GNS3 and ”no ip cef”

Once in a while i experience situations that make me question my sanity. Today i had one of these situations. I was doing a presentation at a local pub for a few people about Cisco, what it means to be an admin etc. I thought i was going to show a few cli commands etc and came up with this brilliant (?) idea to set up a simple company network in a few minutes using GNS3 while explaining the concepts as I go along.

The plan worked beautifully right until the moment when I configured two SVIs on a switch and tried to ping from host A on vlan 10 to host B on vlan 20. The ping failed no matter what I did. I thought my brain was going to explode because i’d never seen anything like that before and the config was super simple.

Finally, a friend of mine who was in the audience said: disable CEF (God bless you Kacper!)

And of course, just like that it worked. You can read more about this bug at

Any lessons learnt? I thought long and hard about this but I guess that not this time. Being an admin means having to deal with the unexpected. I guess I should have labbed it up before but c’mon – labbing up a ping between vlans?

Anyways, GNS3 had always worked fine for me so i couldn’t have seen this coming but it was still one of those excruciating moments where you’re supposed to be ”the expert” and you can’t make a simple ping work through one switch.

Do carpenters have to deal with bugs? or lumberjacks? I need to look it up on google 🙂



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