Re: question about traffic eng/ Cisco CEF.

[Lincoln Dale <ltd () interlink com au>]

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.