So now it's time to talk about GLBP, Gateway Load Balancing Protocol.
So the difference between GLBP and VRP and HSRP is that GLBP allows you to use both gateways
at the exact same time for a single VLAN, for a single subnet, right?
And so some people have trouble wrapping their head around this concept.
Let's just think about HSRP for a second.
Could my routers both be used for HSRP?
Yes, but for different groups, right?
So for HSRP, I could have this as active for VLAN 10 and this would be standby for VLAN
10.
So they both can't be active for VLAN 10.
But this could be active for VLAN 20 and this could be standby for VLAN 20.
So that is perfectly acceptable.
That is perfectly fine, right?
This one's active for 20, that one's active for 10.
No problem with that.
Okay, what I'm talking about GLBP is that they're both essentially active.
They're both forwarders, as we call it, for VLAN 10, okay?
So the word is not even active now anymore.
The word's going to be where I'm forwarding for VLAN 10.
I'm forwarding for VLAN 10.
So why is this so awesome?
Because now, for example, if this is VLAN 10, this user's traffic could go this way and
this user's traffic can go this way.
But like I mentioned in our overview of first hop redundancy protocols discussion, you're
only going to be able to do that if you have the right layer 2, layer 3 setup.
This link here cannot be layer 2.
It has to be layer 3 between DSW1 and DSW2.
And then you're going to have to do some filtering here to make sure that you have a port here
that's in the forwarding state and a port here that's in the forwarding state, again,
or it's not going to work.
So you're going to have to manipulate Spanishy protocol, convert that to routed ports.
You're going to have to do some trunk filtering in order to get GLP to do what you really
want it to do, and that is have traffic go this way and have traffic go this way.
So Spanishy protocol gets in the way.
You have to adjust Spanishy protocol to make it work.
You won't have to do that on the exam, by the way, adjusting Spanishy protocol, but
you might have to set up GLBP.
All right, so what's going to happen now when you set up GLBP?
You are going to have something called the active virtual gateway, the AVG.
You'll notice here that only one device can be the active virtual gateway, right?
The other device is not the active virtual gateway right now.
So with the active virtual gateway, you're going to have one active active virtual gateway
and one standby active virtual gateway.
So what is the active virtual gateway?
So think of the active virtual gateway as the one responsible for making all the decisions
about what all the other routers should be doing that are part of the group.
So it is the overseer of the actual group.
It is the one that's also going to be responsible for responding and answering those ARP requests.
It is responsible for handing out MAC addresses.
Now in this case, there's going to be more than one MAC address.
There's going to be one IP address for the group, but there's going to be one MAC address
for each physical router.
So for example, this physical router gets this MAC address.
This physical router gets that MAC address.
Why?
Because when we build a frame, we need a destination MAC address in it.
So when PC1 sends out an ARP request, the AVG, let's say, hands out 0101, this one right
here.
When PC2 sends out an ARP request, the AVG responds back with 0102, 0102.
So you see what the AVG is doing here.
It's doing a load balancing of those MAC addresses.
So what that means now is PC1, when it builds its frame, is going to be destined to this
MAC address, and this switch is going to be pointing this way for that MAC address.
But this PC here gets this destination MAC address for the default gateway, and this
switch is going to be pointing this way to get to that MAC address.
And that's why it's important that Spanning Sheet Protocol is not blocking any of those
ports.
So that way, we can have this occur.
So now, because of that, all frames built from PC2 go this way.
All frames built from PC1 goes this way.
So let's say you had 250 users in VLAN10.
That means half of them could have traffic going this way.
The other half could have traffic going this way, because the AVG is going to hand out
the MAC addresses round-robin, half of them.
So I go this and this, this and this, this and this.
I'll just keep flip-flopping through them.
So if you ask for the MAC address, I'll give it to you.
The next one, I'll give the next one.
And then back to the first, second, first, second, first, second, first, and so on and
so forth.
That's all we do.
Now, there's other ways we can manipulate that, but that is by default essentially what's
going to happen.
So what does that mean?
It means that every other router is an active virtual forwarder.
An active virtual forwarder, you can have a MACs of four only, four routers, four physical
routers forwarding, but each of those active virtual forwarders get assigned one of those
MAC addresses from the actual router.
So that way, whenever they receive frames destined to that MAC address, they're going
to be able to forward them and process them.
So the configuration looks something like this.
Now, I can't do a demo because my switches, my multilayer switches don't actually support
GLBP in this case.
But there are multilayer switches out there that support GLBP and routers support GLBP.
What am I doing here?
So let's go to interface VLAN 30 in this example on both devices.
Notice we typed in GLBP, gave it a group number, and same IP address.
So this is the IP address that's going to be given to the clients.
