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At work we use an old (10 year old) linux box with 4 interfaces to act as router/firewall for the network. There's never really been a need to change it since it's stable and handles all our needs.

I'm wondering, though, would replacing the network interfaces with newer ones provide a benefit? Besides the obvious bandwidth increase (eg. 100MBit to GBit), would there be a latency reduction, or do newer cards pretty much do the same thing as old ones?

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10 year old machine will not perform well. Even 5 year old machine can do only 300MBps. You would need to use at least dual-core machine to have bandwidth of 1GBps at minimum. Pentium D dual core can make 300MBps if used with a proper network card. So core2duo with 1GBps network card of the server kind would do the trick. You can try something very power efficient and new kind of architecture like 22nm, so you can use also for VPN etc. –  Andrew Smith Oct 23 '12 at 16:29
@AndrewSmith 1GBps or 1Gbit? Bits or Bytes? My 5 year old server can serve nearly a 1 Gigabit (around 100 Megabytes/sec) of traffic from a RAID array so has no trouble shifting packets through the CPU from one card to another. –  Alastair McCormack Oct 23 '12 at 16:59
Sorry I meant 1Gbit. Serving files is entirely different than forwarding packets. You push large chunk of files thru the network card in one direction only. Now in forwarding, that would already be a half of it. Then, on router you have many connections with various packet sizes. In practice, 5 year old server (like single core xeon), does drop packets at 300MBps. If you have core2duo type of tech, this is much faster indeed. –  Andrew Smith Oct 23 '12 at 18:28
Pls write what is your chipset and network card - so I can tell you exactly what is the network performance for router. –  Andrew Smith Oct 23 '12 at 18:29
I don't know who is smoking what but 10 year old machines are more than adequate for the task. So are 10 year old 1Gb NICs. –  John Gardeniers Oct 24 '12 at 10:06

2 Answers 2

up vote 6 down vote accepted

If your expansion bus is just a single plain PCI then you'll probably saturate its 2.1 Gbps theoretical limit assuming you're not doing too much packet inspection.

Any latency would probably be introduced from the CPU (TCP checksum offloading has been available for years) so I don't think it's a cost-effective upgrade.

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"conventional" PCI at 33 MHz and with 32 Bit of bus width would have its theoretical limit even lower at 133 MB/s (or ~ 1Gbps). The practical limit would be significantly lower as with many devices exchanging data over the bus (which is the case with 4 NICs sending and receiving data simultaneously) the protocol and synchronization overhead is likely to be as high as 40%, leaving just 60% of the theoretical data rate for transmissions. –  the-wabbit Oct 24 '12 at 8:15

There's never really been a need to change it since it's stable and handles all our needs.

Personally, I would be slightly worried to have a 10 years old machine as a single point of failure for a service as important as internet connectivity. Your priorities might vary of course, but a PC's life frame typically ends after 5-7 years. Mechanics (fans, HDD) are a likely candidate for a failure, although the electronics will show signs of "wear" too (electrolytic capacitors used in voltage converters all around the device might dry out over time, plug-in contacts would oxidize resulting in raising transition resistance, similar effects might be observable on low-quality soldering points).

Not much will be won in terms of either performance or reliability by replacing the NICs, although routing latency theoretically would benefit slightly due to shorter bit-times of Gigabit networks. The effect would be negligible however, as a receive-send cycle of a 512 Byte FE frame amounts to roughly 0.01 ms while average host latency is larger by the order of a magnitude at approximately 0.5 ms.

BTW: if you are looking for a cost-effective replacement and are comfortable with Linux, have a look at OpenWRT. It runs on a variety of embedded commodity hardware and allows for redundant setups. The faster MIPS- or ARM-based models (>=500 MHz CPU, >=64 MB RAM) perform rather well and would be able to do wirespeed routing (and probably also filtering, but this would depend on your ruleset) for 100 Mbps interfaces.

You would likely save the device's purchase price worth on electricity after 1 year of operation (assuming 100 W of power for your current setup and 10 W for an embedded setup you would save roughly 2 kWh each day which amounts to $100 each year at 0.15 $ per kWh).

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