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There are some protocols that are used so often they become synonymous with IP

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networking.

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And one of those is an application layer service called DHCP, the dynamic host

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configuration protocol.

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And primarily after all said and done, the job of dynamic host configuration

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protocol,

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or as its friends call it DHCP, is to hand out IP address information to

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clients that ask for that.

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So let's draw a typical network right here.

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So this could be for example VLAN on a local network and on top of that VLAN,
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So this could be for example VLAN on a local network and on top of that VLAN,

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let's see we're using the IP dressing space of 10.30.0.0 with a 24-bit mask.

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So that's our IP dressing space for this network.

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Now some devices on this network are very likely going to have a statically

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configured IP address,

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like this router right here.

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Let's say it's IP address, we've configured it as .1 on this 10.30.0 network.

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And we may have other devices, for example, like a server that we statically

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configure.

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And perhaps we give this server the IP address statically configured of .2 on

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the 10.30.0 network.
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the 10.30.0 network.

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And for other devices that are on that network, for example, just have all

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these orange colored devices represent clients,

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which could be Windows, they could be Linux, it could be Mac OS,

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or if it's a wireless network, it could be mobile devices, for example, like

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Android's and iPhones and things like that.

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And for client devices that come on the network and go off the network and so

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forth,

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instead of having to manually configure details such as, okay, the correct IP

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address space for these clients

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and their default gateway and which DNS server to use, etc, etc, instead, we
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and their default gateway and which DNS server to use, etc, etc, instead, we

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can use the wonderful protocol called dynamic host configuration protocol.

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And then train these clients that when they boot up or when they connect to

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this network to dynamically ask for an IP address

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and the process would go something like this.

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The clients could issue a discover message.

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And the discover message in DHCP is sent as a layer two broadcast.

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So effectively, our rails have that same layer two broadcast domain is going to

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have to listen to that broadcast and then see what's inside.

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Now, hopefully one of those devices that sees that broadcast, this discover
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Now, hopefully one of those devices that sees that broadcast, this discover

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message being sent by a client,

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hopefully one of those devices is a DHCP server.

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In fact, let me redraw this just a little bit and let's go ahead and imagine

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these are clients and this device here is acting as our DHCP server on the

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local segment.

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So from a communication path perspective with DHCP, I'm going to have the

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server right here on the right.

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So it's right here.

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Well, they're on our network again here for good measure.

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We'll connect our server to that.

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And again, that server is at dot two.

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And then over here, let's go ahead and just use one of our clients as an

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example who's also connected to that network.
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example who's also connected to that network.

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And on the client computer, we're going to have some option based on the type

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of operating system we're working with, but some flavor of the question.

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Do you want to get an IP address automatically?

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Well, the answer is yes, that's implying in the background that it's going to

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be using DHCP services.

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And here's the play by play.

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The client is going to start the process by setting in a DHCP discover message

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into the network.

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Now, it's hoping that someone on this layer two network is a DHCP server, can

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respond on behalf of that discover message.

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And if there is a local DHCP server, that DHCP server can go ahead and respond.
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And if there is a local DHCP server, that DHCP server can go ahead and respond.

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And when the receiver responds, this can respond with an offer.

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So put an O there, I'll put an offer there just to write out what it is.

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So this D over here in parentheses, I'll just make a little note that that's a

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DHCP discover message.

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So in the offer coming from the DHCP server, it's going to have some

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information for that client.

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For example, let's say, hey, here's an IP address you might like to use on the

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1030 zero network and here's a default gateway you can use while you're on this

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network.

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Oh, by the way, here's a DNS server you can use and reach out to if you have

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need to do name resolution from names to IP addresses.
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need to do name resolution from names to IP addresses.

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And the list goes on.

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There's other options as well that we can hand out all as part of this offer

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coming from the server.

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Then when the client gets that offer, if they want to use it, they'll go ahead

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and they'll send a request message.

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I'll put that here in parentheses as well.

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And the client will send that request in the network saying, you know what that

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offer is made?

