How to safely transform a routing domain

As part of my job as a Senior Network Engineer, I develop procedures for undertakings of varying complexity. In this post I’m describing a technique that greatly simplifies any project where a routing domain is expected to churn (i.e. neighborships going up and down, routes flapping), when such event is undesirable. Motivation I developed this technique for a… Read More »

Paper: Scanning the Internet for Liveness

An interesting paper where the authors are building a better way to scan the Internet. https://sheharbano.com/assets/publications/ccr18-scan-liveness.pdf Shehar Bano et al. Scanning the Internet for Liveness // ACM SIGCOMM Computer Communication Review, Volume 48 Issue 2, April 2018 Liveness—whether or not a target IP address responds to a probe packet—is a nuanced concept without a simple yes/no answer. Responsiveness… Read More »

How many spares do you need?

In designing a network, there is a question that is often missing an answer or at best, answered using some rule-of-thumb. How many spare units you should include in your BOM? Actually, do you need them at all? Disclaimer: I won’t be covering any of the really complex models. People who need them probably know about spare part… Read More »

Why Hulc LED process consumes so much CPU on 2960 platforms?

In this post I’ll try to make an educated guess about what happens with Hulc LED process and why it appears to consume 20-30% CPU on Cisco 2960(S/X/XR/RX) switches. (N.B.: the issue appears to be present on Cisco 3750 / 3560 platforms as well) Symptoms If you monitor your switch via SNMP, you may quickly notice constantly elevated… Read More »

How Cisco ESA treats email traffic

I was working with Cisco ESA (previously – Ironport) lately and would like to write down some notes about how it works (at least the basics). I’ve already covered spam and antivirus testing techniques in a previous post. Here I’ll try to walk through the message filtering steps performed by a Cisco ESA appliance.

Cisco UCS HyperFlex FE class notes

Recently I attended a two-day class on Cisco HyperFlex by Tomaz Klancnik from NIL. It was quite interesting and packed with information about this new Cisco’s system. Here are the notes I took during the lectures and the labs. General Notes about Cisco HyperFlex Not developed internally; Not a complete acquisition / spin-in; software developed by Springpath https://springpathinc.com/resources.php… Read More »

Etherchannel, LAG, portchannel and friends

An EtherChannel is a way to use multiple physical interfaces as a single logical one. That logical one appears as a single interface to MAC table, STP and management plane. That solves two main concerns: it adds bandwidth and it prevents STP from considering several parallel links a loop, thus preventing it from being blocked.