Skip Links

Go to page content
Go to site-wide navigation
Go to Contact Us page

Network World

Back to Cisco Subnet

Brad Reese on Cisco

by Brad Reese

Previous Article Next Article

How to configure a Cisco Catalyst switch to act as a DHCP relay agent

By Brad Reese on Fri, 03/21/08 - 2:40am.

Newsletter Signup

Share
Tweet This
Email this page
Comment
Print

How to configure a Cisco Catalyst switch to act as a DHCP relay agent in the following Cisco Catalyst switches:

2940 - 2970 - 3550 - 3560 - 3750 - 4000 - 4500 - 6000 - 6500

Relay agents are used to forward requests and replies between clients and servers when they are not on the same physical subnet.

Relay agent forwarding is distinct from the normal forwarding of an IP router, where IP datagrams are switched between networks somewhat transparently.

Relay agents receive Dynamic Host Configuration Protocol (DHCP) messages and then generate a new DHCP message to send out on another interface.

The 2900XL, 2950 and 3500XL switches cannot be configured as a DHCP helper, or DHCP relay.

This requires a router or a switch with a Layer 3 capability that can do InterVLAN routing.

These switches are Layer 2 devices, which can only do DHCP snooping.

The ip helper-address command is present on a configurable Layer 3 interface.

The general rule is to configure the command on the Layer 3 interface closest to the client.

In order configure a Cisco Catalyst switch to act as a DHCP relay agent, issue the ip helper-address command in interface configuration mode and specify the IP address of the DHCP server on the remote subnet.

This ensures that all DHCP broadcasts are forwarded to that address.

More than one helper address can be configured per interface.

You may also wish to investigate:

Understanding and Troubleshooting DHCP in Catalyst Switch or Enterprise Networks

View more Cisco How-To Tutorials.

http://www.BradReese.Com


	trixbox vs. Cisco: Enticing Cisco SMB channel partners to change their allegiance to trixbox
	Nortel vs. Cisco: The battle heats up
	He's back, Frank Quattrone the exonerated banker who brought Cisco public
	Invitation to Network World readers to attend ChicagoCon from the Ethical Hacker Network
	Expert warns of scam to blackmail companies for cash to get back access to their Cisco routers

Cisco Power Supplies

Cisco Authorized Factory Refurbished List Pricing

Cisco Repair and Hardware Troubleshooting

Tags

Welcome, visitor. Register Log in

Advertisement:

About Brad Reese on Cisco

Brad Reese cofounded BradReese.Com Cisco Refurbished which offers one year warranties on Cisco Refurbished and Cisco Repair.

Contact Brad Reese

Contact Requires Login

Archives: July 2009; June 2009; May 2009; April 2009; March 2009; February 2009; January 2009; December 2008; November 2008; October 2008; September 2008; August 2008; July 2008; June 2008; May 2008; April 2008; March 2008; February 2008; January 2008; December 2007; November 2007; October 2007; September 2007; August 2007; July 2007; June 2007; May 2007; April 2007; March 2007; February 2007; January 2007

