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Re: Load balancing in routers

  • From: Lincoln Dale
  • Date: Mon Apr 08 05:04:10 2002 
At 10:50 AM 8/04/2002 +0200, Iljitsch van Beijnum wrote:

> load balanced across those links. Some mechanisms (for example Cisco CEF)
> can do this on a per-destination (flow-based) basis, to prevent packet
> reordering.

I seem to remember fast switching was per-destination, and CEF was
round robin. But it seems CEF is now per-destination as well in IOS 12.2.
Round robin is optional.
you remember incorrectly.

by default, CEF uses a hash based on both src & dst to determine the path to take.
somewhat
paradoxically this is referred to as "per-destination" load-balancing (or "deterministic").

on many platforms, you can reconfigure CEF to use a per-packet distribution.

"ip load-sharing XX" is the interface command to set the policy.

"per-destination" historically is what fast-switching used to do -- and it did a particularly bad job of handling large amounts of traffic sourced to one ip-address (such as a news-server or proxy-server). this was particularly apparent for multiple <E1 links load-balanced using equal cost routes.


cheers,

lincoln.