Troubleshooting IP Helper Address Issues
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.
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