Multicast support

[robinsmidsrod]

If you have access to managed switches I'd wager you'd be able to get much more information from them, as they report transport errors and such more properly. Bad cables (or network cards) can indeed cause all sorts of problems. I've once seen a single NIC take down an entire network segment just because of extreme amount of bad data it pushed out onto the network. The switches got completely overloaded! Switched it out and everything got back to normal. Pay attention to those error counters.

How many DHCP requests your network can handle at once I'm not sure, but 100 seems to be a reasonable number. The interesting part is how well iPXE can deal with that, considering it's broadcast traffic. Maybe some of the core developers can give some feedback here?

Just be aware that if you're using 10/100 switches out to the clients routed via a central gigabit switch, you'll still be limited to 100/8 = 12.5MB/s theoretical maximum throughput per client when downloading your boot image. And if you said it was around 130MB that should take about 10-15 seconds for each client under optimum conditions. But if you push 24 machines over a single gigabit uplink you're obviously going to get only half of that, as the gigabit uplink will be saturated, just for one switch (and you said you had 7). That means if only 24 machines boot you should at best get 6.25MB/s download. If you divide that with 7 (a completely full network) around 0.89MB/s which means about 2 and a half minute to boot all of the machines at the same time.

If those cisco 10/100 switches supports trunking so you can have two gigabit cables running at gigabit full-duplex you'd double your performance. Another option would also be to put as many gigabit network cards in the server as it can handle, bridge those network cards together and connect each of them to a 100/100 switch. Don't forget to enable packet forwarding. If you don't have enough PCI slots you can use one NIC to the unmanaged gigabit switch which can take the rest.

My best suggestion would be to buy a fiber module to the gigabit switch and use that between your central switch and the server. That should allow the server to push out a good amount more of data.