The many ‘modes’ of multimode

Fiber types are differentiated as multimode or single mode. Single mode was always easy for me to understand but I could never quite understand what ‘multimode’ actually meant. I’m written some notes for myself on this topic that I thought I’d share. I’m sure some physicists will have an allergic reaction to my take, but I’m happy to pay that price to learn a little more.

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Packet pushers podcast – Hardware Resources

It’s been quite a while since I’ve posted here but I wanted to highlight some work I’ve been doing with Greg Ferro and Simon Chatterjee on the Packet Pushers podcast. We recorded a three part series where we dive deep into the guts of networking hardware. All three shows are now published on the packet pushers podcast.

Show 186 – The Silicon Inside Your Network Device – Part 1

Show 187 – The Silicon Inside Your Network Device – Part 2

Show 190 – The Silicon Inside Your Network Device – Part 3

If you’re just arriving here from the Packet Pushers site, I’ve put together a dedicated page to capture all of my hardware-related posts.

The Network Sherpa – Hardware page 


Using IPMI Serial-over-LAN for server consoles

I’ve been trying to learn linux networking and virtualisation using a donated server in a remote lab. The server didn’t have an IP-KVM attached but it did have a working IPMI connection. Not that I’d need it of course; I was experimenting with network settings whilst ssh’d into a server that was four and a half thousand miles away. What’s the worst that could happen?

Of course the inevitable happened and I haplessly disabled my eth0 interface. I was locked out of the server, but was happy to learn that the Intelligent Platform Management Interface (IPMI) was a powerful tool indeed.

Both the IPMI protocol and HP’s iLO allow you to connect to the Baseboard Management Controller (BMC) on high-end servers. The BMC is a micro-controller embedded on the server motherboard which allows remote management without relying upon the server OS. The two specific functions I would need were the ability to remotely reset the server, and a view of the server console. Continue reading

Is CPU or ASIC responsible for forwarding?

I received the question below from reader Ned as a comment on my 24-port ASIC post and thought that the discussion was worth a post of it’s own.

…Would you be able to speak a bit about the actual physical path or packet flow a packet takes inside the switch itself and how does the hardware forwarding take place within the switch and asic. When does packet get sent to the Asic. Is it happen on ingress or on egress? When does packet get analyzed by CPU or control plane. If the CPU never sees the actual packet how does asic know where to forward the packet and does that mean the packets stay within asic itself and is that what is meant to be hardware forwarding. Is Asic = dataplane. Tx

I like this question because it captures a lot of my early assumptions and concerns about data and control plane separation. My response assumes a single-stage modern switch-on-chip ASIC without backplane or fabric.  Continue reading

Hardware – What’s the ‘holdup’?

iStock_000001593430SmallThis post discusses power supply ‘holdup’, and how it can impact network or server hardware uptime. The holdup time or ‘output holdup time’ is the length of time that a given power supply can maintain output power to the switch or server after it’s input power supply has been cut. The dependent host will shut down if the power supply isn’t restored to the PSU before the hold-up time expires. I like to think of holdup time as a power buffer.

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Planning projects instead of burning benjamins

iStock_000007817002SmallEngineers are often unstuck by poor planning and get hit with large financial penalties as a result. Projects can become mired in delays and complications due to unforeseen costs and expenses. There are some unavoidable bumps in the road, but most could be foreseen and eliminated in advance. I want to share a few tips based on some experiences I’ve had over the years. Continue reading