So let's optimize the links between the two core switches
using ether channel. Once again show spanning tree VLAN 10 shows us that on core 1 all ports are
forwarding but if we have a look at VLAN 20 notice port 24 is blocking something similar is
shown on core 2, show spanning tree VLAN 1 port 24 is blocking for VLAN 10 port 24
is blocking. The big issue here is if traffic for example from VLAN 10 is sent to the Internet that traffic
is gonna traverse this link gigabit 1 0 23 because port 24 is blocking. If traffic is
sent from VLAN 20 to this server that traffic is gonna go to core 2 and then go across gigabit
1 0 23 to the servers.
In other words, you may have many devices at the access layer sending traffic to servers on the left
hand side or to devices on the right hand side such as the Internet or other servers any traffic that
goes via core 1 to core 2 can only use port 23 and any traffic that goes to core 2 and then
to core 1 to get to the servers can only use port 23, port 24 even though it's plugged in
in this topology is not used at all for sending traffic from core 1 to core 2. So for VLAN 20 core
1 is blocking port 24, for VLAN 1 core 2 is blocking port 24.
Same is true for VLAN 10, for VLAN 100 core 2 is forwarding but for VLAN 100 core 1 is blocking. So port
24 is not used even though it's plugged in. So to resolve that we want to bond these two links together
in an ether channel. Now in this video, I'm gonna show you some of the issues that you could encounter
with ether channels or link aggregation or port channels, you'll see different names used for this technology.
So as an example one of the things you need to remember is that when you add physical interfaces to
a port-channel you need to do the configuration on the port-channel rather than on the physical interfaces.
The configuration that you do on the logical port channel should filter down to the physical interfaces.
In other words, the config that you apply on the logical port-channel should be copied and applied on
the physical interfaces by the switch.
That doesn't always work or doesn't always happen.
So if you have problems with port-channel you need to verify the configuration on the port-channel
and the physical interfaces and then you need to make sure that everything is the same on both sides
on both logical port-channel and the physical interfaces then shut down
the physical interfaces and then no shut them. So I'm gonna show you a bit of troubleshooting in this
video as I configure port-channels and I'm gonna show you some of the issues that you could encounter
if you do configuration on the physical interfaces but not on the logical port-channel.
So if you configure the physical interfaces are not the logical port-channel,
you can have issues where the port channel is for example an access port rather than a trunk port even
though your physical interfaces are configured as trunk ports.
The moral of the story is make sure that the configuration of the port-channel and the physical
interfaces is the same.
Once you've shut down the physical interfaces if needed confirm that the configuration is the same and
then no shut
the physical interfaces. So to set up ether channel we're going to go on to the interfaces and in this case
I'll use a range command and I'll specify both interfaces I'll shut both down.
It's a good idea to shut your interfaces down when making changes like this.
In our example, we gonna use what's called a layer 2 ether channel, so we going to run 802.1q
tagging across the links.
It's gonna act as a traditional layer 2 interface rather than a routed or layer 3 interface.
So switch port mode trunk means that it'll act like a trunk
and now we can use the command channel,
group and select a number.
I'm going to select channel-group one, switches can support multiple ether channels or multiple ether
channel groups.
The reason why is that you could use group one to bond these two links together and if you had multiple
ports from your core switch to your access switch let's say from core one to access one you could bond
both of those links together in ether channel 2. So multiple ether channels can be configured.
In this example I'm simply gonna use ether channel 1, I'm gonna specify the mode
and notice here we have five different modes.
Now before we talk about modes you need to understand that we have two link aggregation protocols, LACP
or link aggregation control protocol and PAGP or port aggregation protocol. LACP is an industry standard
protocol, PAGP is a Cisco proprietary protocol. So you can either use LACP or PAGP or you can disable
the use of a protocol and just enable link aggregation on a port.
So if you set the mode to on the switch is not going to negotiate with the switch on the other side
to set up an ether channel.
It's simply gonna turn it on. If you want to use LACP,
you can either use active or passive LACP. Now active means that the switch will use LACP messages
to negotiate with the other side and actively try and form an ether channel with the switch on the other
side of the link.
So if I said active on switch 1 it's gonna try and get switched 2 to form an ether channel on
these ports.
Passive means that the switch doesn't try and actively form an ether channel.
It waits for ether channel messages and if it receives them then it will form an ether channel with
the device on the other side. So active means it's actively trying to form an ether channel.
Passive means it's just passively waiting until the other side initiates the forming of an ether channel.
So you need to specify which protocol to use LACP or PAGP and then with LACP you have the option to
specify either active or passive PAGP does something similar.
We have either desirable or auto, desirable is like active in LACP the switch is gonna actively or
desirably form an ether channel, auto in PAGP is like passive in LACP it's only gona form an ether
channel if the other side initiates the forming of an ether channel.
So in this example, I'm gonna specify active to use LACP and to actively create a link aggregation.
Notice the switch tells us that a port-channel interface has been created, a port-channel interface
is an ether channel interface.
So if we type do show run what we see now is that this port-channel has been created and if we scroll
down we can see that this interface gigabit 1 0 23 is part of the port-channel or ether channel
and so is gigabit
1 0 24. Now I won't no shut the interface yet
I'll go to switch 2 and do something similar.
On switch 2 these interfaces have gone down because we shut them down on switch 1. I'm gonna do something similar
here.
Interface range gigabit 1 0 23 to 24,
I'll shut them down anyway.
Set the switch port mode to trunk, channel Group 1 notice we have options 1 to 48 mode active.
We need to use the same protocols.
So either LACP or PAGP on both sides.
I could have set this side too passive so that switch 1 initiates the ether channel but in this example
I'm gonna get them both to actively try and form a link aggregation. I'll no shut the interfaces on
this side and I'll no shut them on core 1 both physical interfaces have come up and so has the port
channel. So show ether channel summary in the output here.
From the show ether channel summary command we can see that gigabit 1 0 23 and 1 0 24 are part of the
port
channel, so both ports have been put into the port channel.
We can see that port
channel 1 or ether channel 1 is used, S means that it's a layer 2 channel not a layer 3 ether
channel.
In other words, it's using switching rather than routing U means that the ether channel is in use.
So that's great
both ports are part of the ether channel and the ether channel is in use.
Show ether channel summary on this side we see something very similar.
Show ether channel, port channel.
This gives us a lot of information so this command shows us that we have a group.
Group 1, port
channel number is 1.
The channel has been up for this length of time.
There are two ports in the ether channel.
We are using LACP as the protocol on the channel and these two ports gigabit 1 0 23 and 24 are active
in the channel.
This is the time since the last port was bundled so gigabit 1 0 24 was added to the ether channel about
1 minute and 38 seconds ago. On this side
we should see something very similar. So show ether channel port channel.
Notice we can see that both ports are active in the channel.
That's good and that's what we want to see.