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Posted by anoop on October 8, 2006, 10:56 am
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Timo Neuvonen wrote:
> Currently I have two offices connected with a single G.SHDSL
> link over a leased line, the link operates at 2.3 Mbps
> which is a good result with this old cable having length
> of appr. 6 km (~20000 ft).
>
> Now I'm trying to increase the (total) link speed, and
> one way would be using eg. 4 lines with 2.3 Mbps each,
> with similar cheap consumer-class(* modems, and
> combine these links with port trunking (a.k.a link
> aggregation?) in ethernet switches.
>
> Now, the problem:
> The connection is mostly used by a single computer at
> a time, to connect to a file servers etc located at the
> main office.
What protocol are they using?
> The following citation is from
> Allied Telesyn's AT-FS750/16 manual:
>
> "----
> A port trunk always sends packets from a particular source
> to a particular destination over the same link within the trunk.
> A single link is designated for flooding broadcasts and packets
> of unknown destination.
> ----"
This is done so that packets from a single flow don't get
reordered. This can happen when links are of unequal
length, or packets within a given flow are of different size
(e.g. a max sized frame which gets put on one link
followed by several min-sized frames on the other
links; the min-sized frames would be received before
the max-sized frame). Reordering can cause problems with
some protocols, and with TCP, it could potentially lead
to retransmissions resulting in loss of performance.
As a result typical switches allow you to do load balancing based
on one or more of the following criteria:
- The source/destination MAC addresses.
- The source/destination IP addresses (if the packet is IP).
- The source/destination transport port (if the packet is TCP or UDP).
> How I see this, it would effectively keep one line fully used,
> and the other lines free, since there would be only two
> machines communicating with each other.
That's correct.
> ****
> Is anyone aware of ethernet switches that I could use to
> overcome this limitation?
>
> In other words, switches that would effectively use all the (slow)
> lines in a port trunk also while there are only two computers
> communicating with each other?
> ****
>
> What I'm looking for is preferably a lower-end (less than $500
> each) switch with 8 to 16 ports, not a huge modular system
> costing thousands of dollars.
>
>
> *) I would prefer using as cheap terminal equipment as
> possible, since in this area lightning strikes breaking
> any equipment connected to public telephone network
> cables are far too common :-(
Cheap switches typically won't look deep enough in the
packets. But regardless, if you have only two machines
talking to each other with one active flow between them
(defined by a combination of the 3 criteria mentioned above),
then it would be hard to find a switch that will be able to
load balance the traffic. You would have to find a switch
that does some form of proprietary load balancing. I'm
not aware of any.
On the other hand, if the two machines are talking
over several flows, i.e. different transport ports, then
you might want to look for a switch that can load balance
traffic based on the source and destination ports in
addition to the IP addresses. With that you would get
some amount of load balancing although it would be
far from perfect.
Anoop
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