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In a previous video, we optimize Spanning Tree

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to make switch 1 the  root for VLANs 1 and 10

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and switch 2 the root for VLAN 20.

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On switch 1 as an example sh spanning-tree vlan 1

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shows me that the switch is forwarding on all ports

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and the same is true for VLAN 10 but for VLAN 20

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the switch is blocking on gigabit 0/1.

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The same is true on switch 2 sh spanning-tree vlan 1

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switch is blocking on gigabit 0/1 for VLAN 10

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switch is blocking on gigabit 0/1

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but is forwarding on all ports for VLAN 20.

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So in other words, traffic that sent by say this PC to NPM is gonna be sent

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to switch 2 and then its gonna be forwarded across gigabit 0/0 to get to NPM.

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That’s ok if you only have a few PCs

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but if you had multiple access switches with their host in VLAN 20

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their traffic would be sent to this switch

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and traffic would have to go across this gigabit link

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which will become a bottleneck to get to say multiple servers on the left-hand side.

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So traffic that's traversing from 1 core switch to another

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is being limited to using this gigabit 0/0 link.

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So what we're going to do is we're going to bond or bind these 2 physical interfaces

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into a logical EtherChannel or link aggregation

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so that Spanning Tree sees the 2 physical ports as a single port

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and doesn’t block either of the ports.

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So once again on switch 1 notice that gigabit 0/1 is blocking

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that will change in a moment when we create our link aggregation or EtherChannel.

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So to configure an EtherChannel I’m gonna type

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conf t type interface range gigabitEthernet 0/0 - 1

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so I’m gonna make configuration changes on both of these interfaces at the same time.

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I’m gonna shut the ports down and then I’m gonna type

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switchport trunk encapsulation dot1q switchport mode trunk

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channel channel-group 1

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in other words, I’m going to put both of these interfaces into EtherChannel 1

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you can create multiple EtherChannels on a switch.

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So as an example, I could have 2 interfaces down to this access switch

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and bonded them together in link aggregation 2 or EtherChannel 2

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but in this example, I’m gonna use EtherChannel 1

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I’m going to specify a mode

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and in this example, I’m going to use LACP active mode.

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Now in EtherChannel you have 3 ways of doing this

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if you set it to on it means that you manually create an EtherChannel

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and there’s no negotiation with the other side

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the interface are simply added to an EtherChannel, you could also use LACP or PAgP

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now LACP or Link Aggregation Control Protocol is an industry standard protocol

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that allows switches to negotiate

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the forming of link aggregated ports or EtherChannels as Cisco calls it.

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Port aggregation protocol or PAgP is a Cisco propriety protocol

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That allows you to configure link aggregated ports.

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So you either set it on with no negotiation with the neighboring device.

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So switch 1 as an example, will not negotiate with switch 2

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to form a link aggregation or an EtherChannel or you specify LACP or PAgP

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now when using LACP you have 2 options.

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Active means that it will negotiate with the other side to form a link aggregation.

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So it's actively trying to form a link aggregation with the remote end.

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Passive means that the device is waiting for LACP messages neighboring device

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before it forms a link aggregation.

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So you want to ensure that you don’t set both sides to passive

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because that means that both sides are waiting

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for the other side to form the link aggregation

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and because neither of them are initiating the link aggregation it won’t be formed.

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So you can either set both sides to active or 1 side to active

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and the other side to passive

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but don’t set both sides to passive.

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In our example, we're going to set both sides to active

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now with PAgP you have a similar kind of idea.

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PAgP desirable mode means that the switches going to ask

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the other side to set up the link aggregation.

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Auto means that it will wait for the other side to initiate the link aggregation.

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So in our example

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we’ve set link aggregation or the port channel mode to active.

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So were using LACP or LACP or link aggregation

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to form a link aggregation with the remote side.

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Now from a Spanning Tree point of view

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we're going to set the link type to point-to-point

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to allow Spanning Tree to  negotiate things quicker

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we don’t want to use a shared link

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we want to use point-to-point links to enhanced the Spanning Tree convergence timers.

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In Rapid Spanning Tree if a link you shared

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in other words, the duplex is half it uses timers

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such as blocking, listening, learning and forwarding

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but if it's a point-to-point link

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Spanning Tree doesn’t have to wait for timers to expire

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for ports to start forwarding

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so you need to use point-to-point links

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if you want to make use of rapid convergence and Rapid Spanning Tree.

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So I’m going to no shut the interfaces at this port.

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Let’s do something similar on switch 2

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now you might want to wait till both sides

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are I configured before you no shut the interfaces

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because otherwise, you get messages such as the following.

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LACP is not enabled on the remote end

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so my link aggregation or EtherChannel hasn’t been formed,

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so let’s look at the output of the show EtherChannel summary command

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Notice at the moment we have 2 ports that have been added to EtherChannel 1

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we're using the LACP protocol but notice D means that the ports down.

