Re: EIGRP Unequal Cost Load Balancing

Hey Dominik and team,

Interesting question.

EIGRP will calculate the paths and choose the best one. If multiple equal
cost best paths exist then EIGRP can choose more than one. Nothing new or
special about this. So when you have multiple paths, unequal ones, you can
tell EIGRP to use these other paths also. The variance command as you
mention below will allow these to be installed to the routing table.

So how does CEF learn the next hops and prefixes? It learns it from the
routing table. So there is a relation to the routing table, routing
protocol, and CEF.

CEF uses a load sharing table and distributes the traffic accordingly based
on the method you choose; it is a pretty smart protocol. By default the
distribution will be in relation to the metric, in other words it will be
proportional. I think this link will help some (watch the word wrap):

http://www.cisco.com/en/US/tech/tk827/tk831/technologies_tech_note09186a0080094806.shtml

When using CEF and EIGRP or really any load ballancing with CEF, there are
some caveats (of course ...) depending on the amount of destinations your
traffic is going to. Here is a helpful link with describes this some and in
particular discusses CEF and EIGRP:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009437d.shtml

HTH,

Andrew Lee Lissitz

On Wed, Apr 22, 2009 at 10:18 AM, Dominik Nowicki <freemaxis_at_gmail.com>wrote:

> Hi GS,
> I have doubts regarding EIGRP Unequal Cost Load Balancing.
>
> I have a questions:
>
> 1. If we have CEF enabled on both interfaces and just add variance 2
> command
> under EIGRP, then for example we achived traffic share count like 2:1, but
> how really traffic is sending to the remote devices. With CEF on interface
> and variance we have per-destination sharing load sharing.
> Does it mean that for examle if we have just one route that appears in the
> routing table with 2:1 ratio and we ping from our router (always with the
> same source IP to the same destination) then we really dont achived any
> load
> balancing because CEF sends the packets based on the created flow (src and
> dest IP, so always the same).
> So what exactly this variance + CEF do for us? Does it mean that to take
> adavantage from the variance we have to disable CEF?
>
> 2. If we add ip load-sharing per-packet command under interface then we
> have per-packet sharing under show ip cef X.X.X.X, does it mean that thanks
> to this command we disabled CEF on this interfaces and we have packets that
> are process switched.
>
>
> Thanks for clarigfications
>
>
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-- 
Andrew Lee Lissitz
all.from.nj_at_gmail.com
Blogs and organic groups at http://www.ccie.net