DHCP question

Posted on 2011-05-10
Medium Priority
Last Modified: 2012-05-11
Hi I am familiar with DHCP but have a question about how our network is setup and things we are adding. Currently we have Cisco switch that acts as DHCP relay agents that send specific floors(floors setup in different VLANS) and sites to a scope on our DHCP server. We are planning to add a new site (we have a different team that handles the Cisco Switches) and my part is the DHCP server. So I would only need to add the new scopes. I want to understand how the whole system works, so I was told that our network team has to provide IP addresses for me and then I can create the scopes dependent on how many subnets we have. I have never configured a relay agent before so how does a machine know to go to a specific scope on the dhcp server? For example how does floor 1 know to go to 192.168.1.x scope instead of the 192.168.2.x scope? Is this a configuration that is placed in the switch and nothing done at the DHCP server? Thanks
Question by:Thomas N

Expert Comment

ID: 35729640
When you configure VLANs there is typically a gateway for that IP scheme IE VLAN 1 = 192.168.1.x with the default gateway as VLAN 2 The DG is a L3 device. Within that L3 device there are VLAN interfaces which are assigned that IP address (interface vlan 1 ip address, (interface VLAN 2 ip address Via routing tables etc these VLANs can talk. In Cisco speak there are commands that can point DHCP requests to helper address whcih you can read about here: http://www.cisco.com/en/US/docs/ios/ipaddr/configuration/guide/iad_dhcp_rly_agt.html

That is my 2 cents.

Clear as mud I am sure.

LVL 14

Expert Comment

ID: 35729686
Usually the network team can put the IP Helpers on the switches.  All they need is the IP address of your DHCP server

Expert Comment

ID: 35732649
You create the scopes on your server, then provide the IP address of that server to the networking team so that they place that IP address in their Switch configuration (ip dhcp-server X.X.X.X)
NFR key for Veeam Agent for Linux

Veeam is happy to provide a free NFR license for one year.  It allows for the non‑production use and valid for five workstations and two servers. Veeam Agent for Linux is a simple backup tool for your Linux installations, both on‑premises and in the public cloud.

LVL 17

Expert Comment

ID: 35738433
The DHCP server "sees" that the DHCP request comes from a particular VLAN because it is sent (relayed/forwarded) from the VLAN interface in the switch that intercepted the client's DHCP request (broadcasted). Use a network sniffer (Wireshark, MS Netmon) on the DHCP server (flter on UDP 67 and 68) and take a look at the DHCP requests that are originating from a specific VLAN.

If that was not clear, take a look at this pages:

Also  check what is the use of the "Relay Agent IP" field in DHCP packets.
LVL 17

Accepted Solution

vivigatt earned 2000 total points
ID: 35738466
To make it clearer: When the router/L3 switch forwards the DHCP request from a particular LAN, it fills the "Relay Agent IP" field in the DHCP request packet forwarded from the original client to the dhcp server. The router puts the IP address of its own interface in the VLAN that the clients which sent the DHCP requests is located in.
Then the server "knows" what scope to use by matching this "relay agent IP" address with one of its scopes.

Author Closing Comment

by:Thomas N
ID: 35739017
Very clear thanks.

Featured Post


Modern healthcare requires a modern cloud. View this brief video to understand how the Concerto Cloud for Healthcare can help your organization.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

This is my 3rd article on SCCM in recent weeks, the 1st (http://www.experts-exchange.com/OS/Microsoft_Operating_Systems/Server/Windows_Server_2008/A_4466-A-beginners-guide-to-installing-SCCM2007-on-Windows-2008-R2-Server.html) dealing with installat…
Issue: One Windows 2008 R2 64bit server on the network unable to connect to a buffalo Device (Linkstation) with firmware version 1.56. There are a total of four servers on the network this being one of them. Troubleshooting Steps: Connect via h…
Exchange organizations may use the Journaling Agent of the Transport Service to archive messages going through Exchange. However, if the Transport Service is integrated with some email content management application (such as an anti-spam), the admin…
This lesson discusses how to use a Mainform + Subforms in Microsoft Access to find and enter data for payments on orders. The sample data comes from a custom shop that builds and sells movable storage structures that are delivered to your property. …

850 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question