In the process of writing the article series about the DHCP Snooping, I realized that DHCP Relays are a subject that is often misunderstood by network engineers. The aim of this article, which we can say is an appendix to the “Understanding DHCP Snooping” series is to explore the functionality of DHCP Relays, as implemented by Cisco IOS.
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The following is an excerpt from the upcoming IPexpert book – IPv4/IPv6 QoS Operation and Troubleshooting

One of the points that many Cisco QoS texts fail to make clear is that a typical Cisco network device interface possess a software queue and a hardware queue for outbound traffic transmissions. When we read that a particular QoS tool will engage and work in times of congestion only, this is a reference to packets overflowing the hardware queue into the software queue. The software queue is where the magic of Quality of Service happens. In fact, one of the QoS mechanisms called Link Fragmentation and Interleaving (LFI) directly addresses the split between these two queues. LFI ensures that large packets (Jumbograms) do not clog the hardware queue. These large packets are chopped up (fragmented) so that higher priority, smaller packets (like voice) can be interwoven between the fragments in the hardware queue. Chapter 11: Link Fragmentation and Interleaving (LFI) details this process for you.
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While I enjoy working on complex issues, sometimes they are best explored in simpler environments. We don’t need dozens of switches and routes to learn how things work. With this in mind, I will expand our original network with a single switch and router.
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In the previous article in the series, we examined the problem that DHCP Snooping aims to resolve. In this article, we’ll take a look at how DHCP Snooping works when a single switch is involved in the network.
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Few months back, my esteemed blogger colleague and friend, Anthony Sequeira touched on the subject of DHCP snooping in an article you can find here. In this series of articles, we’ll take a deeper look into this valuable technology, which is often misunderstood and by CCIE candidates – underestimated.
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