Jumbo frames accross trunk & sites

[m.egan]

Hi -

There's some really good information in this forum on jumbo frames. I understand that jumbo frames need to be enabled end-to-end.

I have two ESX hosts connected at each site. I want to enable jumbo frames for those ports, but what if not all host on the ESX are using jumbo frames, will I have drops and connection failures?

So if i have two sites, each with a 6509 connected via a trunk and need to enable jumbo frames for a vlan between the sites how do I accomplish this?

If I enable jumbo frames on the trunk link how does that impact other traffic between the sites?

Thanks for the help.

./Matt

[darren.g]

MATTHEW EGAN wrote:
Hi -
There's some really good information in this forum on jumbo frames. I understand that jumbo frames need to be enabled end-to-end. 
I have two ESX hosts connected at each site. I want to enable jumbo frames for those ports, but what if not all host on the ESX are using jumbo frames, will I have drops and connection failures?
So if i have two sites, each with a 6509 connected via a trunk and need to enable jumbo frames for a vlan between the sites how do I accomplish this?
If I enable jumbo frames on the trunk link how does that impact other traffic between the sites? 
Thanks for the help.
./Matt

Matt.

Enabling jumbo frames doesn't exclude smaller frame sizes from being used - if the frame generated is a smaller one, then a smaller frame will be used.

Jumbo frames only affects the maximum MTU available to be used for transport.

If you are sending jumbo frames to a device which doesn;t support jumgo frames, then the switch will fragment them - unless the DF bit is set, in which case the frame will be dropped  - under Windows you can test this by issuing the following command

ping -f -l 9000

You may suffer some performance hits if you're fragmenting a lot of frames (I.E. sending a lot of jumbo frames to a device/host which doesn't support them), but unless you're *really* flogging the switch fabric (or running a low-capacity switch) it shouldn't be noticable.

One thing you should remember is that if you associate a layer 3 interface (SVI) with the VLAN using jumbo frames, you have to specifically set jumbo frames on *all* components in the VLAN concerned - individual ports. etherchannels, SVI, system support - the lot.

I had a situation once where I was using some devices which absolutely required unfragmented jumbo frames (HD video transcoding servers), and I left the jumbo frame specification off the SVI configuration - which broke the whole thing - I had the ports setup, had the port-channels setup - but left the MTU specification out of the SVI - and then spent half a week trying to figure out why it wouldn't work! :-)

Cheers.

[Joseph W. Doherty]

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Posting

As Darren noted, having smaller than jumbo frames on jumbo enabled segments is fine.  As he also noted, if along the path the jumbo is too large, it will follow the rules for too large MTU for that network segment, which include packet fragmentation or dropping the packet; neither desirable.

Something to be careful of, there's no real standard for how large a jumbo must be.  Additionally, two devices could use different maximums for their jumbos.  If a frame is received that's too large it will be dropped.  I.e. make sure devices on the same segment agree on their jumbo size settings.

[m.egan]

Darren & Joseph,

Thanks so much for the clarification. I wanted to make sure I understood the implications before I made any changes.

Thanks again.