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I would love it.

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At which point the DHCP server is going to finalize the conversation with an

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acknowledgement.

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So put an A there and just write out a CK short for acknowledgement.

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And that's the back and forth for traditional DHCP services on a local network.
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And that's often referred to as the DORA process.

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We're representing the first letter of each of those exchanges for a client

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getting an IP address from a DHCP server.

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So the discover, the offer, the request and acknowledgement.

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And those will be packets one, two, three and four.

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So let's modify the topology a little bit and let's imagine that we still have

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these clients here, but we do not have a local DHCP server connected to this

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local 1030 zero network.

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So I'm going to remove this server here because it's no longer on the local

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segment.

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And let's put another network over here. And let's imagine just for grants,
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And let's put another network over here. And let's imagine just for grants,

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this is the 23.1.2 network.

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And let's imagine we do have a DHCP server connected right there.

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And let's imagine it's at dot one. So maybe our router down here is at dot two

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on the 23.1.2 network.

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And our DHCP server is right here.

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So the problem with not having a local DHCP server is that this client right

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here right there, when it does a discover message when it's looking and trying

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to discover a DHCP server, it senses a broadcast.

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And broadcast only goes as far as that layer two broadcast domain.
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And broadcast only goes as far as that layer two broadcast domain.

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So if this is the 1030 zero subnet and it's on top of a VLAN, that broadcast is

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not going to make it past this layer two segment right here.

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And as a result, this DHCP server up here is not going to see that DHCP

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discover message.

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So it can't respond. In addition, if we had 20 or 30 or 50 different subnets in

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our networks, it'd be pretty tedious to make sure we had a logical or physical

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interface from a DHCP server in every single one of those subnets.

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So one of the features that we can use to get around the problem of having to
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So one of the features that we can use to get around the problem of having to

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have a local DHCP server with its interface on every single one of our networks

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is we can use a feature called DHCP relay.

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And most of the time this feature of DHCP relay is going to be enabled or

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configured on a router interface that's directly connected to this network

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segment.

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And so think of the DHCP relay like the proxy, the person or entity that goes

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between two things.

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So for example, if this is the DHCP client over here, and is the DHCP server,

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the DHCP relay is like the middleman who can forward those messages back and
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the DHCP relay is like the middleman who can forward those messages back and

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forth between this local segment and the server.

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So part of enabling DHCP relay on this router is to also tell this router and

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this interface where DHCP relay is enabled.

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We need to tell where the real DHCP server is.

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So we'd spend a moment configure this router and tell it that on this interface

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, we want to enable DHCP relay and that the real DHCP server is at 23.1.2.1.

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Now once we tell the router that the play by play for DHCP relay goes something

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like this.

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This client issues a discover message, which is a layer two broadcast and the
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This client issues a discover message, which is a layer two broadcast and the

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relay function here on this interface.

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If it's enabled, it's going to take that broadcast, wrap it up, encapsulate it

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and ship it over as a unit cast over to the DHCP server.

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At which point the DHCP server de-encapsulates that packet and sees that it's a

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DHCP discover message.

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Now one of the mysteries of the DHCP relay is that what if this server up here

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has multiple scopes or pools of addresses that it can use to hand out IP

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addresses.

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For example, let's imagine that this DHCP server has a pool of addresses

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appropriate for this subnet right here, the 10100 network.
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appropriate for this subnet right here, the 10100 network.

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So it may have a pool of addresses that will say it's 10.10.0.151 through and

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let's go ahead and go through 250.

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And let's imagine it has yet another pool of IP addresses configured and that

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would be over here for the 1020 subnet and that's 10.20.0.0.

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Let's go ahead and use the range 151 through 250.

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And that's also because we're talking about the subnet here, the 1030 subnet.

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Let's imagine the same DHCP server has another pool of addresses in the range
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Let's imagine the same DHCP server has another pool of addresses in the range

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of 10.30.0.151 all the way through 250.

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Now this router on this interface right here, it has the IP address of 10.30.0.

