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 »
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 »
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 »
For iBGP number of peers (i.e. the number of BGP routers inside an AS), the only significant limiting factor is that iBGP peers must be fully meshed (N.B.: not directly interconnected! An iBGP peering can span all the hops you can fit into the IP TTL field) – because it is the only way for iBGP to prevent… Read More »
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 »
Recently, I tested a Cisco 6500 switch in a fan-less configuration, to see how long it can go. DISCLAIMER: DO NOT TRY TO DO IT. This is a stupid idea and it will void warranty / would be a perfectly valid reason for Cisco to decline RMA (in my opinion at least). Running a switch without fans will… Read More »
There is an interesting problem with Cisco 6500 VSS clusters: generally, switchover between nodes is fast enough and only a few packets are lost. NSF&SSO algorithms help a lot to achieve that. But if you configure a feature that doesn’t support SSO for some reason, the flap becomes more noticeable. In this post I’m trying to make an… Read More »
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.
It might come in a greenfield antispam / antivirus deployment or during an audit that one needs to make sure that the protection (against spam or viruses) is enabled (N.B.: measuring protection efficiency is a completely different problem) and the configured policies are applied as expected. As with any program testing, for that task we need sample input… Read More »
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 »
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.