LinuxSA Mailing list archives

  From: Mark Newton <newton@atdot.dotat.org>
  To  : Dale Long <dalel@loftuscomp.com.au>
  Date: Wed, 6 Dec 2000 23:46:54 +1030

Re: "Host based" routers and BGP

On Wed, Dec 06, 2000 at 09:56:40PM +1030, Dale Long wrote:

 > What defines a router? Hardware containing as OS optimised for routing
 > running only routing based services? Or A generic OS (perhaps compiled in
 > a way that optimises it for routing) running only routing based
 > processes/services? Or hardware and OS optimised for routing? Surely
 > network load and so on would dictate what would be be better solution.
 
Sigh. 

Ok, here's why you'd use a dedicated router in a routing role:

  1.  PC hardware sucks arse.  You have no idea about exactly how badly
      it sucks arse until you're an ISP with tens of thousands of customers
      who can't use the Internet because a crappy Taiwanese power supply
      has borked itself *again*.  A truly excellent reason to run a dedicated
      router is because it's not a PC, and that reason is serious enough to
      be all the convincing you should need even if you can't find any other
      reasons.

  2.  General purpose operating systems aren't good at routing.  If you
      asked a 2nd-year computer science student to come up with the most
      buggy and least efficient TCP/IP routing system they could possibly
      think of, a UNIX box with a couple of Ethernet cards in it would 
      be pretty close to the top of the heap (an NT system with a couple
      of Ethernet cards would pip it at the post, though :-).  There are
      far too many reasons for me to go into here to describe why a general
      purpose OS is bad at routing;  Allow me to summarize by drawing attention
      to overheads caused by layer upon layer of I/O abstraction which isn't
      really relevent to routing, overheads caused by trying to stuff
      tens of thousands of small, variable-sized packets per second through
      kernel buffers which are optimized to work with 8kbyte buffers, and
      the fact that there are essentially no general purpose operating
      systems which support hot-swap network interface cards.

  3.  Scalability.  The PCI bus is simply not scaleable enough to handle
      a large ISP's core.  End of story.  There is no argument on this.  If
      you're serious about network performance, you can't use a PC as a 
      router.

  4.  Features.  Want a single box to run OSPF, BGP and legacy RIP?  Simple;
      Give it a few configuration commands and you're away.  Want to add
      a bit of stuff to make transparent HTTP proxying work?  Add one more
      configuration command.  Want to set up a hot spare for redundancy and
      arrange for the failover to be completely automatic and transparent?
      Easy; there are dozens of configuration examples on www.cisco.com.
      The reason I'll pick a real router over a general purpose OS any time
      is because I can pick up a black-box piece of hardware, spend five
      minutes typing a dozen or so configuration commands, and end up with
      something which is perfectly capable of serving as an essential part
      of my network core, with no kernel compiles, no patches, no additional
      software to install, and no hassle.

  5.  Support.  Usually the open-source model wins out big time here, because
      the support given by commercial software companies is so woeful, any
      ol' peon in a newsgroup is bound to be providing better support info
      than your commercial software vendors.  Router vendors, on the other
      hand, offer truly stellar support.  If you want to make something work
      and you don't know how, call the vendor and you'll have the info in less
      than an hour.  If something is breaking, call the vendor and the bug
      will usually actually be fixed.  At work we've had Cisco software 
      engineers flying in from Sydney and Perth to look at problems we've been
      having which they haven't been able to duplicate in the lab;  Try getting
      that kind of support from any software vendor.

There are lots of other reasons, but I'm starting to get boring.  Suffice to
say that there are really very few reasons to use a PC in the role of a 
router, unless the only thing you really care about is how much it costs.
And, if you have an "important" network, you probably figure that the amount
you're prepared to spend on it has some kind of relationship to the loss
you'll suffer if it goes down and the ongoing support load to keep it running.

 > This rule can also apply to database servers, print servers, firewalls and
 > so on. But it also depends on capacity and load and security, among other
 > things.

It depends a whole lot more on exactly how important your network resources
are to you.  

Sure -- If you want a cheap solution and you don't care much about how well
it works, how suitable it is to the task, or what to do when it fails, 
cobble together some spare parts and build whatever kind of server or 
router you want, and laugh all the way to the bank -- Just don't try to
do anything critical with it, because it'll break and it'll be your fault.


   - mark

--------------------------------------------------------------------
I tried an internal modem,                    newton@atdot.dotat.org
     but it hurt when I walked.                          Mark Newton
----- Voice: +61-4-1620-2223 ------------- Fax: +61-8-82231777 -----

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