That is a virtual IP address, so one virtual IP.
So that's what they're going to be getting.
Your default gateway is 10.0.3.254.
So now, when they send out, and these MAC addresses are going to be created based on
the fact that there's two devices that we've configured this on.
If we also had DSW3 as an example, let's say there was another multilayer switch here that
was part of this and we had three default gateways, well then, when we configured on
that one, well then, the AUVG would know that and the AUVG would create another MAC address
for it.
So basically, the MAC addresses are going to be 0.0.0.7.B4.0.0.0.1.0.1.1.0.2.1.0.3.1.0.4.
That's it.
That's how it works for GLBP.
That's the four different MAC addresses.
So now, we are going to have – notice the priority here?
So the priority is automatically 100, but I included in the config down here.
The priority is used for what?
The priority is used for the AUVG election process.
That's what the priority is used for.
It's not for the forwarders.
It's for the AUVG election process.
Likewise, the timers are also for what?
AUVG.
So basically, I am the standby AUVG and you are the active AUVG, and so hello, PAC.
It's back and forth between us.
So that way there, I know you're the active AUVG and I'm the standby AUVG.
So essentially, right, the commands we see here and preemption is disabled by default,
so let's turn on preemption.
So now, those first four commands are essentially the initial setup for the whole AUVG process,
and so once the active AUVG is determined, then everybody else are just active virtual
forwarders based on the number of distribution gateways you have, then that's the number
of MAC addresses the AUVG is going to create, and then as users start asking, hey, can I
have the MAC address for 10.0.3.254, well, then the AUVG hands out the first one to
this one, the second one to this one, the third one to this one, or sorry, the first
one to the third, and the second to the fourth, the first to the fifth, the second to the
sixth, and so on and so forth.
And then you'll see at the bottom here that we have the opportunity to control.
Round Robin is typically the default, which is what I just described to you, but if we
want to turn around and we want to force, for example, that the same MAC address is
always given to a certain host, then we can use host-dependent instead, meaning that once
I give PC1 that MAC address, they're always going to use that MAC address going forward.
Once I give PC2, so if they ask for it again, then I'll give it to them again.
And that way, they're the same MAC is always used, and then those users always use the
same default gateway over and over and over and over again.
And then we have weighted as well.
What if this default gateway, we had greater bandwidth than this default gateway?
Then I could weight that so that 75% of the traffic goes through here, and maybe 25% of
the traffic goes through here.
We could do that.
And how would you do that?
Well, if you set the weighting now, the weighted option instead of doing Round Robin or host-dependent,
then that means I'll hand out one MAC address, so the MAC address that is associated with
this active virtual forward, I'll send it out more often than the one down here.
So let's say I wanted 75% of the traffic to go through here, 25% to go through here, well
then for every 75 I hand out of this, I'm only going to hand this one out 25 times.
Approximately.
Now also something to keep in mind is what happens if we have a failure?
So if we have a failure here of the actual device, well then this device is going to
know about it because hello packets cease to exist.
So what happens?
This one remains the AVG, but now it's also the AVF for that other MAC address.
So it would take over that MAC address.
So now it's two AVFs and one AVG.
And then when this comes back online, then we've got, again, normalcy here.
So what happens if this device fails, then I become the AVG and I become the active forwarder
for all of this.
And I'm also the active forwarder for myself.
So whenever a failure happens to the device, depending on who fails, if the AVG doesn't
fail, no big deal.
It stays the AVG.
But anytime an AVF fails, then that AVF, everything it was doing has to be taken on by some other
device in that environment, in that group, I should say.
And again, just like with HSRP or VRP, you would have to create a different group for
each and every single subnet size, VLAN size, broadcast domain you have.
Does the AVG always have to be an AVF too?
Yeah, it automatically makes itself an AVF, automatically.
And you would want that, because you don't want to waste it, right?
You don't want to waste it, because your whole point is to use all of the routers in the
group at the same time for that same subnet.
So yeah, the AVG will be an AVF too.
And that's good.
So folks, we are done.
Our coverage of first operandency protocols.
What can I tell you to kind of let you know how important it is to ensure that you are
very familiar with all three of these?
Not just from a theoretical standpoint, but from a configuration standpoint as well.
I have taken many Cisco exams over the years.
At the CCNP level, every time I have taken a Cisco exam that had in the exam objectives,
something like HSRP, VRRP, or GLBP, you've always had to know more than the theory of
those options, meaning can you configure it?
Can you verify?
Can you troubleshoot?
So it is exceptionally important that you don't under-prepare for these first operandency
protocols for the exam.
Let's just put it this way.
The probability of you maybe having to hype stuff in regards to first operandency protocols
is extremely high in your future.
As a result of that, make sure you are ready for that.