Chuck,
Round times will be roughly the same regardless of whether there's 1
T1 or 8 T1's in the multilink bundle. There is a limit to the speed
bits will move in copper.
However, the more T1's you have in the bundle, the more bits you can
send at the same time.
I'd suggest you retry your test using the "ttcp" command, which tests
TCP throughput rather than latency. Throughput and latency are two
decidedly different beasts.
I've extensively tested Multilink PPP in the lab (since cisco broke CEF
per-packet load-sharing on OC48 Line cards), and I can verify that
Multilink PPP does increase throughput.
I might be able to dig out some of my data, if you're interested.
Regards,
--phil
On Mon, Oct 01, 2001 at 11:33:39PM -0400, Chuck Larrieu wrote:
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:
http://www.groupstudy.com/form/read.php?f=7&i=21661&t=21623
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