No Safe Changes

Settling into a new job – I was working on what I thought was a routine change. Setup a spare switch in a temporary location with a basic config. Easy enough, right? The time came for me to configure a port on the upstream device. The device in question was a legacy Catalyst 65xx – a big chassis switch that I had read about but never had any experience with. The port I was going to use had a dozen lines of configuration already applied – mostly related to queuing. My first instinct was to issue a ‘default interface <slot/port>’ command and start from scratch. This is almost always the right thing to do, as it ensures no confusing stale configuration remains (I’m looking at you ‘switchport access vlan # / switchport mode trunk’).

Leaning over, I asked my co-worker if the ‘default interface’ command worked on these things. After being assured that it would be fine, I held my breathe and pressed ‘enter’.

I was greeted with several lines of output related to quality of service (QoS) being set to default values on a range of interfaces. Crap! Had I just wiped out the configuration for an entire line card?

No – it turns out the architecture of these switches is such that the queuing must be configured identically on specific groups of ports. I forget if it was all 48 ports on the card, orĀ  16, or whatever, but the point is that I made a simple change and there were unintended consequences. At least Cisco was kind enough to leave me a message about it. And it didn’t bring the network down.

This was a reminder that even the most mundane, routine, everyday changes can go sideways when you least suspect it.

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