Question

How to configure DHCP Snooping on a Cisco layer 3 interface

Asked by: dharrell74

My company is finally going to start to switch from static IPs (on over 300 user PCs) to DHCP. I've started configuring DHCP snooping on my 3560s and 3750s, however I can't seem to figure out or find online how to configure a layer 3 interface with DHCP snooping.

My company has two satellite locations connected by dark fiber. I've added my ip helper-address statements in, but I also want to restrict DHCP offers at these two sites so that only our two DHCP servers at HQ can lease IPs to these two sites. I've had no problems configuring my L2 links and VLANs for DHCP snooping, but these L3 interfaces are giving me problems.

This Question has been solved and asker verified All Experts Exchange premium technology solutions are available to subscription members.

Subscribe now for full access to Experts Exchange and get

Instant Access to this Solution

  • Plus...
  • 30 Day FREE access, no risk, no obligation
  • Collaborate with the world's top tech experts
  • Unlimited access to our exclusive solution database
  • Never be left without tech help again

Subscribe Now

Asked On
2009-11-04 at 14:42:47ID24872837
Topics

Network Routers

,

Network Switches & Hubs

Participating Experts
2
Points
250
Comments
15

Trusted by hundreds of thousands everyday for fast, accurate and reliable tech support.

  • "The time we save is the biggest benefit of Experts Exchange to Warner Bros. What could take multiple guys 2 hours or more each to find is accessed in around 15 minutes on Experts Exchange." Mike Kapnisakis, Warner Bros.
  • "Our team likes having a resource that is more secure than just using Google and most experts using this service really know their stuff. It's nice to look here first versus using Google." Dayna Sellner, Lockheed Martin
  • "Anytime that I've been stumped with a problem, 9 out of 10 times Experts Exchange has either the accepted solution or an open discussion of the potential solution to the problem." Kenny Red, eBay Inc.

See what Experts Exchange can do for you.

Got a question?

We've got the answer.

Experts Exchange has been collecting answers to technology questions since 1996…3 million and counting! If you have a question, chances are we already have your answer.

Screenshot of Experts Exchange Knowledgebase

Need individual assistance?

Our experts are ready to help.

If you can't find the exact answer you're looking for, ask our exclusive community of 50,000 experts. You’ll get a personalized answer from a trusted professional.

Screenshot of Experts Exchange Knowledgebase

Want to learn from the best?

Read articles from industry experts.

Thousands of free tech tips, tricks, how-to’s and tutorials are available in our peer reviewed articles section. See for yourself how smart our experts are, no login required.

Screenshot of an Article

Working on a long term project?

Store your work and research.

Save solutions to your questions, answers you’ve discovered through searching plus helpful articles in your personal knowledgebase for easy future access.

Screenshot of Experts Exchange Knowledgebase

Access the answers to your technology questions today.

Subscribe Now

30-day free trial. Register in 60 seconds.

What Makes Experts Exchange Unique?

Members of the expert community talk about why the experience at Experts Exchange is different than what you will find anywhere else.

Trusted by the world's most respected brands.

image of each brand's logo

Faithfully serving IT professionals since 1996.

Experts Exchange Logo

Try it out and discover for yourself.

Subscribe Now

30-day free trial. Register in 60 seconds.

Related Solutions

  1. Cisco - IP helper for a specific vlan
    I have a Cisco 6509 running 12.1 IOS - its the core switch on the network. It has multiple ultiple Gigabit Ethernet ports on it, each of which is connectect to 3550 switches located on each floor of our building. Each of the gigabit eithernet ports has two VLANs configured ...
  2. VLans and ip helper-address
    I am having issues setting up vlans. My first issue is that I have set up 4 vlan 10,20,30,40, At this point I only have computers setup on vlan 10 I have a dhcp server setup on the default vlan1. Vlan 1 is fine it is able to surf the internet and can ping all sub interface of...
  3. Cisco 2621 VLAN IP Helper Address
    Hi Experts, I have a Cisco 2621 router with several VLAN's setup on it. I have ZERO experience with cisco routers or VLAN's. I'm just looking for the command line to change the IP Helper address and the bootfile on VLAN2. I have the security parameters, and after logging ...
  4. DHCP Snooping
    Currently i have a network with 6 switches (4) 2950 and (2) 3508 switches. We are expanding the switched network with 2 more switches 2960. Currently DHCP snooping is not enabled on any switch, i would like to enable DHCP snooping on the new switches. So far i have only con...

