increasing MTU in a Metro Ethernet environment

[Michael Garcia]

I'm working with a company who has a need to increase their MTU size to 9000 on 2 of the 5 interfaces connected to the Metro Ethernet ring. These 2 interfaces are for replication traffic mostly but the other 3 are for internal data and email. Their question is that if they do not increase the MTU to 9000 on the other 3 interfaces, what might the implications be? Their biggest fear is that they will have congestion issues inbound on those unchanged interfaces because of fragmentation.

Your input is greatly appreciated.

Michael

[Joseph W. Doherty]

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Posting

Not supporting Jumbo Ethernet everywhere shouldn't, alone, cause too much congestion because of fragmentation, although the fragmentation itself can be more of a problem.  If PMTUD is correctly supported, and especially if the MSS adjust feature is also available, you shouldn't have much fragmentation (except in some unusual circumstances).

One thing to insure is that both sides of a link are MTU configured alike (and support the configured MTU).

[Michael Garcia]

Thank you for your response JosephDoherty. I believe the origin of the packets and how they are assempled and placed on the line at the source determines packet size. Example would be a packet place on the line at 1500 bytes could travers an interface set at 9000 without being fragmented. However a packet size of 4500 entering an interface with an MTU of 1500 would be fragmented. What we are proposing is changing the MTU size of only 2 of 5 and requesting the Provider to increase MTU size to 9000. Although fragmentation can occurr in this scenario, would it be best practice to configure all interfaces involved to 9000 to avoid any possibility of fragmentational congestion?

[Joseph W. Doherty]

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

Again, unclear how you see fragmentation causing significant congestion.  It will, of course, increase bandwidth usage because of additional IP headers and L2 framing.  What's your understanding of "fragmentational congestion"?

Also again, if you send frame too large for the receiver's NIC MTU (i.e. on same link), receiver will not fragment, it will drop frame.  Only if frame egresses a L3 interface with a smaller MTU will it be fragmented.