1 Linux PC with two network cards cannot produce network traffic for test system

Question

These questions relate to the basics of networking, but I have not found an answer yet!

Question 1: Can a 100Mbps switch handle 2 simultaneous TCP streams running at 100Mbps? To illustrate this if you have 4 PC's on 1 switch (100Mbps with no other connections) and you had two separate streams, what speed would they reach? Would one stream impact on the others speed, or is a switch fast enough internally to deal with many streams?

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Question 2: I have set up 1 PC (Linux) on a dedicated network switch, the PC has 2 Network Interface Cards (NIC). Both NIC's have different IP address' and work fine. If I send any data from one NIC to the other absolutely no traffic is send on the network switch. It seams that the kernel is intelligent and works out there's no need to send anything on the network as the IP's are on the same PC. How do I turn this off?

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As I don't have access to 4 PC's I cannot test question 1 in the real world. These are very simple questions, but at the same time very difficult to answer.

Thanks!

Answer 1

Question 1: Yes, any 100Mb/s switch should be able to handle two independent 100Mb/s streams, as each port is 100Mb/s and not the switch itself. Traffic does not hit the CPU unless it absolutely needs to, at which point the switch may bottleneck and slow down. Normal traffic should be fine, though.

Question 2: You need to write a program to do it manually, or find one online. I recommend looking at Libnet.

The problem is that the kernel knows that it owns both of those cards, so it doesn't bother actually sending the data out on the network. Using Libnet, you can manually construct packets and send them out of a specific interface.

The main trick, though, is that you need to send traffic in both directions (not much; you shouldn't need more than 1 packet a minute going in one direction, with everything else going in the other direction). The problem is that the switch will not know where the packets are supposed to go and will flood them to all ports, slowing everything down.

Answer 2

The answer to question 1 depends upon the backplane bandwidth of the switch - this is the total rate of traffic it can handle internally. If it has at least 200Mbps of backplane bandwidth, it can sustain two independent 100Mbps streams.

Cheaper switches tend to have less backplane bandwidth than more expensive "big iron" switches.

As for question 2, you could try and add manual routes to your own IP addresses, explicitly specifying the external devices (if you check route -C, you'll see that the routing cache has routes via the loopback device for your own IP addresses).