[46658] in North American Network Operators' Group
Re: Load balancing in routers
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Mon Apr 8 04:50:19 2002
Date: Mon, 8 Apr 2002 10:50:29 +0200 (CEST)
From: Iljitsch van Beijnum <iljitsch@muada.com>
To: Richard A Steenbergen <ras@e-gerbil.net>
Cc: abhijit bare <abhi1999us@yahoo.com>, <nanog@merit.edu>
In-Reply-To: <20020407225117.GH523@overlord.e-gerbil.net>
Message-ID: <20020408103623.X76423-100000@sequoia.muada.com>
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Errors-To: owner-nanog-outgoing@merit.edu
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
