NETWORKING

  • 09/10/2015
    8:00 AM
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Troubleshooting IP Helper Address Issues

IP helper addresses are often used to manage broadcast protocols, especially when it comes to DHCP, but be on the lookout for potential problems they can cause.

I always seem to end up involved in network "cleanups." These cleanups can involve physical equipment removal, but they also include equipment configuration validation and optimization.

In this video I cover the Cisco ip helper-address command and how it can affect hosts. This commonly used command forwards various UDP broadcast protocols to a specific device. I have actually seen IP helper addresses configured with a broadcast address (not recommended), which has caused some pretty weird issues. There are a few other alternatives you can also use to optimize this configuration from blocking unwanted protocols. Those include access control lists and using the no ip forward-protocol udp, service command, and dhcp relay.

In the video, I started with a model of my network in the software tool GNS3. Then I used Wireshark to simply capture some packets from a Microsoft computer broadcasting its typical protocols and how the network equipment reacted to it. I term these exercises "PC bootup baseline," "PC idle baseline" and, of course, "PC login baseline." I can’t stress the importance of performing these baselines to get a true picture of how your network and equipment behave before you are actually experiencing an issue.

I always sarcastically tell my clients that if you perform a baseline correctly, you will have some research to do. And in most cases, you will have some changes to make. I like using captures or anything quantitative so I can see the differences my changes make.

 


Comments

DHCP Client

I guess the use of IP helper-address command for broadcast applications is mainly used remote nodes such as DHCP client.

Re: DHCP Client

yes that is correct.  thanks for the feeedback