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1 based on our plan here.

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So when that DHCP discover message is seen and DHCP relay is enabled right here

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, as part of the encapsulation and shipping that up to the unicast address, the

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single IP address of the DHCP server, it's going to include information

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regarding which subnet this discover came from,

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including sourcing the packet from that IP address on this interface right here
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including sourcing the packet from that IP address on this interface right here

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.

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And that way when the DHCP server receives it says, "Oh, I see this is coming

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from the source address in the 1030 subnet."

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And based on that and also some of our details inside of the DHCP request, the

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DHCP server can say, "Oh, you know what?

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I need to go ahead and hand that IP address from this pool of addresses of the

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1030 range because that's appropriate for this network down here."

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At which point the DHCP server would make that offer, it would unicast it down

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to that IP address and then the relay agent would forward that back out to the

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client.
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So all the client knows is that it made a discover message and it got an offer.

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So that's the offer coming back.

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And if the client wants to accept that offer, it would go ahead and do the R

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for the request and that would go into the network.

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The relay would take it and capsulate that request, forward it to the DHCP

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server, who then followed up with a final acknowledgement.

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So there's an A there for acknowledgement that would be unicast back to the IP

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address of the DHCP relay and then the relay agent would continue forwarding

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that back to the client.

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So with the relay agent, we're still going through the discover offer request

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acknowledgement process, but we have the relay agent in the middle of that
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acknowledgement process, but we have the relay agent in the middle of that

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conversation,

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acting as the middleman between the DHCP server, which can be centralized

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somewhere, reachable on our networks, and the local network segment where the

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DHCP clients are.

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Now DHCP is not the perfect solution for every type of device. So if we have

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things like routers and servers, we probably want to have a specific IP address

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on those devices.

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And very likely we're going to configure that IP address, whatever we want them

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to use directly on those respective interfaces.

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So for things like servers and routers and other devices that you want to know
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So for things like servers and routers and other devices that you want to know

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exactly where they're going to be, for example, maybe a printer, we can go

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ahead and do static or manual configuration for those IP addresses.

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And then for most of our clients, we're going to go ahead and use DHCP services

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.

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And if we happen to have local DHCP servers connected to those subnets,

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fantastic.

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If not, we can enable DHCP relay services on the router interfaces that are

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connected to those subnets where we are going to have DHCP clients.

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Now there is one other option that's kind of cool.

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And that is what if we want to use DHCP services, but we want to specifically
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And that is what if we want to use DHCP services, but we want to specifically

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hand out a specific IP address to a certain device.

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Well, every device on our network has a layer two MAC address, and so we can

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create what's known as a reservation.

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And in the world of NSX, that's referred to as a static binding.

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Let's imagine we have this device right here, this client with a MAC address

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thing in 1234, we can create a reservation that says, you know what, for that

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MAC address, ending in 1234.

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I want to assign the IP address on this network, 10.30.0.

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Let's go ahead and use 95.
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Let's go ahead and use 95.

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So if we specified that we want to reserve this IP address for that specific

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layer two address, when this device makes a discover message, part of that

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discover is going to include its layer two address.

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And so if the DHCP server is local, it can see that directly, and if the DHCP

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server is remote, the relay agent will encapsulate all that information,

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including the client's layer two address, wrap it all up and unicast it as a

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discover up to the DHCP server.

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At which point the DHCP server, if we have the static binding, this reservation

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in place, they'll say, oh, you know what, I've got the perfect IP address for
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in place, they'll say, oh, you know what, I've got the perfect IP address for

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you.

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I'm going to hand you this specific IP address right here, based on your layer

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two MAC address that's included as part of the discover message.

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So using the technique of static bindings, we could specifically pre plan for a

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certain IP address being associated with a specific client.

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But most of the time for most of these clients, it's just a look at the draw,

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we have a pool of addresses, the DHCP server makes an offer from one of those

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addresses in the pool, the client says, I'll take it, and the server follows up

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with the acknowledgement saying, yep, I know you got it, have a good day.
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with the acknowledgement saying, yep, I know you got it, have a good day.

