Load balancing in routers

Mon Apr 8 08:50:29 UTC 2002

On Sun, 7 Apr 2002, Richard A Steenbergen wrote:

> Layer 3 devices usually do a form a load balancing called "equal cost"
> forwarding. If you have two routes to a single prefix (say you have two
> physical links), and both have the same routing "cost", packets may be
> 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.

> But some protocols can't support this, for example UDP or ICMP
> traceroutes usually don't get grouped into a "flow", so you can see this
> kind of load balancing in practice on the internet when you get back
> traceroute answers from different probes on the same hop.

Routers usually don't really take full flow information into account, but
only look at the destination IP address or do a hash over some fields. So
usually traceroute doesn't behave differently from regular traffic.

This link answers the original question for another router vendor:

http://www.juniper.net/techpubs/software/junos51/swconfig51-policy/html/policy-actions-config10.html#1015470