t3 MTU size versus a 100MBps MTU...

t3 MTU size versus a 100MBps MTU...

Post by Bill B » Thu, 06 May 2004 00:22:17


Hello,
Today we have a T3 link that connects two buildings together. We are
currently using an MTU of 4352 for this particular link. I am considering
replacing this WAN T3 link with a true 100Mbps Ethernet link (Which would
now set my MTU size down to ~1500) to connect the same two sites. Currently
our T3 link is being utilized no more the 45% at any one time. The benefits
of having a higher MTU, as I understand it, is that more stations could talk
at once... or better put, instead of everyone standing in one long line to
make it across the WAN link, there are multiple lines (I guess 4:1 or 3:1)
that can be used. We currently have Catalyst 6006s on both sides. I guess my
questions are:

- Am I describing the benefits of having a higher MTU correctly?

- Should I expect Higher Latency, and more bandwidth? Or Same Latency and
more bandwidth?

- Are my buffers for the Ethernet ports going to possibly overload with
these 3 or 4 lines being squeezed into one?

- Should I do this <grin>?

- Where should I look for information into this?

Thank you in advance...

Bill Bushong
 
 
 

t3 MTU size versus a 100MBps MTU...

Post by roberso » Thu, 06 May 2004 02:14:14

In article < XXXX@XXXXX.COM >,

: Today we have a T3 link that connects two buildings together. We are
:currently using an MTU of 4352 for this particular link. I am considering
:replacing this WAN T3 link with a true 100Mbps Ethernet link (Which would
:now set my MTU size down to ~1500) to connect the same two sites. Currently
:our T3 link is being utilized no more the 45% at any one time. The benefits
:of having a higher MTU, as I understand it, is that more stations could talk
:at once... or better put, instead of everyone standing in one long line to
:make it across the WAN link, there are multiple lines (I guess 4:1 or 3:1)
:that can be used.

:- Am I describing the benefits of having a higher MTU correctly?

Not even close.

When there is a large burst of data to send (e.g., ftp, but not
telnet or NFS version 1) then having a large MTU allows more of that
data to be sent at one time. In some cases, that can make a big difference
for performance, as you eliminate the packet-header overhead and
intra-packet gap times and reduce fragmenting.

If there does not happen to be large bursts of data to send, then
the longer MTU makes no difference.

When a large burst of data is sent by virtue of the larger MTU, then
everyone else who is trying to send data has to wait longer for
their turn, because it takes longer to send the larger packet over the link.
Especially if the large packets tend to come from one particular source
and not the other sources, that -increases- latency for everyone else.

Whether you should expect lower latency or not when you go to
an MTU of 1500 is not a simple question. It depends on how your
link is being used. For example, if those large packets are not
sent often, but there a lot of smaller packets, then sending that
large packet in a burst might then allow that particular sender
to go quiet, leaving less contention for the line, and less collisions.
With the smaller MTU, then that large packet would have to be
fragmented, potentially leading to increased collisions as that
sender tries to gain primacy on the link. But the timings are
crucial to the behaviour -- if you research the topic of the
"ethernet capture effect" you will see some hints of how the
exact packet timings can make a big difference in how different
systems are granted access to the line.


You don't happen to mention whether the new line will be full duplex or
half duplex. Switches and a full duplex link should result in orderly
queuing and buffering rather than in the kind of contention scenarios I
describe in the previous paragraph. The capture effect occurs only
with shared collision domains, which doesn't happen if you are fully
switched, every NIC going directly to a switch port. The capture effect
-can- occur if you have hubs, or if you have a shared media such as
thinwire instead of Cat5.
--
Take care in opening this message: My grasp on reality may have shaken
loose during transmission!

 
 
 

t3 MTU size versus a 100MBps MTU...

Post by Barry Marg » Thu, 06 May 2004 03:51:50

In article <c78j16$an9$ XXXX@XXXXX.COM >,




However, if the MTU of the inter-building link is larger than the MTUs
of the LANs within the buildings, the extra MTU will never be used.
Routers don't ever combine frames, so if the devices on the LANs never
send anything larger than 1500 bytes, the frames sent over the T3 will
never be larger, either.

--
Barry Margolin, XXXX@XXXXX.COM
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
 
 
 

t3 MTU size versus a 100MBps MTU...

Post by Captai » Fri, 14 May 2004 05:56:37

On Tue, 04 May 2004 14:51:50 -0400, Barry Margolin






Interesting point!

I use a MTU of 1500 thoughout my network, but most users connect
to me through dsl connections,(terminating on a FastEthernet card on
cisco 3640 box).

I heard that adsl use a smaller MTU.

Should I lower the MTU on my 3640,(and thoughout my network)?
 
 
 

t3 MTU size versus a 100MBps MTU...

Post by roberso » Fri, 14 May 2004 06:41:41

In article < XXXX@XXXXX.COM >,

:I use a MTU of 1500 thoughout my network, but most users connect
:to me through dsl connections,(terminating on a FastEthernet card on
:cisco 3640 box).

:I heard that adsl use a smaller MTU.

Often use, but not always. Consumer-level ADSL often uses PPPoE, but
business level doesn't always, especially for SDSL.

:Should I lower the MTU on my 3640,(and thoughout my network)?

If you never use the jumbo packets, you might as well lower the MTU
to long enough to support tagged (possibly MLPS) frames, but there's no
point in going any higher. Just confuses people about what's going on
over the link. But other than the confusion, it doesn't really hurt.
(But it might potentially get you into a different pricing model to drop
down to MTU ~ 1500, as that opens up more technologies.)
--
*We* are now the times. -- Wim Wenders (WoD)
 
 
 

t3 MTU size versus a 100MBps MTU...

Post by Captai » Fri, 14 May 2004 06:59:10

On 12 May 2004 21:41:41 GMT, XXXX@XXXXX.COM (Walter






Yes, we are using PPPoE,(I now it's smaller then 1500, but I'm not
sure what it is exactly)?

Is there any standard out there?
 
 
 

t3 MTU size versus a 100MBps MTU...

Post by roberso » Fri, 14 May 2004 07:05:31

In article < XXXX@XXXXX.COM >,

|On 12 May 2004 21:41:41 GMT, XXXX@XXXXX.COM (Walter


|>In article < XXXX@XXXXX.COM >,


|>:I heard that adsl use a smaller MTU.

|>Often use, but not always. Consumer-level ADSL often uses PPPoE, but
|>business level doesn't always, especially for SDSL.

|Yes, we are using PPPoE,(I now it's smaller then 1500, but I'm not
|sure what it is exactly)?

|Is there any standard out there?

Not really. 1492 is common, but it depends on the ISP. I've seen 1450
as common, and I've seen MTUs as low as 1392 for some of them.

My residential ISP used to use 1476, but now allows higher.
--
Are we *there* yet??
 
 
 

t3 MTU size versus a 100MBps MTU...

Post by Captai » Fri, 14 May 2004 08:14:08

On 12 May 2004 22:05:31 GMT, XXXX@XXXXX.COM (Walter







1472 looks like the size that our local cable company uses.