Free Tech Articles

  1. WARNING: 5 Reasons why you should NEVER fix a computer for free.
    It is in our nature to love the puzzle. We are obsessed. The lot of us. We love puzzles. We love the challenge. We thrive on finding the answer. We hate disarray. It bothers us deep in our soul. W...
  2. SCCM OSD Basic troubleshooting
    SCCM 2007 OSD is a fantastic way to deploy operating systems, however, like most things SCCM issues can sometimes be difficult to resolve due to the sheer volume of logs to sift through and the dispe...
  3. Migrate Small Business Server 2003 to Exchange 2010 and Windows 2008 R2
    This guide is intended to provide step by step instructions on how to migrate from Small Business Server 2003 to Windows 2008 R2 with Exchange 2010. For this migration to work you will need the fo...
  4. Create a Win7 Gadget
    This article shows you how to create a simple "Gadget" -- a sort of mini-application supported by Windows 7 and Vista. Gadgets can be dropped anywhere on the desktop to provide instant information, ...
  5. Outlook continually prompting for username and password
    There have been a lot of questions recently regarding Outlook prompting for a username and password whilst using Exchange 2007. There are a few reasons why this would happen and I will try to cover t...
  6. Backup Exchange 2010 Information Store using Windows Backup
    There seems to be quite a lot of confusion around the ability to backup Exchange 2010 using the built in Windows Backup feature. This stems from the omission of this feature prior to Exchange 2007 s...

Cloud Class Webinars

  1. Avoiding Bugs in Microsoft Access
    Alison Balter takes and in-depth look at avoiding bugs in Access. In this webinar you will learn about using the immediate window to debug your applications, invoking the debugger, using breakpoints to troubleshoot, stepping through code, setting the next statement to execute, ...
  2. Top 10 Best New Features in Visio 2010
    Scott Helmers gives live demonstrations of the top 10 new features in Visio 2010. This webinar will teach you how to create compelling diagrams by adding shapes to the page with a single click, linking the shapes in a diagram to data in Excel (or SQL Server, or SharePoint), ...
  3. IT Consultant Business Secrets Revealed
    Michael Munger, Experts Exchange tech pro and IT consultant, pulls back the curtain on his very successful businesses and answers question on every IT consultant and business owner should know about. He shares secrets on what he did to solve the 5 most common problems in IT, ...
  4. Disaster Recovery and Business Continuity
    Quest CTO, Mike Billon, gives an overview of the steps involved in building a dunamic disaster recovery plan. Through case studies and an examination of software/hardware tooles for monitoring and testing, you'll gain a better understandin of where you are, where you want ...
  5. Organize Your Visio Diagrams with Containers and Lists
    Scott Helmers uses cross functional flowcharts, wireframe diagrams, data graphic legends and seating charts to teach you: how to ustilize all three new structured diagram components in Visio 2010, the best practices for organizeing shapes in previous version of Visio, how to organize ...
  6. How to Us Objects, Properties, Events and Methods in Microsoft Access
    Alison Dalter gives an in-depbth look at objects, properties, events and methods in Microsoft Access. In this webinar you will learn about using the object browser, referring to objects, working with properties and methods, working with object variables, understanding the ...

Join the Community

Give a Little. Get a Lot.

Join the community of experts here and help other tech pros by answering question in your area of expertise. You can earn FREE access to all Experts Exchange's premium features and resources.

Join the Community

Answers

 

by: rochey2009Posted on 2009-11-04 at 15:12:08ID: 25745135

Hi,

You don't need to to configure DHCP snooping at layer 3. It's only done at layer 2 on the VLANs where you've got your PC's connected.

 

by: dharrell74Posted on 2009-11-04 at 15:14:54ID: 25745169

Thanks for the quick reply rochey, but if I don't configure the interface connecting our remote site as a trusted interface then wouldn't having DHCP snooping on that switch block all DHCP request/replies going to our DHCP servers on the other end of the L3 link?

 

by: rochey2009Posted on 2009-11-04 at 15:18:08ID: 25745193

Which interfaces do you have as trusted at the moment?

 

by: dharrell74Posted on 2009-11-04 at 15:20:02ID: 25745211

I have all my L2 trunk interfaces between my switches at the main site. I don't have any interfaces trusted at the remote sites at this time.

 

by: rochey2009Posted on 2009-11-04 at 15:24:45ID: 25745259

And the ports connecting to the DHCP servers, they are trusted as well?

Where do you have the ip helper statements at the remote site.

Is this a layer 3 switch or a router connecting to a stack of switches?

 

by: dharrell74Posted on 2009-11-04 at 15:28:50ID: 25745295

Yes, the ports to the DHCP servers are trusted

IP helper statements at the remote sites are on VLAN 1 interface, all hosts there are in vlan 1. main site has the helper statements for all vlans.