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So let me clean this up a little bit.

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Now that we've taken a look at some basic concepts regarding DHCP and DHCP

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relay, and let's talk about a few options that we have in the world of NSX

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related to DHCP services.

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And let's start off with a network segment right here.

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And let's imagine we want to use on this network segment 10.30.0.0 with a 24

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bit mask.

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Let's also imagine that we want to use this network address space.

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We want to use it for some testing of some servers.

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So there's server one and there's server two.
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So there's server one and there's server two.

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Maybe this is an application server, maybe this is a database server, and the

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list can go on.

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Let's also imagine we have some clients, so I'll put a few clients here as well

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.

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And so on our servers is very likely we want to statically configure those, and

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then for the clients very likely we're going to use DHCP services.

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So if we have just an overlay segment like this one right here and we're using

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the 10.30.0 address space and we're not connecting it to any of our gateways in

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NSX.

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One of our options is to enable DHCP services for a segment, which is not

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connected to a router.
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connected to a router.

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And that's referred to as a segment DHCP server.

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So what we would do is simply say, you know what, I want to enable a segment

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DHCP server to support this subnet.

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So as part of setting up a segment DHCP server with no logical routers attached

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to it, we would create a DHCP profile.

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In that profile, we'd specify an IP address of a DHCP server that we want to

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use.

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And again, it's all virtualized, so we can specify something like six dot seven

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dot eight dot three with a 29 bit mask.
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So I want to use a kind of a crazy number there because you don't have to use a

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real reachable IP address for a real DHCP server because that DHCP server is

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going to be virtualized.

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So we set up a profile.

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We specify an IP address we want to use that represents the server.

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So with that profile on place, we then go to the segment and we associate this

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segment.

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Let's call this segment 30.

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So I'll have label here.

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So if this is segment 30 as part of the configuration of segment 30, there's an

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option to say set up DHCP services.

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And so we would say, you know what, we want to use DHCP services.
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We'll use this profile, which represents this logical DHCP server at six seven

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eight three.

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And then as part of the configuration for the segment, we'd also specify the

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details.

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So we say, I want to use the range of addresses 10 dot 30 dot zero dot, let's

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use 151 all the way through 200.

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And if we need more addresses, we simply define a larger pool.

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And as part of the configuration, we could also specify default gateway.

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But if we don't have a default gateway, we don't want to default gateway.

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We just want DHCP services for this one logical segment.

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And in addition to the range of addresses, we want to hand out for DHCP
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And in addition to the range of addresses, we want to hand out for DHCP

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services.

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We'll also hand out a logical IP address on that subnet that the clients will

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believe is the DHCP server.

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So we could say, you know what, the DHCP server address is going to be 10 dot

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30 dot zero dot two, for example,

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because from a client's perspective, when they go about doing things like renew

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ing their IP address and so forth,

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they're going to want to communicate directly with their DHCP server.

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So we're also going to give an IP address that they can play with to support

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that function.

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Now, if we don't want NSX to provide the DHCP services directly, we want to use
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Now, if we don't want NSX to provide the DHCP services directly, we want to use

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, for example,

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maybe our own DHCP server that we already have in place.

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Let's go ahead and put this guy at, for example, dot two on the 23 dot one dot

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two network.

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And let's imagine that DHCP server has a pool of addresses for the 10 10

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network.

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I'll go ahead and label that 10 10 and we have a pool of addresses for 10 dot

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20.

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We could then go to these segments and we can enable effectively the DHCP relay

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services right here.

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And that would support any DHCP clients here on segment 10.
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And that would support any DHCP clients here on segment 10.

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We could also enable DHCP relay for segment 20 to support all the clients over

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here on the 10 20 network.

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So each of these gateways would logically on these interfaces connected to the

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10 10 and 10 20 networks respectively,

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would see those discover messages encapsulate them and then route them to the

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IP address of our DHCP server.

