Category Archives: Technical

Express Setup is BAD; or, The Importance of Physical Security and Good Configuration

Cisco Catalyst switches include a feature called Express Setup which is designed to simplify the configuration process for users that don’t spend their lives at an IOS command prompt. A laudable goal, but the way the feature works brings with it some serious consequence.

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Going VIRL

For at least the last decade, something that experiences extraordinary popularity on the Internet has been said to go “viral.” Was Cisco anticipating the potential for enormous popularity when coming up with the name for the Virtual Internet Routing Labs? After getting a chance to try it out at Cisco Live, I’m thinking so.

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Openly Passionate

This year while at Cisco Live, I had the opportunity to join the Tech Field Day crew for a mini Networking Field Day round table event. At the event, we heard from Opengear, maker of advanced and highly-capable in- and out-of-band serial console management products.

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Can I See Some ID Please?

I had an interesting issue yesterday where I had to prove that a firewall was injecting TCP RST packets to drop active connections. The details of the firewall problem aren’t relevant, but in order to tie several packet traces together and prove beyond doubt that the mysterious RST packets were being injected and not coming from the end host I had to turn to a little-remembered field in the IP packet header — the identification field.

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Cisco CSR1000V on VMWare Fusion

The release of the Cisco CSR1000V is definitely garnering some buzz around the Internets. The platform is intended for deployment in a VMWare ESXi environment, but it seems to work just fine using VMWare’s desktop virtualization products as well. Join me as I boot up a couple of cloudy routers on my Mac Mini! Continue reading

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The Morning-After Command

Recently, during a very carefully planned, meticulously scripted change, I hit a bump in the road. It ended up not being a total show-stopper, but it did derail the change by about 30 minutes while we figured out a work-around. I think this kind of thing has probably happened to anyone who has been working on networks for a while. Invariably you get through (or abort!) whatever you’re working on and the following morning you start digging into what you hit. That’s when you discover that one command that would have saved your bacon last night. I call this the Morning-After command.

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Disabling Gigabit Link Negotiation on Fiber Interfaces

File this one under “things I’ve missed so many times I should write a blog article about them.”

I’ve Been Here Before

Here’s the scenario: You’ve ordered a new Ethernet-delivered circuit from your ISP to connect to equipment in your cabinet at a colo facility. The carrier has dropped the circuit, the colo staff has done the cross-connect and left you a fiber pigtail in your cabinet. You’ve configured your port, inserted your SFP optical module, and plugged in the fiber. You “no shut” your port and….. nothing. Nada. Zilch. Not even a link light.

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Tags, Tunnels, and Encapsulation

A common discussion I hear among seasoned network geeks and newbies alike is what is different between the ideas of tagging/labeling, tunnels, and “regular” encapsulation. Usually the newbies are trying to grok the entire concept while the old hands are trying to wing curveball exception cases at one another. While it seems nothing is absolute in networking and the astute engineer can always find a case that violates the usual rules, here are the working definitions I use which I think are right for about 98% of cases.
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Jonathan Davis (@subnetwork) keeps layers 1-7 working so that Layer 8 can be productive.