L3 switch, a 3750 at the main site and one remote site,  and the other remote site uses a 3560.

 

by: rochey2009Posted on 2009-11-04 at 15:32:54ID: 25745330

I don't think you need to do anything else other than enabling dhcp snooping globally and for the vlan. Since the vlan interface is on the same switch, there is no actual interface to trust. Have you tried this?

 

by: dharrell74Posted on 2009-11-04 at 16:03:32ID: 25745533

I tried that on my switches connected at L2 and it blocked the requests/replies. I had to trust each interface on both sides of the trunk to get it to work. It did block my "rogue" dhcp server from leasing IPs though. Only my authorized servers can lease IPs through these switches, but I'm betting that my little "rogue" server will be able to lease them out at these remote sites. I'll be testing it tomorrow.

 

by: nickbarauPosted on 2009-11-04 at 21:31:16ID: 25747000

Hi dharrell74,

With DHCP snooping enabled, all ports are by default "untrusted". Snooping can be applied globally onto all your Vlans if needed. Snooping also only happens on Layer2 on your user ports. Hence, the Offer/Ack responses from the rouges never get past the layer 2 switchport. They'll never even make it to your layer 3 interface.

Trusting only the ports connected to your Authorized DHCP's is sufficient.

 

by: dharrell74Posted on 2009-11-05 at 04:20:52ID: 25748666

Nick, these remote sites are connected via L3 ports over leased fiber. How can I turn on DHCP snooping at these remote sites and put the L3 port in trusted status? That L3 port is the only interface for those offer/ack packets to travel through to get to my server.

 

by: rochey2009Posted on 2009-11-05 at 04:45:38ID: 25748811

You don't need to put the L3 port in a trusted state. DHCP snooping doesn't exist for a layer 3 port. Just enable DHCP snooping globally and for the VLAN. All of the L2 ports at the remote site will automatically be put in non trusted state.

 

by: dharrell74Posted on 2009-11-05 at 05:09:53ID: 25748985

I guess what is throwing me off here is that the switch seems to indicate that L3 ports can be put in a trusted state.

ch-3750-stk-core>sh ip dhcp snoo
Switch DHCP snooping is enabled
DHCP snooping is configured on following VLANs:
1-2,150,160
DHCP snooping is operational on following VLANs:
1-2,150,160
DHCP snooping is configured on the following L3 Interfaces:

Insertion of option 82 is enabled
   circuit-id format: vlan-mod-port
    remote-id format: MAC
Option 82 on untrusted port is not allowed
Verification of hwaddr field is enabled
Verification of giaddr field is enabled
DHCP snooping trust/rate is configured on the following Interfaces:

 

by: rochey2009Posted on 2009-11-05 at 07:28:18ID: 25750460

I see what you mean. I really don't think that you need to worry about it. Have you attempted to implement DHCP snooping at the remote site?

 

by: dharrell74Posted on 2009-11-05 at 11:44:47ID: 25753128

Just got back from one of the sites. It appears to be working with DHCP Snooping enabled on the VLAN.

I wonder why the line "DHCP snooping is configured on the following L3 Interfaces:" even shows on the switch??

 

by: rochey2009Posted on 2009-11-05 at 12:08:15ID: 25753384

I'm not sure either. It wasn't always there. There are references to this on cisco's website but it doesn't offer any explanation.

20120131-EE-VQP-002

3 Ways to Join

30-Day Free Trial

The Experts

98% positive feedback on 31,087 answers since March 2000. angeliii is a Microsoft Most Valuable Professional for his work with MS SQL Server & Develoment.

He has also proven his knowledge of Visual Basic Programming, PHP Scripting and Oracle Databases.

The Experts

97% positive feedback on 10,752 answers since July 2000. lrmoore has more than 18 years experience in the networking industry.

The six-time Mircosoft MVPs specialties include firewalls, virtual private networking, and network management.

Testimonials

"...and excellent source for support... Kind of like having your very own IT dept." Electriciansnet

Testimonials

"I was apprehensive at signing up at first. However... it has already made my life as an IT administrator much easier." JaCrews

Testimonials

"WOW! You guys have great, active, and knowledgeable people on here." moore50

Business Clients

Business Clients

In the Press

"If you’ve got a question... Experts Exchange can supply an answer.”

In the Press

"...an invaluable aid for both IT professionals and those who require tech support."

In the Press

"where IT professionals provide quick answers on just about any topic"

Business Account Plans

Loading Advertisement...