[30484] in North American Network Operators' Group
Re: question about traffic eng/ Cisco CEF.
daemon@ATHENA.MIT.EDU (Lincoln Dale)
Wed Aug 9 00:55:12 2000
Message-Id: <4.3.2.7.2.20000809004305.04e9cae0@mailhost.interlink.com.au>
Date: Wed, 09 Aug 2000 00:50:50 -0400
To: Jaideep Chandrashekar <jaideepc@cs.umn.edu>, nanog@merit.edu
From: Lincoln Dale <ltd@interlink.com.au>
In-Reply-To: <Pine.SOL.4.21.0008081545420.22522-100000@plato.cs.umn.edu>
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At 16:47 08/08/00, Jaideep Chandrashekar wrote:
>When I enable cef and set it to share load on a per destination mode, for
>some reason, all the traffic to a destination takes a single route
>.. though the route table shows two equal cost paths.
CEF (by default) will load-balance based on a hash of the src/dst
ip-addresses for a flow ("per-destination load balancing").
this means that a single ip flow will always take the same [pre-determined]
path. (actually, strictly speaking, it'll always take the path that is
reflected by the hash index).
this is generally considered to be 'a good thing<tm>' as it help avoid
out-of-order packets and the like.
due to the fact that a link typically has hundreds/thousands/millions of
flows/sec across it, you will find that the load-balancing is very even
under normal [internet] traffic loads.
you can make CEF perform "per packet" load-balancing using the interface
command "ip load-sharing per-packet".
>Also ... is there any way to clear the routing table so that when a new
>link comes up, (and a new route is added to a list of equal cost
>paths) all the connections are again redistributed across the
>links. Couldn't seem to be able to make existing connections switch
>paths. With fast switching ... was able to do this by flushing the
>routecache .. but cef doesn't use a cache (says the documentation)....
if you use per-packet load balancing this will occur automatically (without
any requirement to clear the route-cache).
for per-destination load balancing, this will also occur, but with 'some'
of the flows picking the new path if the hashing determines it that way.
under normal 'internet' traffic-patterns, you can happily keep
load-balancing at per-destination. only if your traffic patterns are such
that you have a small number of long-duration flows does per-packet
load-balancing become necessary.
cheers,
lincoln.