Categories: A government official in possession of a large corporate stockholding while that corporation is subject to administrative rulings by that same government official; After contacting Cisco; Agito adds that its enterprise fixed mobile convergence (eFMC) platform enables low-cost in-building voice coverage; Agito introduced Agito for BlackBerry; Agito's BlackBerry smart phone functionality for Cisco VoIP; Agito's RIM BlackBerry support announcement; Americas Vice President and General Manager of HP ProCurve - Karl Soderlund; An assortment of communications companies; Andreessen plans to watch what companies Cisco acquires; Andreessen sees as the next market transition; As a matter of Cisco policy; Back in April the CCIE Security track changed; Black Hat attack on Cisco's network admission control (NAC); Boas also led an educational session at the Gartner Security Summit; Boas shares his insight on the most prevalent threats to the enterprise network; Bragging about Cisco's ability to catch market transitions; Brings enterprise VoIP over WiFi for dual-mode BlackBerry smartphones; But neither felt it as bad as Cisco did; Can Cisco VARs profitably match HP ProCurve pricing on a deal-by-deal basis?; Careers; Chairman and CEO of Cisco China - Jim Sherriff; Cisco; Cisco CEO John Chambers has never been shy; Cisco Chief Technology Officer Padmasree Warrior; Cisco NAC design flaws that the folks at Black Hat so alarmingly described; Cisco VARs no longer have enough margin left to profitably match HP ProCurve pricing; Cisco customer a discount; Cisco has held this share position since CY02; Cisco has produced a new CCIE count; Cisco is celebrating its 25th anniversary this year; Cisco is the revenue-leading vendor overall with 38% of total network security appliances and software in 1Q09 (down 2.8 points from 4Q08); Cisco might do very well with the Flip; Cisco offer price-matching discounts to existing customers in order to equal ProCurve prices; Cisco only counts your CCIE number once; Cisco released its new worldwide CCIE count; Cisco's executive biographies web page; Cisco's loss of market share has in itself become the networking industry's newest market transition; Compromised the Cisco agent installed on the end system; Confirmation testimony before the U.S. Senate noteworthy; Customer-proven best practices of network access control (NAC); Data Center; Didn’t RIM already support voice over WiFi?; Doesn’t RIM’s Ascendent acquisition give them this?; Douglas Gourlay - Vice President of Cisco Data Center Solutions; Douglas Gourlay is as sharp as a tack and one of the most impressive people I've ever had the honor of collaborating with on Cisco stories; Dual CCIE #18532 Routing and Switching/Security - George Morton; Dual-mode BlackBerry smartphones; Enables BlackBerry to be integrated into corporate PBXs and Unified Communications systems; Enterasys NAC is agent-less assessment based on a network scan; Enterasys security expert Dennis Boas; Enterasys uses multiple criteria beyond end system health assessment to assign and limit access granted to an end system; Enterprise concerns about the financial and management aspects of NAC; Enterprises that have standardized on the BlackBerry platform; Exactly what's causing everybody to quack?; Famous networking industry journalist; Flexible options with Enterasys NAC; Flip video below of Chambers' doing his quacks; Flip video of Cisco CEO John Chambers performing his duck quacks; Further details on why Cisco has lost market share in network security; Gourlay was featured just 7 days earlier in the Cisco Data Center Networks blog story; Gourlay's office is located at Cisco's nearby headquarters in San Jose; HP ProCurve continues to win business and gain market share; HP ProCurve is calling the shots now; HP ProCurve is whacking Cisco in its most vulnerable spot; HP ProCurve pricing on a deal-by-deal; HP ProCurve will take 20% off its list price; Half the smartphones in use in the US today are BlackBerry devices; Hogtied by Cisco's new committee culture; How Cisco was working overtime AGAINST the Buy America provisions of the $7.2B broadband stimulus fund; I can still see those wishing to cash out selling to Cisco; I have worked for a handful of telecommunications companies of varying sizes; I voted for President Obama seeking change; If you trade-in your Cisco equipment; Independent companies operating in Silicon Valley able to out innovate Cisco; Interesting CCIE news from around the world; Is Cisco likely to innovate? No; Its been proven that a government official can be bribed with free dinners; Jeff Wilson - network security analyst at Infonetics Research; Joel Bion - Senior Vice President of Cisco's Product Resiliency Research; Juniper and Check Point are second and third with 10.4% and 9.5% respectively; LANs / WANs; Larry Strickling is confirmed as the new Administrator of the National Telecommunications and Information Administration (NTIA); Last month Cisco missed the multiple CCIE numbers; Made by Strickling during his March 19; Manny Rivelo - Senior Vice President of Cisco's Development Organization; May 2009 vs. June 2009 Worldwide CCIE Count Comparison; Mobile features integrated into the BlackBerry; My previous government service at the FCC provide me a unique background for the position of Assistant Secretary; My source suggested Gourlay had left Cisco; Network security vendor Enterasys; Not only among the Cisco workforce; Not too many senior executives are around from Cisco's early days; Now it appears the Internet is quacking too; Omitted the years of Cisco service for both John Morgridge and Richard Justice because they are no longer full-time Cisco executives; Only 66% of all applicants who passed were for the CCIE Router and Switch track; Only one CCIE is a member of Cisco's 59 strong senior executive team; Out innovate Cisco simply because they have no desire to be acquired; Pacific Rim CCIE numbers didn't change over the last 39 days; Pejman Roshan - Chief Marketing Officer of enterprise fixed mobile convergence (eFMC) vendor Agito Networks; Ponemon Institute reported; Post a comment to Cisco's blog that contains a link to your duck quacking video; ProCurve wants to make all Cisco customers aware of this offer; ProCurve’s momentum and market share gains; R & S + Security this year as the most popular dual CCIE track; R & S + Service Provider was 49% of the successful attempts for dual CCIE; RIM offers only data services over WiFi on their dual-mode smartphones; Responsible for Cisco's IOS Software; SMB; Security; Security mechanisms are used to validate the integrity and authenticity of the Enterasys agent for all server/agent communications; Security sales will come back for Cisco; Showed that Stickling owned a large Cisco stock position; So we had 251 new CCIEs; Subject of Cisco's senior executive team came up; The CEO of Cisco quacked; The Cisco workforce quacked; The IOS 12.4 track with ISR routers is slowing down the Security CCIE track; The National Telecommunications and Information Administration (NTIA) granted Cisco its coveted Buy American Exception; The average tenure would be of the 61 executives listed on Cisco's Mount Rushmore; The best duck call will win a new Flip Mino HD from John Chambers; The change in the CCIE Security track has had a major impact on new security CCIEs; Tremendous benefits HP ProCurve provides by offering the best value; VoIP / Convergence; We build our culture at Cisco around catching market transitions; We can confirm that Doug Gourlay is no longer with Cisco and we wish him well in his future endeavors; We're also now starting to see the CCIE Wireless track; We've experienced a new low for CCIE Security track; What exactly has Agito Networks announced this week?; What's your take on the implications of the new worldwide Cisco CCIE count?; What's your take on why Douglas Gourlay left Cisco and where do you think he will land?; Which had Gourlay inviting guests to Cisco Live in the following video; Which video quacks you up the most?; While Cisco salivates over 30 to 50 new market adjacencies; Why does Cisco lack pricing flexibility?; Why is cellular-only PBX and UC integration incomplete?; Why the Enterasys NAC solution is doing so well; Why the Enterasys NAC solution is in such high demand; Wireless / Mobile; Without ducking my question; Would you back the technology vision of Marc Andreessen; You too will understand why duck quacking has become such an overnight sensation

On The Web: Twitter

Daily Newsletter Get the most important tech news every day: Terms of Service

Network World, Inc The Leader in Network Knowledge

Other IDG Sites

Copyright © 1994 - 2009 Network World, Inc. All rights reserved.

Terms of Service