Jumbo Frames - over cross-over cable, and over GigE switch

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

Hi,

I was talking to someone yesterday about some testing that I've been
doing with between a pair of computers with Intel GigE NICs. I've been
doing some of my testing with a cross-over cable and some other testing
going through a GigE switch.

Re. the cross-over cable connection, he said that the NICs may not be
able to negotiate Jumbo Frames when they're connected with a cross-over
cable, i.e., they needed to be talking to a GigE switch.

Is this true? I haven't heard this before.

Also, re. the GigE switch: In general, assuming that the GigE switch
can support Jumbo Frames, do GigE switches need to be configured
specifically to enable support for Jumbo Frames? Or, is this usually
automatically detected/enabled by the switch(es)?

Thanks,
Jim

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

ohaya <ohaya@cox.net> writes:

>Re. the cross-over cable connection, he said that the NICs may not be
>able to negotiate Jumbo Frames when they're connected with a cross-over
>cable, i.e., they needed to be talking to a GigE switch.

>Is this true? I haven't heard this before.

It is complete news to me that the use of jumbo frames (or other
frame size contraints) would be in any way negotiated between
communicating Ethernet stations, with or without using a switch.

>Also, re. the GigE switch: In general, assuming that the GigE switch
>can support Jumbo Frames, do GigE switches need to be configured
>specifically to enable support for Jumbo Frames?

Yes.

best regards
Patrick

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

Patrick Schaaf wrote:
>
> ohaya <ohaya@cox.net> writes:
>
> >Re. the cross-over cable connection, he said that the NICs may not be
> >able to negotiate Jumbo Frames when they're connected with a cross-over
> >cable, i.e., they needed to be talking to a GigE switch.
>
> >Is this true? I haven't heard this before.
>
> It is complete news to me that the use of jumbo frames (or other
> frame size contraints) would be in any way negotiated between
> communicating Ethernet stations, with or without using a switch.

I was kind of surprised to hear that myself ...


> >Also, re. the GigE switch: In general, assuming that the GigE switch
> >can support Jumbo Frames, do GigE switches need to be configured
> >specifically to enable support for Jumbo Frames?
>
> Yes.


Thanks. That might partially explain something that I'm puzzling over.
I had set Jumbo Frames in the (Intel) NICs to 9014 bytes, and tried
some "ping -f" tests, and am getting a "fragmented" message for
anything with "-l" greater than 1472 bytes.

I've been searching through the documentation for the switch (a GigE
switch that's been re-branded by IBM), but I can't find anywhere to
enable/disable Jumbo Frames.

Assuming that there IS somewhere in the GigE switch configuration to
enable/disable Jumbo Frames, would the Jumbo Frames being disabled in
the switch cause the "ping -f" test to behave this way?

Jim

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

ohaya wrote:

(snip)

> Thanks. That might partially explain something that I'm puzzling over.
> I had set Jumbo Frames in the (Intel) NICs to 9014 bytes, and tried
> some "ping -f" tests, and am getting a "fragmented" message for
> anything with "-l" greater than 1472 bytes.

You set the NIC to 9014, but what is the MTU that IP is
using on the interface? Fragmentation is normally an IP function.

> I've been searching through the documentation for the switch (a GigE
> switch that's been re-branded by IBM), but I can't find anywhere to
> enable/disable Jumbo Frames.

> Assuming that there IS somewhere in the GigE switch configuration to
> enable/disable Jumbo Frames, would the Jumbo Frames being disabled in
> the switch cause the "ping -f" test to behave this way?

As far as I know, only a layer three switch would do IP
fragmentation. Though there are stories about ethernet/FDDI
bridging doing it, also. (FDDI allows frames larger than 1500,
I believe somewhere around 4096.)

Where are the messages coming out?

-- glen

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

ohaya <ohaya@cox.net> writes:

>Thanks. That might partially explain something that I'm puzzling over.
>I had set Jumbo Frames in the (Intel) NICs to 9014 bytes, and tried
>some "ping -f" tests, and am getting a "fragmented" message for
>anything with "-l" greater than 1472 bytes.

On both machines:

ifconfig eth0 mtu 9014

>Assuming that there IS somewhere in the GigE switch configuration to
>enable/disable Jumbo Frames, would the Jumbo Frames being disabled in
>the switch cause the "ping -f" test to behave this way?

No. Packets would appear to be sent from the local machine, but mysteriously
disappear in the switch. Some counter you'll find in about five weeks while
studying documentation, will have increased by one.

best regards
Patrick

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

Patrick Schaaf wrote:
>
> ohaya <ohaya@cox.net> writes:
>
> >Thanks. That might partially explain something that I'm puzzling over.
> >I had set Jumbo Frames in the (Intel) NICs to 9014 bytes, and tried
> >some "ping -f" tests, and am getting a "fragmented" message for
> >anything with "-l" greater than 1472 bytes.
>
> On both machines:
>
> ifconfig eth0 mtu 9014
>
> >Assuming that there IS somewhere in the GigE switch configuration to
> >enable/disable Jumbo Frames, would the Jumbo Frames being disabled in
> >the switch cause the "ping -f" test to behave this way?
>
> No. Packets would appear to be sent from the local machine, but mysteriously
> disappear in the switch. Some counter you'll find in about five weeks while
> studying documentation, will have increased by one.
>
> best regards
> Patrick


glen and Patrick,

I'm going to respond to both of your posts in this one post. Hope that
that's ok.

Sorry that I forgot to mention that both machines are running Windows
2003.

Patrick, your comment above about the packets appearing to mysteriously
disappear might explain something else that I found when I did more
testing yesterday.

When I did "ping -f" from machine 1 to machine 2, I got timeouts. When
I did "ping -f" from machine 2 to machine 1, I got the fragmentation
msg.

Now, with your post, I'm wondering if MTU is set larger than 1500 on one
machine and not the other?

glen and Patrick, re. MTU, does that mean that I should have set the
MTU? In my case, with Windows 2003, I think that there is an "MTU" item
that I can set in the Registry, but in Windows, I think it's under the
individual "Interfaces" or "Adapters".

I'll try that, but in any event, since I'm getting the timeouts when
doing the "ping -f" from machine 2 to machine 1, does that seem to
indicate that the GigE switch is NOT passing the Jumbo Frames?

Maybe I should go back and test with the cross-over cable again, but
this time manually set the MTU? I had given up on that earlier because
of the comment that I was told that the NICs might not negotiate the
Jumbo Frames when going over a cross-over.

Jim

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:
> As far as I know, only a layer three switch would do IP
> fragmentation.

In which case it should be called a router

> Though there are stories about ethernet/FDDI bridging doing it,
> also. (FDDI allows frames larger than 1500, I believe somewhere
> around 4096.)

More like nightmares - because the device wanted to be "just a little
bit IP/pregnant" and didn't support all the things it must for an IP
router. Should all be in the netnews archives from the mid '90's and
earlier.

rick jones
--
denial, anger, bargaining, depression, acceptance, rebirth...
where do you want to be today?
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to raj in cup.hp.com but NOT BOTH...