC address.

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so lowest MAC address will win

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switch 1 has a lower MAC address when compared to switch 2

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once again 0011 is the same on both switches

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but notice c6ea is greater and c6ac in hexadecimal

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so switch 1 becomes the root of the Spanning Tree.

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So that’s the first decision determine who the root bridge is.

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Once again priority 32768 is the default.

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So we’ve now determined who the root bridge is or root switch is.

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The next decision is that every non root switch needs to determine its root port

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the root port is at its best port to get to the root bridge.

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The root port is chosen based on lowest path cost.

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If there’s a tie-breaker on that

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then it's based on lowest neighbor bridge ID if path cost are the same.

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if that can't be used to determine the root port

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then the lowest port priority is used

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the port priority is 128 by default

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and if that can't be used then the lowest port ID is used as a tie-breaker.

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So first decision is based on lowest path cost.

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Here’s a table showing you the path cost of Cisco switches

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they are either based on a 1998 IEEE cost or 2004 IEEE cost.

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In the 1998 cost values, a 10 Meg link has a cost of 100, 100 Meg 19, I gig 4

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and 10 gig 2 and the IEEE cost in 2004 and later the cost changed to the following.

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So in our topology, we have gig interfaces on the switches

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and if we look at the path cost of various ports

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notice the value associated is 4

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so these gigabit links have a path cost value of 4

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which means that the switches are using the old path cost method

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to determine the best path to a destination.

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now the first decision is to determine the root port based on the path cost

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in this topology, we have gigabit 0/0 connected directly to switch 1

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gigabit 0/1 is also directly connected to switch 1

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gigabit 0/3 is connected to a hub

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which inturn connected to switch 1.

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So the path cost of gigabit 0/3 would be 8

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if there was a switch connected here

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but at the moment the path cost is 4 because we have a hub instead of a switch.

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So we have 3 ports with the same path cost to get to switch 1.

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on switch 2 we can type sh spanning root as an example

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and we can see that gigabit 0/0 was chosen as the root port to get to switch 1

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but that couldn’t have been determined based on the path cost

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it would need to be determined base on something else.

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So once again sh spanning-tree

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so on switch 2 its chosen gigabit 0/0 as its root port.

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Can path cost be used to determined the best path to the root bridge

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based on its port numbers and the answer is no

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the path cost of this link is 4, the path cost of this link is 4

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the path cost of this link is 4

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but that can’t be used as the determining factor.

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So the next choice is neighbor bridge ID.

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Now in this example switch, 2 is connected to switch 1

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on 2 ports that are directly connected to switch 1.

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so the neighboring bridge ID on both this port is the same

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so that cannot be used at as the tie-breaker

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the next decision criteria is based on priority

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but the priority of the ports are the same

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so that can't be used as a tie-breaker.

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So the port number is used as the tie-breaker.

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1 is a lower number than 2

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so hence gigabit 0/0 is chosen as the root port in this topology.

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now once the root ports are chosen on a per segment basis

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a designated port needs to be chosen.

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The easiest way to work this out

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is imagine that you have a PC in the middle of this cable

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and it needs to get to the root bridge

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using either the port on the left or the port on the right.

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So if I had a PC in this topology

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which port would it used to get to the root bridge

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and hopefully, it's fairly obvious

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that this port is closer to the root bridge than this port.

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and hence on this segment gigabit 0/0 to gigabit 0/0

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this port, port 0/0 on switch 1 is the designated port

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a designated port is the best port to use

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on a per segment basis to get to the root bridge.

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So this port is the best port to use on this top segment

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to get to the root bridge. What about this segment?

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So on this segment imagine once again that you had a PC here

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what's it's best port to use to get to the root bridge?

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Well it would be this port here on switch 1

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and once again on switch 1, we can see that by typing sh spanning-tree

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notice gigabit 0/1 on switch 1 is the designated port for this segment.

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The same it's true for this segment

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which is the best port to use to get to the root bridge?

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It's gonna be gigabit 0/3 on switch 1.

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And on this segment looking at layer 2 switches running spanning 3

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this port is the best port to use to get to the root bridge.

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So we've now chosen a designated port for this top link

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the 2nd link, these links through the hub as well as this link.

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The last remaining link is this link

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and the best port to use to get to the root bridge is this port on switch 2.

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Any other ports on the network will go blocking.

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So this port gigabit 0/1 is put in the blocking state

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and so is gigabit 0/3 also put in the blocking state.

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Now in Rapid Spanning Tree or Rapid PVST these are known as alternate ports.

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In other words, on these hubs as an example, if we have a PC connected to it.

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If this link went down

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PCs could send traffic into the network using this alternate port

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because it would now transition to the forwarding state when this link goes down.