LinuxSA Mailing list archives

  From: Glen Turner <glen.turner@aarnet.edu.au>
  To  : John Edwards <isplist@pinnacle.net.au>
  Date: Thu, 07 Dec 2000 12:36:20 +1030

Re: "Host based" routers and BGP

John Edwards wrote:
> 
> PC's make up for it with grunt. While a purpose built router might have
> an efficient RISC cpu, the PC will have a cheaper x86 thing running
> around 5 to 10x faster. That more than makes up for any inefficiency,
> although extra ram and bus accesses may cause more latency.

I run both Cisco 7500-series and Linux PCs as gigabit routers,
so I think I can comment :-)

Cisco uses SRAM for packet buffering, so the CPU MHz rate is
very misleading when comparing potential packet forwarding
performance.

There are other architectural tweaks too.  For example, there
are multiple PCI buses.  And importantly the CPU can mark
control transfers so that routing table updates can occur
even though the bus is congested.  The serial port isn't
on the PCI bus, so you can configure the router when it
is hosed.

All this means that the Cisco runs at 100% utilisation
whereas the Linux PC has problems, especially with
avoiding starvation of the routing daemon (which will
cause a flapping black hole).

If you are buying a Cisco low end router and a PC to act as a
web server, then using Linux to do both is attractive.

For anything faster or more robust, a designed router is attractive.
There is a wider range of interfaces and the supporting software
for them works (unlike ATM or MPLS for Linux).

Since the person wants to multihome, they are after a degree
of reliability, so a made-to-fit router is the better choice.

If you don't like Cisco and are looking at a 7500-class router,
then the Juniper range is an alternative.

Zebra isn't really intended to be a routing daemon on a generic PC
that is also doing packet forwarding.  Rather its designed to provide
a platform for route servers (where the routing daemon runs on one
machine but forwarding is done by specialist machines) or for
Linux/BSD systems that are embedded in hardware.

Glen

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