[c-nsp] load balance over two unequal cost links?

[Andre Beck]

On Thu, Apr 14, 2005 at 03:23:36PM -0700, Matt Bazan wrote:
> Got the following:
> 
>   A----------B
>    \        /
>     \      /
>      \    /
>       \  /
>        C
> 
> Cisco 3640's.  T1's between sites.  

So it's a redundant setup and you would be running a dynamic routing
protocol to make use of it, I assume OSPF as IGP here.
 
> I'm looking for a way to load balance traffic from site A to site B
> using the link between A <> B and A <> C <> B.

Possible, but not always a good idea. C will introduce additional
traffic on the A-C and C-B links which will cause equal distance load
sharing to have some problems with the path A-C-B (higher RTTs etc).
If you're balancing by flow or destination, that won't disturb too
much (except in the case where A-C-B is congested while there is still
free bandwidth on A-B but a certain flow/dest is going A-C-B anyway),
but with per-packet, it will cause severe packet reordering and
there are L4+ protocols/apps that choke on it.

> Can I split traffic
> based upon a particular port on the A router and load balance it using
> both paths to B from A?  Thanks.

For the assumption that it is OSPF, just make the interface costs up in
a way that A-B has the same cost as A-C + C-B. For instance, make A-B
100 and A-C/C-B 50 each. This way, you still have redundancy while also
having equal cost load balancing/sharing. Rumours are that with EIGRP
you even could do unequal cost load balancing, but that's generally
not a good idea stability-wise (and a proprietary IGP).

HTH,
Andre.
-- 
                  The _S_anta _C_laus _O_peration
  or "how to turn a complete illusion into a neverending money source"

-> Andre Beck    +++ ABP-RIPE +++    IBH Prof. Dr. Horn GmbH, Dresden <-