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Christopher Hart Profile picture
Christopher Hart
@_ChrisJHart
Consulting Engineer at Cisco, ex-TAC. Interested in systems & network administration, creating extremely Pythonic dumpster fires! Tweets are my own
Mar 21, 2022 • 56 tweets • 12 min read
Lots of things can ruin the average person's Christmas holiday. In 2019, one network engineering team ruined their Christmas by combining IP SLA operations, track objects, and static routes.

Let's find out how! 🧵

Prefer a blog post format? Click here: chrisjhart.com/TAC-Tales-How-… It was Christmas Day of 2019, and I was working the holiday shift in Cisco TAC. Working Christmas is enjoyable - it tends to be quiet, and in the rare case you need to assist with an issue or outage, customers are nice and in good spirits.
Jan 23, 2022 • 37 tweets • 7 min read
A #network administrator's worst nightmare can be intermittent network congestion - it's impossible to predict, short-lived, and has major impact. Can #Python help us find and fix it?

Let's find out! 🧵

Prefer a blog post format? Click here: chrisjhart.com/Practical-Pyth… A case I've seen in TAC is where customers observe intermittently incrementing input discard counters on interfaces of a Nexus 5500 switch. This is usually followed by reports of connectivity issues, packet loss, or application latency for traffic flows traversing the switch.
Jan 23, 2022 • 7 tweets • 2 min read
Excellent thread from Nick on this topic! A big point I'm a fan of:

"...most juniors don't have an immediately accessible lab on their laptops or cloud environment, because they don't spend much time labbing. Most mid-levels can spin up a topology on demand." Labbing something does not have to be a arduous, time-intensive process. Being familiar with lab resources available to you and knowing how to efficiently use them is paramount to getting definitive answers to questions quickly.
Oct 24, 2021 • 35 tweets • 7 min read
A common misunderstanding engineers have about Equal-Cost Multi-Pathing (ECMP) and port-channels is that they increase the bandwidth that can be used between two network devices. This *can* be true, but isn't *always* true.

Curious why? 🧵 First, let's review our topology. Three Cisco Nexus switches are connected in series. Traffic generators are connected to Switch-1 and Switch-2 through physical interface Ethernet1/36. Switch-1 and Switch-2 connect to Router through Layer 2 port-channels.
Oct 17, 2021 • 19 tweets • 6 min read
"I see a lot of packet loss when I ping my switch" 🚩🚩🚩🚩🚩🚩🚩🚩🚩🚩🚩🚩

Wait, why is this a red flag? Let's dig into this behavior in a bit more detail... 🧵 First, let's take a look at our topology. We have two hosts in different subnets that connect to a Cisco Nexus 9000. One host connects via Ethernet1/1, and the other connects via Ethernet1/2. Ethernet1/1 has an IP of 192.168.10.1, while Ethernet1/2 owns 192.168.20.1.
Oct 15, 2021 • 12 tweets • 3 min read
On Cisco Nexus switches in production environments, avoid working within a configuration context on the CLI unless you're actively configuring the switch. Otherwise, you might accidentally cause an outage by trying to run a show command.

Curious how that's possible? 🧵 Cisco IOS and IOS-XE require you to prepend show commands with the "do" keyword to execute them within a configuration context.
Sep 4, 2021 • 32 tweets • 10 min read
Discovered an interesting issue at home today - when I ping a Nexus 9000v running in CML from an Ubuntu host, I see duplicate replies. At first glance, you might think the Nexus is duplicating replies. Meaning, a single ICMP Echo Request packet enters the switch, and the Nexus sends two ICMP Echo Reply packets.