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Category: switching

Will DPUs Change the Network?

It’s easy to get excited about what seems to be a new technology and conclude that it will forever change the way we do things. For example, I’ve seen claims that SmartNICs (also known as Data Processing Units – DPU) will forever change the network.

TL&DR: Of course they won’t.

Before we start discussing the details, it’s worth remembering what a DPU is: it’s another server with its own CPU, memory, and network interface card (NIC) that happens to have PCI hardware that emulates the host interface cards. It might also have dedicated FPGA or ASICs.

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A Quick Look at AWS Scalable Reliable Datagram Protocol

One of the most exciting announcements from the last AWS re:Invent was the Elastic Network Adapter (ENA) Express functionality that uses the Scalable Reliable Datagram (SRD) protocol as the transport protocol for the overlay virtual networks. AWS claims ENA Express can push 25 Gbps over a single TCP flow and that SRD improves the tail latency (99.9 percentile) for high-throughput workloads by 85%.

Ignoring the “DPUs could change the network forever” blogosphere reactions (hint: they won’t), let’s see what could be happening behind the scenes and why SRD improves TCP throughput and tail latency.

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DPU Hype Considered Harmful

The hype generated by the “VMware supports DPU offload” announcement already resulted in fascinating misunderstandings. Here’s what I got from a System Architect:

We are dealing with an interesting scenario where a customer had limited data center space, but applications demand more resources. We are evaluating whether we could offload ESXi processing to DPUs (Pensando) to use existing servers as bare-metal servers. Would it be a use case for DPU?

First of all, congratulations to whichever vendor marketer managed to put that guy in that state of mind. Well done, sir, well done. Now for a dose of reality.

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Running Routing Protocols over MLAG Links

It took vendors like Cisco years to start supporting routing protocols between MLAG-attached routers and a pair of switches in the MLAG cluster. That seems like a no-brainer scenario, so there must be some hidden complexities. Let’s figure out what they are.

We’ll use the familiar MLAG diagram, replacing one of the attached hosts with a router running a routing protocol with both members of the MLAG cluster (for example, R, S1, and S2 are OSPF neighbors).

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Are DPUs Any Good?

After VMware launched DPU-based acceleration for VMware NSX, marketing-focused websites frantically started discussing the benefits of DPUs. Although I’ve been writing about SmartNICs and DPUs for years, it’s time for another closer look at the emperor’s clothes.

What Is a DPU

DPU (Data Processing Unit) is a fancier name for a network adapter formerly known as SmartNIC – a server repackaged into an interface card form factor. We had them for decades (anyone remembers iSCSI offload adapters?)

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Combining MLAG Clusters with VXLAN Fabric

In the previous MLAG Deep Dive blog posts we discussed the innards of a standalone MLAG cluster. Now let’s see what happens when we connect such a cluster to a VXLAN fabric – we’ll use our standard MLAG topology and add a VXLAN transport underlay to it with another switch connected to the other end of the underlay network.

MLAG cluster connected to a VXLAN fabric

MLAG cluster connected to a VXLAN fabric

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VLAN Interfaces and Subinterfaces

Early bridges implemented a single bridging domain across all ports. Within a few years, we got multiple bridging domains within a single device (including bridging implementation in Cisco IOS). The capability to have multiple bridging domains stretched across several devices was still missing… until the modern-day Pandora opened the VLAN box and forever swamped us in the complexities of large-scale bridging.

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How Routers Became Bridges

Network terminology was easy in the 1980s: bridges forwarded frames between Ethernet segments based on MAC addresses, and routers forwarded network layer packets between network segments. That nirvana couldn’t last long; eventually, a big-enough customer told Cisco: “I don’t want to buy another box if I already have your too-expensive router. I want your router to be a bridge.

Turning a router into a bridge is easier than going the other way round1: add MAC table and dynamic MAC learning, and spend an evening implementing STP.

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Router Interfaces and Switch Ports

When I started implementing the netlab VLAN module, I encountered (at least) three different ways of configuring physical interfaces and bridging domains even though the underlying packet forwarding operations (and sometimes even the forwarding hardware) are the same. That confusopoly is guaranteed to make your head spin for years, and the only way to figure out what’s going on behind the scenes is to go back to the fundamentals.

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Repost: Buffers, Congestion, Jitter, and Shapers

Béla Várkonyi left a great comment on a blog post discussing (among other things) whether we need large buffers on spine switches. I don’t know how many people read the comments; this one is too valuable to be lost somewhere below the fold


You might want to add another consideration. If you have a lot of traffic aggregation even when the ingress and egress port are roughly at the same speed or when the egress port has more capacity, you could still have congestion. Then you have two strategies, buffer and suffer jitter and delay, or drop and hope that the upper layers will detect it and reduce the sending by shaping.

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MLAG Deep Dive: Layer-3 Forwarding

The layer-2 forwarding and flooding in an MLAG cluster are intricate but still reasonably easy to understand. Layer-3 gets more interesting; its quirks depend heavily on layer-2 implementation. While most MLAG implementations exhibit similar bridging behavior, expect interesting differences in routing behavior.

We’ll have to expand by-now familiar network topology to cover layer-3 edge cases. We’ll still work with two switches in an MLAG cluster, but we’ll have an external router attached to both of them. The hosts connected to the switches belong to two subnets (red and blue).

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MLAG Deep Dive: Layer-2 Flooding

In the previous blog post of the MLAG Technology Deep Dive series, we explored the intricacies of layer-2 unicast forwarding. Now let’s focus on layer-2 BUM1 flooding functionality of an MLAG system.

Our network topology will have two switches and five hosts, some connected to a single switch. That’s not a good idea in an MLAG environment, but even if you have a picture-perfect design with everything redundantly connected, you will have to deal with it after a single link failure.

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Select the Best Switching ASIC For the Job

Last week I described some of the data center switching ASIC design tradeoffs and the ASIC families Broadcom created to fit somewhere in that multi-dimensional space.

Next step: how could you design your data center fabric to make the most out of them? To keep things simple, we’ll build a typical leaf-and-spine fabric with a WAN edge layer (sometimes called border leaf switches).

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MLAG Deep Dive: Dynamic MAC Learning

In the first blog post of the MLAG Technology Deep Dive series, we explored the components of an MLAG system and the fundamental control plane requirements.

This post focuses on a major building block of the layer-2 data plane functionality: MAC learning. We’ll keep using the same network topology with two switches and five hosts, and assume our system tries its best to implement hot-potato switching (sending the frames toward the destination MAC address on the shortest possible path).

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