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A part.

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Cost is the cost to get to the route bridge or route switch.

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Path costs are calculated from the cost of a port and the number of links to get to the root bridge.

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As an example, a one gigabit per second link may have a cost of four.

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If a switch is directly connected to the router switch, the path cost would be four.

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Because there's a single link with a cost of four.

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However, if switch three had a link to switch two with a port cost of four and switch two had a link

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to switch one, which is the route with a port cost of four.

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The cost of switch three to get to switch one using these links would be eight because it's four plus

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four.

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To get to the root bridge.

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Support costs are incremented in a similar way to hop count in a protocol such as rep.

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So path cost is calculated from the sum total of port costs to get to the root bridge based on a path

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of multiple links to the root bridge.

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So port costs do have default values on a per port and per VLAN basis.

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You can change the port cost values, but they do have defaults defined by the triple e.

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There are two standards the triple E cost 1998 and triple E cost 2004.

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Some Cisco switches still use the old path cost and you can change the switch to use the new path cost

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by using the command spanning tree path cost method long.

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In this topology.

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I have five switches switch one, two, three, four and five.

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Nothing has been configured on these switches except for the switch names, so they are running a default

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configuration.

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I've connected hubs to the switches so that we can do Wireshark captures in this topology.

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But please note this is not a recommended spanning tree topology.

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It's simply used to demonstrate some of the features and functions of spanning tree.

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So let's determine who the root switches on switch.

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One show spanning tree.

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Is the switch, the root of the spending tree?

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The answer is no.

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We can see that the switch has a mac address of the following, but the route has a mac address of the

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following.

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The local switch also has a path cost to get to the route switch via Port one, which is gigabit zero

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zero.

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In other words, it's cost to get to the route bridge is four based on the triple E 1998 specification.

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Scrolling down, we can see that gigabit zero zero is the root port for the switch.

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So in other words, because the port cost is four, it's most likely that switch two is the root switch.

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Because this port has a cost of fall and the port cost to get to the roots, which is four.

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In other words, there'll be a single link to get to the root bridge.

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One switch two shows spanning tree.

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The switch is the route and we can see it displayed clearly in this output.

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You can also see that the Mac address of the root and the local bridge or local switches the same.

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What you'll also notice is that all ports of forwarding.

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So this is the route switch currently.

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Let's see what BPD use are being advertised by that switch.

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So he has a BPD you.

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The destination of the frame is the well known Mac address for spending tree.

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In the spending tree output.

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We can see that the switch is running rapid spanning tree will change that in a moment, but notice

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the path cost is zero.

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That's the route identifier and the local bridge identifier of the switch and the values are set the

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same.

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Other information, including the max age.

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Hello time and for delay are shown in the output.

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So I'll stop that.

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Why shock capture?

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So Switch two is currently advertising that it's the root of the topology.

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Let's have a look at what switch three shows with regards to path cost.

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Shows spanning tree.

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It also has a path cost of four and it's using port one.

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In other words, gigabit is zero zero as its root port.

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So this port is the root port.

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To get to the root of the spanning tree and this is the length of switches using with a path cost of

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four.

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What about switch for.

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Shows spanning tree.

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We can see here that the path cost is eight, not four.

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And the port that it's using is gigabit is zero zero.

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So in other words, this port is the port that it's using to get to the root bridge.

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It has a path cost of four because this link has a cost of four.

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And this link has a cost of four.

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On Switch three.

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We could use the command show interface spanning tree interface gigabit zero three cost.

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To see that it's advertising a cost of fall on the sport.

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So from its point of view, it's got a cost of four plus four to get to the Route Bridge.

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We can also use the command show spanning tree route to see that the local switch is using gigabit zero

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zero as its root port.

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The cost to get to the root is eight.

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The root ID or bridge ID is the following.

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So four plus four gives us eight.

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What about switch five?

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So switch five shows spanning tree.

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There's the command.

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The output shows us that the cost to the route is 12.

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In other words, four plus four plus four is 12.

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So as you can see, the plot cost is incrementing on a per link basis.

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Once again show Spending Tree Root podcast to get to the root is 12.

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The local root port is gigabit zero zero.

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That port has a port cost of four because the switches are using the 1998 cost at one gigabit per second.

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Port has a path cost of four by default.

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So let's make some changes.

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We'll make switch one the route, see how that affects the BPD use advertised by switch two and we'll

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change the path cost method to long to see how that affects the path costs of the switches.