- [Instructor] Let's review the operation of EtherChannel.
Now Etherchannel, that's a topic we discussed back
in your CCNA studies,
and we're building up to the next video
where we're going to be doing
some EtherChannel troubleshooting.
In this video where we just want to make sure everybody
is up to speed on the basics of EtherChannel operation.
With EtherChannel, we can use multiple links
simultaneously between our switches.
You see, normally if I interconnect
a couple of switches with a single link,
that link could become a bottleneck as we're trying
to push traffic between the switches,
but if I want to add a second link by default
with a spanning tree protocol, we would only be sending
traffic over one of those links.
Because a spanning tree protocol will be blocking
traffic over the other link,
So we didn't have a layer two topological loop.
What we can do though with EtherChannel is logically
burgled together multiple links.
Typically it's in powers of two we can bundle together,
two or four or eight links into an EtherChannel.
And that little oval symbol you see over those two links,
that indicates this is now an EtherChannel,
which means we can simultaneously be sending traffic
over both of those links.
So this is going to give us some benefits.
For example, this is going to give us higher
effective bandwidth between our switches
or giving us load balancing.
And even though Spending
Tree Protocol was great because it gave us redundancy,
this does too.
This also gives us redundant links because if we lose one
or more links in the EtherChannel, the remaining links
in the EtherChannel continue operating.
And we can configure EtherChannel in a few different ways.
We can just say the EtherChannel mode is on
and EtherChannel is just working.
Or we could use one of two protocols.
Cisco has a protocol called the PAgP
the Port Aggregation Protocol,
or there's an industry standard called LACP.
The Link Aggregation Control Protocol.
And this protocols can help switches negotiate
between themselves, the formation of an EtherChannel.
Think of it as being somewhat similar to DTP
when we're setting up a (mumbles) trunk.
DTP can negotiate between two switches.
Well, same thing here, either PAgP or LACP.
They can negotiate the formation
of an EtherChannel between our switches.
But a lot of people get a false sense of security about
how this load balancing works.
A lot of people might envision something like this,
We've got three servers connected off of switch B.
And you might assume if pc one is going to be sending
traffic to those three different servers,
that one time the packet would go over
one link in the EtherChannel.
And the next packet might go over the next link
in the EtherChannel and the next packet will go over
That's not the way it works by default.
By default EtherChannel has a load balancing algorithm
and we can select it and we'll talk about
some of the options.
But it's going to have a load balancing algorithm
that's going to dictate which link we use.
Let's think about this in this EtherChannel,
we have four links, how many binary bits would it take
to give us four different combinations?
Two, we can have two binary bits.
And that gives us four combinations
of binary digits 00011011.
And we can take a look at source or destination
information or both, we can look at IP address information,
MAC address information,
but on a lot of Cisco catalyst switches,
the default load balancing algorithm is to look at the
destination MAC address.
And in this case, let's say that the destination
MAC address of the servers ends
I didn't run out the entire MAC address,
but that first server the last hexadecimal digit
in the MAC address,
it ends in a one, and the next MAC address
ends in a five and the next MAC address ends in a D.
Well, those were different.
But if we break those hexadecimal digits down into binary,
and we look at the last two digits are that they're all 01.
That's right, by default
on many of our Cisco catalyst switches
in a situation like this,
even though we've got an EtherChannel
with four separate links between switches A and B,
we're only going to be using that second link.
No matter which of those three servers we're going
to because each of those three servers
has a MAC address that ends in 01, so personal opinion
I don't like to use just source or destination information.
And I gave a MAC address example here It could be
the source or destination IP address.
What I would prefer to do is use a little randomness.
I like to blend together source and destination information.
So I would probably use source destination IP address
or source destination MAC address.
What this will do if I said source destination MAC address
My load balancing algorithm.
In a case like that, when we were choosing the link to use,
then we would look in this case that the last two bits
of the source MAC address, the MAC address of PC one,
and the destination MAC address of one of the servers.
And we would do an exclusive OR Boolean operation.
That means that this PC might be using one link
to get to the servers.
But another PC might be using a different link,
because we're including source address information,
not just destination address information.
So personal opinion as a best practice,
I like to combine source and destination either IP address
or MAC address information in my load balancing algorithm.
Now let's talk about how we can afford an EtherChannel.
If we have a couple of Cisco catalyst switches,
and we're using a pag p, the port aggregation protocol,
the Cisco proprietary protocol,
what we can do is set both sides too on.
And by the way, if we do that we're not really using PAgP
we're just saying being EtherChannel.
And if we set both sides to on that will work we'll
get an EtherChannel, but on does not send any PAgP frames
because again, we're not really running PAgP we just say on
we're just an EtherChannel.
And then just like DTP had an auto and desirable mode,
we've got auto desirable modes here and they work
the same way.
If one side is set to desirable
and the other side is set to auto,
then the desirable side will initiate the formation
of the EtherChannel,
it will send out the PAgP frame and the auto side
will see that and it will respond and say, Okay,
let's be an EtherChannel, and you'll see the
different combinations and permutations here on the screen
of what will work and what will not work.
Again, the on mode is not really running PAgP.
So if the other side is set to auto or desirable,
so I'm going to work because we're not going to be sending
any PAgP frames to the other side.
If both sides are set to auto.
That's not going to work because neither side
is going to initiate the formation of a channel.
But any other combination,
which would be one side set to auto,
and the other side set to desirable
or both sides set to desirable in those situations,
then yes and EtherChannel would be formed.
And it's very similar with LACP.
Instead of saying auto and desirable,
the terminology changes.
Instead of auto, we have passive and instead of desirable,
we have active, but the logic works the same.
And you might wonder if there's an advantage of using one
over the other PAgP versus LACP they are very similar.
I typically would gravitate towards LACP because
it is vendor interoperable.
And it might even work with some appliances that you have.
Maybe, for example,
you want to have an EtherChannel between a Cisco
wireless LAN controller and your Cisco catalyst switch.
Well, in that case, you would typically use LACP for that.
And another advantage of LACP is it lets us have
some additional links on standby.
I mentioned that we could have EtherChannels
with two links, or four links, or eight links.
That's true for both PAgP and LACP.
But LACP will go a step further,
it will allow us to have eight additional links on standby
in case our original eight links fail,
so we can have 16 ports.
If we have a lot of ports available on our Cisco catalyst
switches, we have 16 ports available
for EtherChannel operation,
eight that are currently active
and eight that are in standby.
And that's a review of the basic operation of EtherChannel.
Now in our next video,
we're going to take a look at some common EtherChannel
troubleshooting issues and go through
a troubleshooting scenario together, I'll see you there.