Load balancing in routers

Richard A Steenbergen ras at e-gerbil.net
Mon Apr 8 17:56:40 UTC 2002


On Mon, Apr 08, 2002 at 12:58:46PM -0400, Alex Rubenstein wrote:
> 
> > Paul's statement about CEF is interesting. It's probably the first public
> > statement I've ever heard where someone was praising CEF. Usually
> > discussions about CEF are accompanied by liberal amounts of swearing...
> 
> I dunno; except for some silliness in 12.1(8a)E[1-4] on a MSFC2, we've
> seen general goodness from CEF from 2600, 3600, 4700, 5300, 7200, 7500.
> 
> Then again, we're not UU or Sprint, and don't have the traffic loading
> they do.

Well most complaints revolve around distributed "dCEF", CEF itself it just
the (tm) of using an mtrie data structure to create a prefix table that is
pre-populated and optimized to be used for forwarding only (a FIB).

When you use dCEF, the individual linecards or VIPs each have their own
copy of the FIB, and make the forwarding decisions on their own processor
without having to consult the central route-processor. The problem comes
when you have a routing change (I've never calculated the average rate of
BGP churn but I'd guess its at least 1-2 changes per second) and need to
rebuild the FIBs, sometimes the individual line cards get "confused" and
put the wrong destination in their FIB. Thus traffic coming in on a
specific source linecard starts being forwarded to the wrong destination
interface, which can make for some very difficult diagnosis (and the
aforementioned swearing).

But other than that, CEF works just fine. Newer architectures (for example
Juniper, which does its FIB work in the IP2) don't have any other legacy
route-caches at all. One of the benefits of having a guaranteed FIB for
doing all longest prefix match lookups is that you can design your RIB so
it is optimized for what it does most, insertions and deletions. Many RIB
applications improve greatly when they no longer need a Patricia tree.

To quote Avi Freedman, "Customer Enragement Feature".
To quote Majdi Abbas, "John Chambers owes me a pony".

-- 
Richard A Steenbergen <ras at e-gerbil.net>       http://www.e-gerbil.net/ras
PGP Key ID: 0x138EA177  (67 29 D7 BC E8 18 3E DA  B2 46 B3 D8 14 36 FE B6)



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