Here's some food for thought on reducing MTU (in an Ethernet network environment)...
You have 1500 bytes of data, with 26 bytes of framing overhead (preamble,
source/destination addressing, type/length field and CRC). Let's put one frame
containing "voice" to follow after this data goes through an Ethernet interface.
Think of the delay that might be encountered.
Now, let's take that same 1500 bytes of data, break it up into 48 byte (ATM payload
size) chunks. To transport this same 1500 byte payload, 32 transport frames are
required--each appended with 26 bytes of framing overhead. The router will now
process over 2200 bytes of data and 32 frames instead of 1526 bytes contained on one
frame. For comparison, let's let the router process the frame of "voice" after all 32
frames of data have been processed. Which would you suspect would have a higher
degree of latency?
While prioritization of traffic could be used to ensure the packet of "voice" is
processed before the 32 packets of data, why not start with prioritization as a
mechanism rather than changing the MTU? As alluded to, a smaller MTU results in an
increased overhead:data ratio resulting in a less efficient transport network. This
increases the latency on your network for other applications.
A few years ago, Alteon Networks spearheaded an initiative to increase the payload
size of Ethernet lessening the ratio of overhead to data transport--in much the same
manner used in 16 Mbps token-ring networks. While it was accepted by many vendors
(including Cisco), it didn't gain much momentum, having been overshadowed by the
development of the Gigabit standard amongst other things. It's an interesting
technology (and political challenge) that's well worth the read.
-- Leigh Anne
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
> Chuck Larrieu
> Sent: Monday, June 05, 2000 12:02 PM
> To: Robert John Lake; Clark, Jason; [EMAIL PROTECTED]
> Subject: RE: setting mtu size on a 2611
>
>
> One reason might be a move to voice over IP. While I am not really up to
> speed on this yet, I recently attended Cisco sponsored AVVID training, and
> this was a point that was made. On the internal network, having a smaller
> MTU helps greatly on the voice over side. Voice packets suffer less delay
> when data packets are smaller rather than larger. Voice packs
> don't have to
> wait around for large data packets to go through. Less delay =
> better voice
> quality.
>
> I asked specifically about the issues on the data side, and the instructor
> did point out that ATM, with a packet size of 53 bytes, was
> highly efficient
> and did not cause data services to denigrate.
>
> I suppose there is the additional overhead of fragmenting and reassembling
> large packets. And a major issue if the DF bit is set. One more
> reason never
> to set the DF bit, I suppose.
>
> Chuck
>
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
> Robert John Lake
> Sent: Monday, June 05, 2000 9:58 AM
> To: Clark, Jason
> Cc: '[EMAIL PROTECTED]'
> Subject: Re: setting mtu size on a 2611
>
> Hi,
>
> Why do you want to change the MTU size.... You are going to walk into
> serious issues if you do.
>
> Robert
>
>
>
> "Clark, Jason" wrote:
> >
> > Good Morning
> >
> > I am trying to manually set the MTU size on a 2611 and am receiving the
> > following message % Interface Ethernet0/0 does not support user settable
> > mtu." Is it not possible to manually set the MTU size on Ethernet
> > interfaces?
> >
> > TIA
> >
> > Jason
> >
> > ___________________________________
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