[c-nsp] Equal cost load balancing between geographically dispersed sites

[David Barak]

--- Dale Shaw <dale.shaw+cisco-nsp at gmail.com> wrote:

> At each distribution switch ("dist1" and "dist2"), I
> need to be able
> to load balance traffic out to the WAN. At this
> stage I'm only
> concerned with outbound (LAN to WAN) load balancing.
> This is the only
> key requirement - effective use of both (expensive)
> WAN links is my
> goal.
> 
> There is no requirement to do traffic load balancing
> in other areas of
> the network.
> 
> Irrespective of the source of the traffic destined
> for the WAN, it
> should be load balanced between the two WAN routers.
> This means
> traffic could be sourced from "MAN office A" or "B"
> or "C" or the
> "servers" cloud or from any of a number of other
> sites in the MAN.
> This means I may need to send traffic arriving at
> the distribution
> switches back out the interface it came in on.

Stepping back a second, let's look at an assumption
which seems implicit in the design:

Is having two WAN links and two WAN routers designed
to give you 1+1 redundancy?  If so, you can't run more
than 1 full link's worth of traffic without losing
your redundancy, so all of this work to do load
balancing becomes additional complexity (and possible
additional latency/jitter/asymmetry) for either no or
very small benefit.

If all of your traffic will NOT fit inside a single
link, and you are okay with not having enough capacity
in the event of a failure, there still may be an
easier way than trying to get a 50/50 split: whatever
classification mechanism you would use to deal with
the oversubscription in the event of a failure can be
put to work with PBR, or you could just route traffic
out the closest exit point, and end up with an unequal
division, but still use both links.

Is there a technical reason you want a 50/50 split?

-David Barak

David Barak
Need Geek Rock?  Try The Franchise: 
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