Jumbo Frames

Answered Question
Jun 17th, 2009

This is what I know about jumbo frames

Every device in a jumbo frame enabled vlan should use jumbo frames.

Please comment.

Thanks,

Mohamad

I have this problem too.
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Correct Answer by Joseph W. Doherty about 7 years 5 months ago

Well it's obvious, if server 1 is jumbo configured and server 2 isn't, server 2 won't be able to process jumbo frames from server 1.

If server 1 it attempting to communicate to server 2 with TCP, server 2 would advertize its smaller MSS (due to non-jumbo support) and TCP should work fine.

What's not as clear (to me), for other IP communication, whether MTU fragmentation and/or PMTU would arise for end-to-end L2 communication. For instance, server 1 sends a jumbo UDP packets to server 2 on the same VLAN.

I tried to (quickly) find some information on how bridges should handle different media MTU but didn't find anything. Even if I did, and even though switches are really multiport bridges, this is the type of issue I would expect switches might not always handle correctly since they might assume media is always the same, Ethernet. (NB: not surprising, different Cisco swithes support Jumbo differently. 6500s appear to support Jumbo per port, lessor switches, Jumbo is global, see: http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml#c10)

Certainly it would be better if all devices within the same logical L2 (VLAN) supported the same frame sizes, but the question is, is this a requirement? (At L3 interfaces, I would expect "normal" MTU and PMTU.)

Perhaps someone else can comment on pure L2?

[edit]

BTW, there's appears to be a major distinction for Jumbo support based on port bandwidths. Jumbo appears to be best supported on gig (which makes sense, due to Ethernet frame timing gap requirements), but this can lead to more issues, even at L2, if you mix gig with non-gig port bandwidths across the VLAN.

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Jerry Ye Wed, 06/17/2009 - 17:04

Hi Mohamad,

I am not sure what type of info you are looking for. Here is the defination for Jumbo Frame:

Jumbo: Jumbo frames are frames that are bigger than the standard Ethernet frame size, which is 1518 bytes (including Layer 2 (L2) header and FCS). The definition of frame size is vendor-dependent, as these are not part of the IEEE standard.

http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml#backinfo1

HTH,

jerry

Mohamad Qayoom Wed, 06/17/2009 - 18:08

Thank you, Jerry.

I understand what jumbo frames are. I would like to know if you guys agree with the statement posted earlier.

Thanks,

Mohamad

prakadeesh Thu, 06/18/2009 - 01:21

Hello Mohamad,

Use of jumbo frames will decrease the CPU processing on the end server. Because lets say the end server will need to use 6 cycles to generate 6 separate 1500 byte frame. But the processing power will be reduced if it will use the jumbo frame so that one 9000 byte frame is generated in much less than 6 cycles. More over to support jumbo frames the complete end to end link should support the Jumbo frame MTU. IMHO there is no hard and fast rule to use only Jumbo frames, but if we dnt use it on a vlan/interface that can support it we loose its effectiveness.

Hope I am clear.

Thanks,

Prakadeesh

dario.didio Thu, 06/18/2009 - 04:37

Hi,

indeed if you want to use jumbo frames, it has to be end-to-end, because devices supporting jumbo frames do forward them, but devices that dont't support them drop them, breaking the end-to-end communication.

HTH,

Dario

Joseph W. Doherty Thu, 06/18/2009 - 04:33

"I would like to know if you guys agree with the statement posted earlier."

I don't, if you also intend end hosts must support jumbo. Depending on the nature of the hosts' traffic, might be anywhere from very little benefit to very much benefit to usage of jumbo (e.g. Citrix server vs. backup storage server).

If there's benefit to jumbo, why restrict to just L2 (VLAN) and not also L3?

[edit]

Even within a VLAN, I believe you could just want to allow jumbo between devices on the same switch but might not need to extend jumbo support to all VLAN L2 transit devices.

Using jumbo, where it's not fully supported end-to-end, I believe will work, but having such will lead, I also believe, to MTU mismatch issues. Then the queuestion becomes, is jumbo's benefit when it's supported outweigh jumbo's issues when end-to-end support varies.

Mohamad Qayoom Thu, 06/18/2009 - 04:58

Thanks guys. Maybe I should have been more clear. Let's say everything is setup as you guys recommended -- end-to-end jumbo frames, l2 jf, l3 jf...

In the scenario below, if server1 and server2 are in VLAN5, which has jumbo frames enables. Server1 is configured for jumbo frames, but server 2 is not. What happens then? What happens when server1 tries to talk to server2 using jumbo frames? I know if there is another server, let's call it server3(doesn't support jumbo frames), in a different vlan, then there shouldn't be any issues in the communication between server1 and server 3.

Server1 ---- L2 Switch ----L3 Switch --- L2 Switch --- Server2

Correct Answer
Joseph W. Doherty Thu, 06/18/2009 - 06:25

Well it's obvious, if server 1 is jumbo configured and server 2 isn't, server 2 won't be able to process jumbo frames from server 1.

If server 1 it attempting to communicate to server 2 with TCP, server 2 would advertize its smaller MSS (due to non-jumbo support) and TCP should work fine.

What's not as clear (to me), for other IP communication, whether MTU fragmentation and/or PMTU would arise for end-to-end L2 communication. For instance, server 1 sends a jumbo UDP packets to server 2 on the same VLAN.

I tried to (quickly) find some information on how bridges should handle different media MTU but didn't find anything. Even if I did, and even though switches are really multiport bridges, this is the type of issue I would expect switches might not always handle correctly since they might assume media is always the same, Ethernet. (NB: not surprising, different Cisco swithes support Jumbo differently. 6500s appear to support Jumbo per port, lessor switches, Jumbo is global, see: http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml#c10)

Certainly it would be better if all devices within the same logical L2 (VLAN) supported the same frame sizes, but the question is, is this a requirement? (At L3 interfaces, I would expect "normal" MTU and PMTU.)

Perhaps someone else can comment on pure L2?

[edit]

BTW, there's appears to be a major distinction for Jumbo support based on port bandwidths. Jumbo appears to be best supported on gig (which makes sense, due to Ethernet frame timing gap requirements), but this can lead to more issues, even at L2, if you mix gig with non-gig port bandwidths across the VLAN.

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