WEBVTT

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 Hello and welcome to this video, which
 is titled, Utilizing Network Devices

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 for DHCP. Now, in previous videos, we
 did a review of exactly what DHCP

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 does between a client and a DHCP server,
 how it operates, we did that

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 review of Dora, which is the discover,
 the offer, the request, and the

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 acknowledgement.

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 We talked about the UDP port numbers
 of 67 and 68 use for DHCP.

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 Now what we're going to look at is, what
 if, usually in a lab environment,

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 but sometimes in a production environment,
 you actually want to configure

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 your Cisco iOS device, like your router
 or your switch as a DHCP server.

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 Instead of using a legitimate
 Linux or Windows DHCP server.

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 You can do that, I'm going to show
 you the basics of how you would do

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 that. So here are the steps for configuring
 your router or switch as a

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 DHCP server. So step number one is you
 enable that functionality by typing

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 service DHCP. Now I've seen in multiple
 lab environments that I don't

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 have to do this.

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 I think service DHCP is already on
 by default, but doesn't hurt, only

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 takes a couple of seconds to type that
 in to ensure that it is operational.

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 Then after that, you're going
 to create your DHCP pool.

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 Now you're going to create one or more
 DHCP pools, depending on how many

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 subnets are going to
 have DHCP clients.

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 Clearly, if in my network, I've got 25
 different subnets, one for payroll,

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 one for marketing, one for HR, one
 for engineering, and on and on and

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 on. I'm going to need a unique DHCP
 pool for each and every one of those

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 subnets. So, but here we're just going
 to do one to make it simple.

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 And then you give your pool a descriptive
 name, it's completely up to

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 you. You might want to name it payroll,
 marketing, engineering, just to

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 keep it separate.

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 Then once you do that, that puts you
 into config-dacp mode, and now you

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 configure the parameters that you're
 going to offer up to your clients

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 in your DHCP offers.

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 You can configure your network.

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 For example, you can say network 10
.10.0, and then you do need to supply

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 a subnet mask. This is one of those
 few places where you can actually

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 supply the subnet mask in a slash notation
 in addition to data decimals.

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 So certainly you could type in 255.255
.255.0, you could do that, or you

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 can just simply type in slash 24.

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 You could actually do it that
 way to make it easier.

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 Then you're going to want to supply
 a default router, like 10.10.10.1,

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 or whatever it is that's appropriate
 for this network.

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 Probably want to give them a DNS server,
 maybe Google's DNS server.

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 And then you want to
 specify the lease.

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 And with the lease, you have three
 numbers you could supply.

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 If you supply all three numbers, that
 first number is days, second number

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 is hours, third number is minutes.


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 And so those are now, if you do a question
 mark, there are other options

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 you can specify as well.

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 But those are probably the minimum parameters
 you want to put into your

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 DHCP pool. Now, lastly, in this particular
 network, for example, let's

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 say I'm doing the network of the 10
.10 network that I'm allocating.

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 Maybe on that particular network segment,
 we actually have a couple of

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 servers already there that
 have static IP addresses.

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 Maybe one server, maybe server A is
 10.10.10.1, and the other server B

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 is 10.10.10.2. And we want to make sure
 we tell this router or the switch

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 don't allocate those addresses.

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 Those addresses are
 already in use.

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 Exclude those from the pool.

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 So to do that, we don't actually do
 that exclusion in the pool itself.

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 We do that exclusion at the global
 configuration level with an IP DHCP

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 excluded dash address command.

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 So in this particular case, we would
 say 10.10.10.1, 10.10.10.2, and that

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 would exclude those addresses
 from being allocated via DHCP.

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 Now, if your DHCP clients are directly
 connected to this DHCP server,

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 this router, or switch acting
 as a DHCP server, you're done.

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 Because this guy will receive those broadcasts,
 those DHCP discover messages,

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 those DHCP request messages
 which come in as broadcast.

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 But what if that's not the case?

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 Or if we have a situation like this
 where we have, let's say these are

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 the clients, these are the laptops
 and PCs, they're probably connected

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 to a switch. But what of here we have
 router one, router two, router three,

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 and router three is going
 to be our DHCP server.

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 Well, we could configure all that stuff
 we just saw on router three, but

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 it's not going to do us any good if
 that's all we do, because we know

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 that those DHCP messages are going
 to be coming in as broadcasts.

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 And once they hit this interface right
 here on router one, router one's

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 default behavior is going
 to be to drop those.

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 So we have to configure on router one
 specifically on this interface right

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 here. We have to configure that as a
 DHCP, what's called a relay agent.

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 We have to give him relay agent functionality
 so that when he gets those

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 broadcasts, he will encapsulate them
 in a unicast and send it directly

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 to our router. And then the next slide,
 I'm going to show you how to do

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 that. One other thing, in the lab that
 we have devised for this, if you

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 choose to follow the lab, the actual
 DHCP clients are routers or switches.

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 We're not going to be using
 an actual client.

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 So how do I go onto a router's interface
 and configure that router's interface

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 to obtain its address, not statically,
 which is what we would normally

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 do, but to obtain its
 address via DHCP?

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 Well, this is how
 we would do that.

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 We go onto the interface and we
 simply say IP address DHCP.

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 So that turns that layer three interface
 into a DHCP client interface.

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 And then lastly, how do we go into that
 router one and configure him to

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 be a DHCP relay agent?

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 Just like this. Go onto the interface
 that's facing the DHCP clients and

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 use the IP helper dash
 address command.

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 In this particular case, this address
 right here is your DHCP server.

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 Wherever that happens to be.

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 So now when DHCP broadcasts, like DHCP
 offers, DHCP requests, when they

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 come in and are seen on interface gigabit
 zero one, instead of just ignoring

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 them and dropping them, he will encapsulate
 them in unicast and unicast

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 those packets directly
 to our DHCP server.

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 Now, how do we confirm that
 all this is working?

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 Well, there's a couple
 of ways we can do it.

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 So we can do show IP DHCP binding.


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 So on the DHCP server, you can do that
 first command and actually see

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 if he has allocated any addresses
 to any DHCP clients.

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 And on the client side, so on the router
 that we're using as a DHCP client,

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 we can do a debug command
 debug DHCP detail.

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 And that will actually show you the
 DHCP transactions in a debug output.

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 So that concludes that video on how
 to configure routers and switches

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 as DHCP servers, as well
 as DHCP clients.