Routers vs. PC's for routing - was list problems?

Thu May 23 18:35:15 UTC 2002

At 02:20 PM 5/23/2002 -0400, you wrote:
>Vinny Abello wrote:
> >
> > I would have to say for any Linux/BSD platform to be a viable
> > routing solution, you have to eliminate all moving parts or as much
> > as possible, ie. no hard drives because hard drives will fail. Not
> > much you can do about the cooling fans in various parts of the
> > machine though which routers also tend to have. Solid state
> > storage would be the way to go as far as what the OS is installed
> > on. You have to have something to imitate flash on the common
> > router. Otherwise, if you can get the functionality out of a PC,
> > I say go for it! The processing power of a modern PC is far beyond
> > any router I can think of. I suppose it would just be a matter of
> > how efficient your kernel, TCP/IP stack and routing daemon would
> > be at that point. :)
>
>I've several comments here.
>
>First off, you're right about moving parts generally being a bad thing.
>However, it is not always necessary to eliminate the hard drive.  Two
>drives in a RAID-0 configuration may be reliable enough.  Especially if
>the failure of a single drive sets off sufficient alarms so that it can
>quickly be hot-swapped for a new drive.

I'm assuming you meant RAID-1. In RAID-0 if you 'swapped' any drive all 
your striped data is toast. ;)

>The real problem with using PCs is bandwidth and hardware reliability.
>
>PCs generally don't have many hot-swappable parts.  You can get hot-swap
>hard drive assemblies without too much work, and redundant power
>supplies can be purchased.  A motherboard that allows hot-swapping of
>PCI cards (usually by having multiple busses and a mechanism to turn
>them off individually) is very rare.  You can get dual-CPU motherboards,
>but not with the processors being hot swappable.  And I don't know if
>any will allow the system to transparently fail over from one CPU to
>another, should the primary fail.

I agree with you on that. Hot swapability for various interfaces is 
something routers obviously have over PC's.

>Then there's the issue of the PCI bus.  Standard PCI (32-bit 33MHz) has
>a theoretical maximum bandwidth of about 1Gbit/s.  But you can never use
>all of a PCI bus's bandwidth, so actual limits will be less than this.

True... unless going for 64 bit PCI at 66MHz... still it's obvious that 
routers are designed for one simple purpose and generally have larger 
backplanes to handle that.

>When you're doing software routing, every packet must cross the bus
>twice - once for the receive and once for the send.  So your standard
>PCI bus (if used for nothing but packets) has a top speed of 500Mbit/s.
>Which is less than three ports of full-duplex fast Ethernet at line
>rate.  Multiple busses and/or 64-bit 66MHz PCI can increase this limit,
>but now you're talking about much more expensive motherboards.
>
>You can dramatically improve throughput if you can get line cards that
>have on-board forwarding chips.  If cards can forward packets between
>each other without getting the CPU involved, then packets can cross the
>PCI bus only once.  But these kinds of line cards are not cheap, if they
>can be found at all.  And you will need some way of downloading your
>kernel routing table into the cards, which may require some serious OS
>hacking.
>
>In other words, a PC equipped to be as reliable and capable as a decent
>router will likely end up costing as much as a router.  And the reason
>has nothing to do with the CPU speed or the operating system.

I agree a router is probably more efficient in just routing packets, but in 
complex filtering or traffic manipulation/packet sniffing, a PC might have 
the edge. :)

Don't get me wrong. I would never use a PC for a router, just as I wouldn't 
try to run my workstation as a 7206. ;) It's all just hypothesizing.

Vinny Abello
Network Engineer
Server Management
vinny at tellurian.com
(973)300-9211 x 125
(973)940-6125 (Direct)

Tellurian Networks - The Ultimate Internet Connection
http://www.tellurian.com (888)TELLURIAN