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During my studies I had a course called "Advanced Network Administration". I learnt about the OSPF routing protocol and its Quagga [1] implementation and I had to prepare a simple installation that consisted of 3 Linux machines. They were connected with cheap USB network adapters.

After everything was configured I started the Quagga daemons and somehow they just didn't want to talk to each other. I've opened tcpdump to see what happens and the OSPF packets were exchanged properly. After a while the communication and routing was established. I thought that maybe the services just needed some time to discover the topology.

I've restarted the system to see if it's able to get up automatically, but the problem reoccured - daemons just didn't see each other. Again, I launched tcpdump, tweaked some settings and now it worked - until it didn't a few minutes later.

It take me a long time to find out that diagnostic tool I've used had actually changed the observed infrastructure (like in the quantum world). tcpdump enables the promiscuous mode on the network interfaces and apparently this was required for Quagga to run on the cheap USB ethernet adapters. I've used the ifconfig promisc and after that the OSPF worked stable.

[1] http://www.nongnu.org/quagga/

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