Caveats of enabling jumbo frames on a VLAN (HP 4512zl switch)?

[qs-it]

We have a 4512zl backbone switch, with one VLAN dedicated to storage (iSCSI).

However, at the initial configuration of this vlan, jumbo frames were not enabled.   

I am presently implementing a new clean VMWare environment, using this same VLAN for storage.  The new environment has it's own SAN and front end servers, so it doesn't interfere with currently active hosts on the storage VLAN.

I want to enable jumbo frames on this vlan, to optimize iSCSI throughput for my new environment, but I don't want it to impact any of the other iSCSI connections currently utilizing the storage VLAN.  

If I enable jumbo frames on this VLAN, will that impact or interfere with any devices currently doing iSCSI on this same VLAN?  Or does it just enable the VLAN to recognize jumbo frame packets from hosts that have their MTU size configured for greater than 1500?

[Vince-Whirlwind]

Enabling jumbo frames on a VLAN on a 5412 running K15 while the switch is in production and passing traffic has no effect on anything that I've ever seen.

(I can't remember if K14 allowed jumbo - probably did, if so I'm sure it worked the same).

All it changes is it allows your 1Gb & 10Gb physical interfaces to accept jumbo frames.

[qs-it]

Thanks, Vince.

Just so I'm clear..

All enabling jumbo frames on the VLAN, while running K.15 firmware, does is allow the switch to properly allow 1/10GbE Ethernet interfaces to be able to identify and send/receive packets with MTU greater than 1500?   Any other interfaces on the VLAN that are passing packets of normal 1500 MTU size continue to behave without interruption?

[Vince-Whirlwind]

Sounds like you should test it for yourself before implementing this into production.

I can't say I ever had anything actually happen when implementing jumbo frames, but of course doing it on a production network was done during a proper change, after hours, and with the change doco showing the results of our testing of the same change on the dev/test network.

The trickiest bit was answering their question on quantifying the exact benefit of using jumbo frames - they weren't very impressed with the maths involved....