This is just a pre-morning-coffee thought. I'm thinking that multiple
links only help if you're actually needing the extra bandwidth. If
you're not generating more than 1.544 Mbps of traffic, I would expect
the round-trip times to be at least fairly similar regardless of which
configuration you use. I'm guessing that if you were somehow able to
generate a lot more traffic across those links that you'd see more of a
benefit from the second link.
I think a good test would be if you could generate around 2Mbps or more
of traffic. I wish I had some testing software that would generate
traffic in a controlled manner. It would be interesting to test this
out and see what happens.
Okay, back to my first cup of coffee. :-)
Regards,
John
>>> "Chuck Larrieu" 10/1/01 9:33:39 PM >>>
A couple of weeks ago there were a couple of discussions on this board
about
using multiple T1's to improve data throughput. If memory serves, there
were
two possible ways to do this: 1) per packet load sharing and 2) PPP
multilink
for no particular reason I decided to do a little study on PPP
multilink.
Well, OK, I do have two particular reasons - an upcoming Lab and a
customer
who is asking about this.
So, I build a scenario as follows:
serial0 token ring
R6--------R5-----------R4
|--------|
serial1
to test throughput, I used extended ping, with multiple pings and
various
size payloads, from a loopback on R4 to a loopback on R6.
the routing protocol was EIGRP, done to assure per packet routing
between R6
and R5 as a control.
My results were interesting, to say the least. unexpected, but so
consistent
that there is no question, in my mind, anyway, about some of the
assumptions
many of us make about various load sharing and multiplexing options.
a summary of the results are using the Cisco router reporting of
min/avg/max round trip times - the middle number is the one to watch.
packet size PPP multilink single serial link configured as PPP
multilink
1000 24/24/132 20/20/104
1500 28/29/52 24/27/112
500 16/19/64 12/13/104
64 12/14/60 4/7/104
note that in every case, the single link, configured for PPP multilink,
is
SIGNIFICANTLY faster than the dual link.
Interesting. So I constructed some further experiments, using extended
ping,
multiple packets of variable size - range 64 to 1500:
PPP multilink per packet load share single T1
8/17/136 4/17/136 4/17/144
these figures are from over 15,000 pings per scenario, so it is not a
case
of random chance here. there is no difference whatsoever between the
results
of a single serial link, per packet load sharing over two serial links,
and
PPP multilink. what is most surprising is that a single serial
connection
proves JUST AS FAST as a dual serial connection.
Now what I conclude from this is an opinion that multiple T1's DO NOT
really
do much for you in terms of more bandwidth. At least for the kinds of
data
flows I am able to generate in the lab. Furthermore, PPP multilink is
actually harmful to throughput. So I gotta ask - is load sharing
really
adding anything to the mix? Really? In real world scenarios and data
flows,
where is it that you are gaining anything?
Lastly, I set up a final scenario in which I sent 5000 byte packets.
this
means fragmentation and reassembly would occur, because the MTU on all
wan
interfaces is 1500 bytes. Here are the results when pinging 5000 times
using
a 5000 byte payload:
single serial link: 64/66/168
per packet load share: 64/64/168
ppp multilink: 48/52/172
note here that the load sharing scenario is slightly faster than the
single
serial link, and that the ppp multilink is FAR AND AWAY faster that
the
other two. I suspect the reason for this is efficiencies gained under
the
multilink scenario when fragmenting and reassembling the oversized
payloads
In any case, I hope this presentation will lead to some good discussion
of
bandwidth and results. would it be fair to suggest that peoples'
efforts to
solve what they perceive as bandwidth issues by implementing multiple
WAN
links is really a study in fruitless activity?
Maybe I should have set up some IPX scenarios?
Chuck
Message Posted at:
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