[clue] a networking riddle

[Mark G. Harvey]

Having worked tech support on several of jumbo frames calls, here's what I learned 

To cover the overhead of Jumbo Frames, the switch & its ports have to be set at a higher MTU to pass the additional headers added when a packet leaves eth0   Important: every interface that handles a packet needs to be set for Jumbo Frames.  At the hosts, 9000 ... at the switch globally & at each port 9216 

       HostOne
eth0 w/ MTU=9000
             |   
         cable
             |   
switch w/ MTU=9216 set globally PLUS 
MTU=9216 set at specific ports passing Jumbo Frames
             |   
         cable
             |   
      HostTwo
eth0 w/ MTU=9000 

Given that it works in your system one way, but not the reverse, there could be just one switch port that still has the default MTU=1500  

Google this:  jumbo frames switch mtu 9216   ... a lot of good info there


Best of luck on solving the problem.  






On Monday, October 14, 2013 4:37 PM, Quentin Hartman <qhartman at gmail.com> wrote:
 
Well, what do the logical groupings of servers have to do with the network topology? What exactly lies between server A and B? Are there any weird tunnels? Do all the switches support jumbo frames? All the routers correctly translating between networks?

MTU is a fiddly thing, and if every device in the path isn't correctly configured you will get weird behavior.

QH



On Mon, Oct 14, 2013 at 3:57 PM, Mike Bean <beandaemon at gmail.com> wrote:

Well, I don't mean OU in the windows sense, I mean logical groupings of servers.  Clusters.   I'm trying to get my head around this, because intuitively, it just doesn't make any sense. 
>
>If you're coming home from the grocery store, and you have more groceries then you can carry, you make multiple trips to the car.   
>
>but I don't think it's quite that simple.  Think networks aren't as smart as I think they are,  intuitively, something configured to send as 1500, should be able to receive 1500 only, not necessary 9000, but something that's configured as 9000 should be able to send and receive as 9000 or 1500.   (scratches head.)
>
>
>In actuality, B (9000) can send to A (1500)
>
>and A (1500) can send to B (9000)
>
>
>but if we set A to 9000, it can no longer send to B (9000)
>
>(scratches head)
>
>
>Boy, if you know someone who's in school looking for a secure job, tell them to become a network person.    REALLY GOOD network people are worth their weight in gold.
>
>
>
>
>On Mon, Oct 14, 2013 at 3:26 PM, Quentin Hartman <qhartman at gmail.com> wrote:
>
>By OU, do you mean broadcast domain? ie - two LANs connected by a router? If so, it sounds like you have the (usually default) MTU of 1500 set on side A, but the 9000 set on B, and your router isn't correctly doing MTU management when crossing from one to the other.
>>
>>
>>Going the other way works because the fragments are already small enough to be handled correctly at the other end. The complexity of making this work reliably is the primary reason so few people bother to use jumbo frames, even though they technically are better most of the time anymore.
>>
>>
>>The wikipedia article on MTU is actually pretty good: http://en.wikipedia.org/wiki/Maximum_transmission_unit
>>
>>
>>QH
>>
>>
>>
>>On Mon, Oct 14, 2013 at 3:15 PM, Mike Bean <beandaemon at gmail.com> wrote:
>>
>>
>>>So I've been scratching my head for a long time trying to understand this, and it makes no sense to me.   We have to discrete OU's.   SCP transmissions of any real size tend to fail from A to B, but not from B to A.   
>>>
>>>My colleagues have been arguing that the fix is to set the MTU at side B to 1500.  And Lo, and behold, when we do, it works, increase the MTU to 9000, and it fails again.
>>>
>>>http://unix.stackexchange.com/questions/14187/why-does-scp-hang-on-copying-files-larger-than-1405-bytes
>>>
>>>What I have difficulty understanding, is if a jumbo frame carries, say, 9000 bytes, and hypothetically, if it's a 5,000 bytes file + your MTU = 1500, which means you split it up into 3 transmissions.
>>>
>>>Same size transmission; capped packet size,  more packets.   So I would naturally conclude that at an MTU of 9000 it could get done in 1 what an MTU of 1500 would do in 3?
>>>
>>>(scratches head)
>>>
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