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So we are setting up a layer 2 link aggregation, not a layer 3

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so we are using switching rather than routing.

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So we'll use trunks across this ports as an example

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but the ports are currently down because the switch cannot negotiate with the other end.

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The link aggregation is not working.

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sh run will show me my configuration

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notice there's my port channel and on my 2 interfaces

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notice these 2 interfaces are part of the link aggregation or port channel

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which is port channel 1

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we're using LACP it’s a trunk port so using a layer 2 interface

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and we're running Spanning Tree across this making them point to point links.

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That’s means that Spanning Tree will converge quicker.

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sh etherchannel port-channel

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shows me some additional information

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such as the number of ports in the link aggregation being 0

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no ports are currently in this port channel.

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So let’s configure switch 2 and see if it makes any difference.

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So here switch 2, conf t int range gigabitEthernet 0/0 - 1

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so these 2 core ports shut the interfaces down.
e

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switchport trunk encapsulation dot1q switchport mode trunk channel-group

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we're going to use the same number in this case so EtherChannel 1

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doesn’t have to be the same on both sides.

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The mode we're going to use is active because we want to use LACP

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and we want this ports to also initiate link aggregation with remote side.

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spanning-tree link-type point-to-point

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sh run

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let’s see what we’ve configured and then I'll no shut the interface.

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So there's our port channel or EtherChannel.

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Here’s our configuration on the first port

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and configuration on the second port.

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Now something that's really important is make sure

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that the configuration on all ports is the same.

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So in other words, the speed and the duplex

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the switch type all your settings need to be the same on both sides

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to guarantee that the link aggregation comes up.

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So I’ve no shut the port or rather no shut the port

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let’s see if the negotiation takes place correctly.

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So we can see the interface came up

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there’s gigabit 0/0 coming up, here’s gigabit 0/1 coming up

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so line status has changed to up

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sh etherchannel summary, our 2 parts are bundled in a port channel.

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So P using the protocol LACP, it’s a layer 2 EtherChannel.

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So S for layer 2 and the ports are in used. So that's good.

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Let’s look at the port channel

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so we see more information now than we saw on switch 1 previously.

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So our port channel is port channel 1

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the number of ports in the channel are 2

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we can see the 2 ports that are active in the channel

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are gigabit 0/0 and gigabit 0/1

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the last port that was bundled is 0/1, so back on switch 1

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previously notice no ports were part of the port channel

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but here the port channel did come up.

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So if we do the command again

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we now see that 2 ports are in the port channel

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or link aggregation or EtherChannel

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and both ports gigabit 0/0 and 0/1 are active.

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So we’ve now bundled these 2 ports in a link aggregation

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how does that affect Spanning Tree?

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so sh spanning-tree vlan 20

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Notice VLAN 20 is forwarding on all ports on switch 1

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Vlan 1 all ports are forwarding, vlan 10 all ports are forwarding.

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Now previously on switch 1 gigabit 0/1 was blocking

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but notice now all ports are forwarding.

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The root port of switch 1 is the port channel or EtherChannel.

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Notice the path cost has gone down from 4 to 3

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because it sees this as a better path.

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Port is forwarding it’s the root port.

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On switch 2 sh spanning-tree vlan 1

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all ports are forwarding

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port channel 1 or EtherChannel 1 is the root port

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and were forwarding, cost has also gone down to 3

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same for VLAN 10 and for VLAN 20.

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All ports are forwarding on this core switches.

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So for redundancy and for higher throughput

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between our core switches, we want to enable link aggregation

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or EtherChannel also called port channels.

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Link aggregation or the bonding of multiple  ports

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will give us the ability to load balance traffic across the link aggregation.

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It also means that Spanning Tree is not blocking 1 of the ports

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so we get much better throughput across the links.

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It also provides redundancy because if one of the ports goes down

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the port channel will still be up.

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So as an example, for VLAN 20 on switch 1 the root port is port 1

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if I go into gigabit 0/0 and shut the port down

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and then type show etherchannel summary

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we can see that one of the ports has gone down

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but one of the port is still in the port channel

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and from a Spanning Tree point of view

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the port channel is still up and still forwarding.

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The difference here, however, is the cost has increased

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because one of the ports has gone down.

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go back into the interface and no shut it.

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Look at Spanning Tree again

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port is coming up, the root port is still port 0/1

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looking at the summary notice both ports are part of the link aggregation now.

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So as we can see the port channel is still used by Spanning Tree

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it may take it a while for everything to converge

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if we'll look at the port channel details.

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Notice both ports are now active in the port channel a minute and 26 seconds ago

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gigabit 0/0 was unbundled from the port channel

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and then 47 seconds ago it was bundled back into the link aggregation or EtherChannel.

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So that’s an example of setting up a layer 2 link aggregation between 2 switches

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which provides for additional redundancy

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additional throughput and stops Spanning Tree blocking ports unnecessarily.