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DHCP  for Wired and Wireless Devices

DHCP  for Wired and Wireless Devices

I would like to have an Expert explain the path Wired and Wireless devices take to get IP address from DHCP  


I put a diagram below, it might not be the right path that devices take to get IP address, but it can be corrected by an Expert.


Thank you


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David Johnson, CDSimple Geek from the '70s
Distinguished Expert 2019

Commented:

Wired  -- network adapter -> switch -> dhcp -> router -> internet

Wireless -- network adapter -> access point -> WLC -> switch -> dhcp-> router -> internet


Author

Commented:
in your path, I am assuming switch is L3...

DHCP has Scopes [pools of addresses]... how does it know from which scope to hand out IP address to device X
Hemil AquinoNetwork Security Engineer
Distinguished Expert 2018

Commented:
I would like to have an Expert explain the path Wired and Wireless devices take to get an IP address from DHCP  

Hi, your diagram looks vague and it does not explain what are you trying to archive. But here is a scenario that might help you.

SERVER:

Your server will have the following subnet pools

[DHCP SERVER---- SWITCH (with its respected network AKA VLAN)---- ROUTER
DHCP SERVER---- SWITCH (with its respected network AKA VLAN)----WLC---ROUTER

Server IP: 192.168.1.2

SWITCH CONFIG


Wired: 192.168.1.0/24 (Every time you create a pool, default gateway and DNS will be implied on your config.)
Wireless: 192.168.2.0/24 (Every time you create a pool, default gateway and DNS will be implied on your config.)

Your switch Will have the VLANS (networks) and will relay the IP address from the server. E.g:

VLAN 10 192.168.1.1 ip-helper 192.168.1.2 (This is the server IP that has all the pools of IPs)
VLAN 20 192.168.2.1 ip-helper 192.168.1.2

ROUTER:

It is the gateway that delivers packets in/out of the internet.

Author

Commented:
OK,

Wired Subnet : 192.168.1.0/24
Wireless Subnet: 192.168.2.0/24

Now a Wired Computer when it boots up, what makes it go and get an IP address from 192.168.1.0/24 instead of 192.168.2.0/24 ?

Also let's say a Laptop that can be sometimes Wired and sometimes Wireless, what makes it go and an IP address from 192.168.1.0/24 when it is wired then go and get and IP address  192.168.2.0/24 from  when it is wireless ?

Thank you
dhcp is a level2 protocol : you need the server and clients to be in the same lan. there cannot be a router unless it implements a dhcp forwarder or arp proxy of some sort.

in your situation, you most likely have either 2 separate dhcp servers or a single dhcp in your access point that is configured with 2 different pools based on  the interface the dhcp packet arrives on.
Hemil AquinoNetwork Security Engineer
Distinguished Expert 2018

Commented:
Now a Wired Computer when it boots up, what makes it go and get an IP address from 192.168.1.0/24 instead of 192.168.2.0/24 ?

By default, computers send a broadcast to any address asking for an IP address, when your DHCP server sees it, then it proceeds to negotiate an IP address.

Example

A computer sends a discover address, the server offers it, the computer requests it and the server acknowledges it.
For this process to work you have to make sure you have the right network pointing to your DHCP server.

If you have a laptop connected to your network, VLAN1 as an example, you need to indicate the VLAN that your DHCP server is located at the server address 192.168.1.2 - same thing with Wireless, if you have VLAN 2 make sure you relay your network to that DHCP server. 192.168.1.2

Author

Commented:
For Wired Computers

Example:
Computer A is  plugged to VLAN 20 on L2 switch ...

L2 Switch  is connected to a Router

DHCP server  IP : 192.168.12.12

The Router will have a sub-interface for each Vlan.. let 's say for VLAN 20 the sub-interface is fa01/20 and will have IP address  192.168.20.254
each sub-interface on The router will be configured with IP Helper-address 192.168.12.12

  when Computer A  boots up , it sens a DHCP request in form of broadcast, it will go through the Switch Vlan 20  then,through the Trunk between the Switch and the Router , when it gets to the router the broadcast stops there, the router will send the request on behalf of Computer A to DHCP server , DHCP server knows that it came from 192.168.20.254 and this IP should exist in one of its DHCP Scope options ,so it will hand out IP address from that Scope to the Router and the router will hand back the IP to the switch then to Computer A

That was my understanding in a nutshell...  Correct me where I m wrong

Thank you
Hemil AquinoNetwork Security Engineer
Distinguished Expert 2018

Commented:
Yes, that’s the whole process for you to provide IP addresses dynamically.
each sub-interface on The router will be configured with IP Helper-address 192.168.12.12

aka a dhcp proxy


DHCP server knows that it came from 192.168.20.254 and this IP should exist in one of its DHCP Scope options

hmm... how would the dhcp server know that ? the ip helper does a little more work than you think but yes that's the overall process. your dhcp config should make it obvious. actually, there are multiple ways to pass the information along.

--

note that it is unlikely ( but possible ) that the same dhcp handles your wireless connections. most of the time, the wireless router takes care of dhcp.

Author

Commented:
skullnobrains

192.168.20.254  is the sub-interface for VLAN 20. in DHCP scope there should be an option that states the Default Gateway(Router) 192.168.20.254
Not sure exactely about the details on how the Router communicates with DHCP considering the Scopes configuring in DHCP.
if you can provide more details that would be great.

this is for Wired computers.... Computer ---L2 Swicth---Router [DG]-----DHCP


what about Wireless? I am not sure about the involvement of WLC and the AP in the process... is AP configured with an IP address and will serve as DG for wireless computers ? is AP connected to WLC and WLC serves just as L2 Switch ? is WLC connected to a Router sub-interface ?
So I need more clarifications in regards  to  Wireless process for a computer to get an IP address from DHCP..
Thank you
most likely,
- the wireless system handles dhcp for wireless users on it's own. this is both simpler and more secure than using an external dhcp.
- the dhcp server has a single scope configured with adequate config for the wired lan so there is no complexity overhead than with a regular setup without a router.

but then, there are many ways to setup a network and i cannot provide you with your own network specifics. you should be able to lookup which scopes are configured in your dhcp server and how. and possibly view the leases it provided as well. that should make things obvious.

if you do not provide me with such information and possibly the router config ( the part related with dhcp should suffice ), there is no way i can answer your question other than with what seems likely.

Author

Commented:
The question has been :


You have DHCP Server configured with Scopes [Range of IP address, Default Gateway,DNS,DHCP ]

You have Network-Wired computers

You have Wireless Laptops


I needed to know how  Network-Wired computers gets their IP address from DHCP Servers
We said  the path is Computers in VLANx will connect to L2 Switch VLANx will connect to Router that has sub-interface configured to VLANx and on the Router IP DHCP-Helper address is configured to point to the IP address of DHCP server.


For Wireless, I am not sure if Wireless LAN Control [where Access Points are connected to] can do the function of DHCP server, whether WLC can connect to the same DHCP server where Wired computers get the IP addresses...?
Every access point can function as standalone since they all feature a builtin dhcp server. This is also better than using a separate one in terms of security. Some of them can use a separate one but won t default to do so. unless you did specific config, the ap uses its own dhcp.

Many ap can also do dhcp for attached wired machines. Not all. Mini routers with multiple lan ports almost always do. Aps with a single ethernet port usually only use that port for upstream traffic and usually rather feature a dhcp client on that interface.

Regards