From: operator sid (ccie1@live.co.uk)
Date: Wed Jan 14 2009 - 15:55:27 ARST
Thanks Paul and group, this really helps explain it, can you just clearify
few more things..
If a jumbo frame hits a device which has less MTU, will it by default drop it
or would it fragement it..
Or does this depend whether fragmentation is enabled ?
And if it does drop it, would it send icmp to host to tell it to send smaller
frames.. or would the packets keep continuously dropping
Also you mentioned path MTU discovery does this only need to be enabled on the
end host (something you configure on the nic) or also on the transit network
devices.. and it sound like its good to enable it anyway (and allow the ICMP
through firewalls) to prevent packet drops dont you think ? are there any
security concerns for enabling it.
> Date: Wed, 14 Jan 2009 16:07:19 +0000> From: paul.cosgrove@gmail.com> To:
ccie1@live.co.uk> Subject: Re: Please explain Jumbo Frames> CC:
pitt2k@gmail.com; ccielab@groupstudy.com> > If end hosts send very large
frames, both the endpoint NICs and the network> equipment between them should
support those large frames. When performing> large file transfers on reliable
links, sending larger chunks can improve> performance as you say. You should
be careful though to ensure that the> packet the host is sending, plus any
required overhead (e.g. MPLS tags), is> supported throughout your network.> >
Keep in mind though that if one of the intermediate (transit) IP network>
devices has a lower configured MTU, then your large packets will be forcing>
that device to perform fragmentation - which will increase its CPU. This> can
cause real problems. Path MTU Discovery running on the end hosts helps> them
determine the maximum end to end MTU supported across transit IP links,> which
helps to avoid this problem (unless administrators block the ICMP type> is
needs as is often the case). If your Layer 2 transit interfaces do not>
support the required MTU then you are not so lucky, as your large frames> will
be silently dropped.> > Depending on how your network gets the packets between
the two endpoints, it> may need to temporarily increase the size of those
packets. If you are> tunneling, you add a tunnel header which increases the
size of the frames.> If the MTU used on your transit network does not allow
for this additional> overhead, then you may again cause IP fragmentation (or
frame drops). So> it is common for a S.P. network to have larger configured
interface MTUs> internally than those configured on customer device
interfaces.> > Transit devices do not bundle together smaller packets to make
Giants. To> do so would be too heavy on processor and buffer resources when
there could> potentially be many different flows on the same link. You would
also be> introducing additional delay to all the bundled packets, and
increasing the> risk of their being dropped further on, without much real
benefit. Doesn't> happen yet as far as I know (but that approach may have
benefits for> tunneling protocols).> > On Switches, Gig ports commonly support
MTUs of around 9000 bytes, with fast> ethernet ports supporting 1546. You will
find that these vary a little> between vendors and in some IOS versions; for
instance I think the Jumbo MTU> of 3750 SEB4 was higher than other releases.
Packets less than 1560 are> referred to as Baby Giants (according to cisco
docs - though I'm not sure if> that figure also varies between vendors).
Larger packets are Giants.> > Paul.> > On Wed, Jan 14, 2009 at 2:57 PM,
operator sid <ccie1@live.co.uk> wrote:> > > so does that mean that Jumbo
frames are generated by the PC Network adapter> > and jumbo frames must be
enabled end to end through the network for them to> > work.> >> > Also can
jumbo frames be used to consolidate smaller frames into bigger> > jumbo> >
frames at the distribution/core for transport ??> >> > ive read a few
documents but still not clear on this. can someone please> > explain...> >> >
> Date: Wed, 14 Jan 2009 15:08:15 +0100> From: pitt2k@gmail.com> To:> >
ccielab@groupstudy.com> Subject: Re: Please explain Jumbo Frames> >> >
(...)While> > it may seem that setting the MTU for a network adapter is>
sufficient to> > use> > jumbo frames (assuming the operating system supports>
it), the reality is a> > lot more complex.> In a controlled network
environment, purchasing> > equipment> > with> identical jumbo frame support
may allow the use of large frames.>> > However, such equipment is typically
much more expensive than>> > standard-only> > equipment(..)> >
http://www.asicdesigners.com/jumbo_enet_frames.html> >> > HTH,>> > Piotr> >
2009/1/14 operator sid <ccie1@live.co.uk>:> > Hi,> >> > can> > someone> >
please explain how and where jumbo frames are generated. I know> > that> >
jumbo> > frames help increase throughput on gig links, but where and how are>
>> > these> > jumbo frames created. Surely there is no point in enabling jumbo
frames> >> > on> > ports, when there are no jumbo frames traversing the
network in the first>> > >> > place.> >> > For example if an application is
sending data across the> > network,> > does the> > application have to
generate jumbo frames, or can smaller> > frames> > be> > consolidated into
jumbo frames on the distribution and core switches> > and> > then> > broken
down into the small frames down at the access.. If so how do> > you> >
configure this.> >> > Please can someone clearify..> >> > Thanks in> >
advance> >> >> >> >
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