01-20-2012 09:01 AM - edited 03-07-2019 04:28 AM
Hi,
I need some clarifcation regarding the MTU concept.
I know that IP MTU is 1500 bytes and that includes IP header+TCP header+payload
What about ethernet MTU?
I am a bit confused when trying to understand QinQ for example. Why should the system MTU be 1504? I know that we are adding a tag of 4 bytes but the tag is in the header right. If you are taking in consideration that ethernet MTU is the IP MTU that is to say 1500 so the 4 bytes of the tag are not taking into account as ethernet MTU would without header.
Would be nice to get some clarification there.
Thanks.
/Laurent
01-20-2012 12:01 PM
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Posting
IP MTU isn't 1500 except on standard Ethernet as such Ethernet's maximum frame payload is 1500. (IP's "MTU" is about 64 KB.)
The system MTU, I believe you're describing, supports enabling Jumbo Ethernet; it increases the maximum frame payload size.
MTU on a trunk shouldn't change as the media allows VLANs tags to extend the frame size.
01-20-2012 12:34 PM
Hi Doherty,
Thanks for trying to explain but it didn´t clarify really my understanding of MTU. Anyone else could help me to clarify my questions in my original post?
Regards,
/Laurent
01-20-2012 12:49 PM
Good question. Ethernet MTU is 1500. The switch is not counting ethernet header or CRC trailer which is 18 btyes. It is only referring to Ethernet payload as Joseph stated. When system MTU 1500 is configured switches support a maximum Ethernet Frame size of 1522. 1500 ethernet payload + 18 bytes of header and trailer + 4 bytes for dot1q. When q-in-q is configured the system mtu must be changed to 1504. This brings the max ethernet frame size to 1526.
Regards,
Ryan
01-20-2012 01:15 PM
Hi Ryan,
Thanks a lot for reply. that means that:
Correct me if I am wrong
/Laurent
01-20-2012 01:43 PM
Correct. We need the 4 extra bytes for QinQ.
Is QinQ a part of the header? The command increases the Ethernet mtu payload. Which leads me to believe (CAUTION: I have entered the realm of speculation) the first q tag, the customer tag, would actually be considered Ethernet payload. The second q tag is added by ISP switch and is a part of the Ethernet header. Again this is me speculating and I could be totally wrong.
Regards,
Ryan
01-20-2012 02:44 PM
Interesting. Could be nice to test. I hought like the switch will just insert the metro tag in the header and then strip it off when sending the frame out to the CE. A bit like in MPLS with labels.
Regards,
Laurent
01-20-2012 06:37 PM
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
Sorry, it might have helped if you had replied what or why what I had explained didn't help to clarify MTU.
However, from reading your follow-up exchange between you and Ryan, it appears the part of understanding MTU that was really confusing you is Q-in-Q; the part of your original post I didn't touch upon. The reason I didn't address that was because you didn't seem clear on MTU for basic frames or single tagged frames.
The key point of understanding MTU it's the maximum transmission unit for L3. This would be, for Ethernet, the frame's payload, and again for "standard" Ethernet, 1500 bytes.
802.1q tags extend the Ethernet L2 frame to support them, leaving the system configured MTU as it was.
Q-in-Q though, instead of once again implicitly extending the frame size to support these tags, considers the prior tag as part of frame's payload. If using IP, and starting with an original payload of 1500, IP could fragment the packet to deal with the additional overhead, as it often does, for example, with GRE tunnels. However, if we can explicitly increase the MTU, this fragmentation can be avoided. As described by Ryan, to allow for just the additional overhead of a single Q-in-Q, increasing the MTU, if supported by the hardware, to 1504, avoids L3 fragmentation. (NB: same thing can be done for GRE if you can increase MTU by 24 bytes.)
Now if you're wondering why 802.1q tags is an implicit frame size extension while Q-in-Q, optimally, requires explicit frame extension; the latter, where you can adjust MTU is more flexible. As I've already noted, it can be used for GRE, but there are other encapsulation protocols that can benefit too. Also keep in mind, L2 frame sizes have hardware implications. Many, but not all, Jumbo Ethernet devices support frames up to about 9K, but other hardware media/interfaces support different MTUs. Even currently, don't recall any mainline hardware media/interface that would support a full IP packet of 64 KB.
Where all of this is confusing, a 1500 byte packet would optimally use a 1500 MTU setting for standard Etherent or 802.q tagged Ethernet, but use a 1504 MTU setting for Q-in-Q Ethenet and a 1524 MTU setting for GRE. The latter two MTU settings are larger because instead of implictly extending frame size for the overhead, it's now included as part of the payload and so we need to extend MTU to allow for this additional encapsulation overhead.
You might find some of the following helpful reading:
http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note09186a00801350c8.shtml
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