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And they would source them from the address base here.

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If this is dot one and this is dot two, the source IP address for those encaps

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ulated and now shipped over to the DHCP server unit cast packets.
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The source address is going to be the interface address connected to the local

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network where the clients are.

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And then with this DHCP server receives those DHCP discover messages, it can go

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ahead and make offers from the appropriate pools based on where that relay

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agent sent it from.

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So over here, hopefully we have an IP address in a pool from 10 10 that it can

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serve down to clients over here.

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And over here, hopefully we have an IP address or two available in our pool in

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the 10 20 subnet, which the server could make an offer for over here.

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And these gateways are then acting as the middleman, the DHCP relay agents
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And these gateways are then acting as the middleman, the DHCP relay agents

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between the real DHCP server up here and our clients on these respective sub

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nets.

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So that is yet another option we have to set up a DHCP relay.

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And our third option we have is setting up gateway DHCP services.

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And the reason we didn't use gateway DHCP services down here is because this

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segment segment 30, we didn't have a gateway.

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We didn't have a logical layer three router connected from an NSX perspective

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to this network segment.

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So as a result, if you don't have a gateway connected, our only option here for

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DHCP services would be either through NSX providing segment DHCP server right
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DHCP services would be either through NSX providing segment DHCP server right

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here, or if we had a VM right here, for example, connected to that segment,

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which could act as a local DHCP server.

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However, if we've got our segments like segment 10 and segment 20, we're

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already connected to those segments with our gateways, we can simply enable

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gateway DHCP services.

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And behind the scenes, we'll reference a profile that points to a virtual DHCP

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server inside of NSX.

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We can use the same IP address as we had up here.

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And then as we configure the details for the gateway based DHCP services, for
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And then as we configure the details for the gateway based DHCP services, for

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segment 10, we can say we want to use, let's say, 10.10.0.151 all the way

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through 200.

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And then over here for segment 20, we can use 10.20.0.151 all the way through

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200.

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And then for the default gateway, we specify whatever the IP addresses by that

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logical router interface.

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So on segment 10, it'd be 10.10.01 and over here on segment 20, it'd be 10.10.0

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.

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I put a do there.

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We were just talking, you know, creatively, but in our existing topology, that
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We were just talking, you know, creatively, but in our existing topology, that

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isn't a two on this logical gateway B.

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It is a dot one on the 10.20.0 network.

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So let me update my notes here as well. And we'll put a dot one there.

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So as part of our options as part of DHCP that we're going to hand out for

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clients on segment 20, we'd hand out the default gateway information of 10.20.0

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.1.

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We'd hand out whatever IP address we want them to use for DNS resolution.

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And it could be, for example, a DNS server out here on the internet or a DNS

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server that we have here in our enterprise.
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And so one of the questions that might be asked is, okay, why would we ever use

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a segment DHCP server to support, for example, any of our segments versus a

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gateway DHCP server?

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A gateway DHCP server. And the answer is, if you've got a gateway connected to

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your networks, you're probably always going to use a gateway based DHCP service

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.

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And in the off chance that you've got some segment with no logical router

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configured to it whatsoever.

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In those situations, the only option you have from an NSX perspective to

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provide DHCP services is a segment based DHCP server.

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And the great news is this. We are going to have a chance in this set of videos
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And the great news is this. We are going to have a chance in this set of videos

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and in the skill to configure and demonstrate each of the devices.

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Now, to make it as successful as possible, what we ought to do also is we ought

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to put a game plan together.

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So in the next video, let's put a specific game plan together for how we're

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going to implement DHCP relay inside our topology.

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And then after we verify that, we'll then move on to segment based DHCP

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services to support a segment where there is no layer three gateway.

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And then last but certainly not least, we'll also walk through setting up
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And then last but certainly not least, we'll also walk through setting up

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gateway based DHCP services to hand out IP addresses to segments where they

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have a directly connected gateway.

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So I'll see you in the next video as we put that game plan together.

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together.