×

Warning message

  • Cisco Support Forums is in Read Only mode while the site is being migrated.
  • Cisco Support Forums is in Read Only mode while the site is being migrated.

MTU Mismatch

Unanswered Question
Nov 11th, 2014
User Badges:

I have 2 hosts connected to the same switch.

 

Each host it set to execute an FTP at 9000 Bytes

Switch ports are both configured for 2500 Bytes

 

What happens when a 9K byte frame is received on the switch interfaces? They drop them, right?

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (2 ratings)
Loading.
Alexey Stytsenko Wed, 11/12/2014 - 01:30
User Badges:

Hi.

 

Check the output of command

show system mtu

On the switches MTU settings for switchports and Vlan interfaces are system wide. So you will derive information what size Jumbo frames allowed on your system currently.

 

To enable support of frames of 9k size, if not enabled yet, use

system mtu jumbo 9000

command.

Tagir Temirgaliyev Wed, 11/12/2014 - 02:37
User Badges:
  • Silver, 250 points or more

it will drop packet and it sends icmp packet to source to inform it to reduce packet size.

so sender can understand and to reduce packet size.

visitor68 Wed, 11/12/2014 - 04:00
User Badges:

Does that need to be configured on the switch? I mean the ability to send an ICMP packet back to source...

Also, lets say a downstream switch has an even smaller MTU than 2500 (1500), will it send an ICMP frame to the sending switch and will it in turn send back to the source once again?

Looking for a good article on this..

danjone3 Wed, 11/12/2014 - 08:53
User Badges:
  • Cisco Employee,

There are a number of things which can affect this but the short answer is that by default the frames will not be dropped, they will be fragmented.  The payload is split into smaller pieces and forwarded, then reassembled at the far end.

In the example you give, 9k frames will probably not be received on the switch interface.  They'll likely be fragmented by the PC before they're sent out on the network. Windows, Linux, OSX and other operating systems that support Ethernet interfaces have their own MTU at each layer of the network stack and/or protocol involved.  If a layer receives a datagram from the layer above it for processing that's larger than that layer can support, it will generally fragment the datagram into an acceptable size.  Unless you've explicitly configured the Ethernet MTU on the host for a larger size, the host's MTU for Ethernet is probably set for 1500 and it will send packets of a maximum of that size, regardless of the size of the chunks that are being sent from the FTP server running on the host.

I'm throwing a lot of "probably" and "likely" qualifications in there because, as I noted, there are a number of factors which can affect this.  But fragmentation as it goes through the network stack is typical behavior.

visitor68 Sun, 11/16/2014 - 13:53
User Badges:

Joseph/Danjone:

I think you are both correct. Well, let me say that I know for sure that Jospeh is correct that a switch port that is configured to send/receive frames of a certain MTU will drop received frames that exceed that setting. At least this has always been my understanding and I have seen this repeated on many other forums outside Cisco, too.

Now, Danjone, what you are saying is extremely interesting. You don't contradict Joseph, per se, regarding a switch port's behavior IF a frame of a larger size than its configured MTU is received. BUT, you contend that that would NOT happen in the first place because certain operating systems will auto-correct, so to speak. Do you have any documentation to corroborate that?

Thanks

Joseph W. Doherty Mon, 11/17/2014 - 12:51
User Badges:
  • Super Bronze, 10000 points or more

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Fragmentation happens at L3, not L2.

If FTP (which uses TCP) is configured to send MSS of 9K, but the interface is configured with a MTU of 1500, it will work, but not as optimally if MSS's size would fit into the MTU.

visitor68 Fri, 11/21/2014 - 16:07
User Badges:

Danjone;

Do you have any documentation to corroborate that?

Joseph W. Doherty Wed, 11/12/2014 - 09:09
User Badges:
  • Super Bronze, 10000 points or more

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of   the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Yes, I believe that's correct - i.e. the ingress port will drop the frame.

However, I also believe, unlike what Tagir has posted, you'll receive no notification because both hosts are on the same L2 media (where they're supposed to agree on MTU).

Tagir Temirgaliyev Wed, 11/12/2014 - 22:01
User Badges:
  • Silver, 250 points or more

you can easily check it

make ping from host to host

ping a.b.c.d  -l  9000 -f

which means 9000 byte packet with dont fragment flag, and

ping a.b.c.d  -l  9000

Actions

This Discussion