Multiple MTU Size in vSwitch and iSCSI Adapter

[isaps]

Hi folks,

i've got a question about connection one esxi hosts to two different storages with two different mtu size configuration.

My configuration looks like this:

iSCSI_1_HP (MTU: 9000) ->                                           -> vmnic1 (HP Storage - MTU : 9000)

iSCSI_2_HP (MTU: 9000) ->                                           -> vmnic2 (HP Storage - MTU : 9000)

                                          -> vSwitch5 (MTU: 9000) ->

iSCSI_3_IBM (MTU: 1500) ->                                          -> vmnic3 (IBM Storage - MTU : 1500)

iSCSI_4_IBM (MTU: 1500) ->                                          -> vmnic4 (IBM Storage - MTU : 1500)

As you can see i've got two adapter for connection to a HP Storage supporting MTU Size 9000 and one IBM storage that does not support Jumbo packets (MTU Size 1500).

Now my question is, how the vswitch handles both connections as the switch is also configured with a MTU Size of 9000. Perhaps it should be configured with 1500? I am not sure so i wanted to start this discussion.

[Mattallford]

‌You need the MTU size to be the same end-to-end on all paths.

I think in this example you would be best creating another vSwitch and moving your vmk adapters and vmnics to it with an MTU of 1500.

Cheers, Matt.

[MattiasN81 · Accepted solution]

Actually, you dont need to change it.

Just as a physical switch the vSwitch just enables jumboframes when setting it to MTU 9000, it doesn't require you to use jumbos, it allows you to use MTUs from 1500-9000

However, the sender and the receiver NICs should always match MTU size (some arrays does accepts MTU1500 when Jumbos are configured though)

Example

Good Configs.

1. vmnic (mtu9000) -> vswitch (mtu9000) -> storage nic (mtu9000)

2. vmnic (mtu1500) -> vswitch (mtu9000) -> storage nic (mtu1500)

Bad Configs.

1. vmnic (mtu9000) -> vswitch (mtu1500) -> storage nic (mtu9000)

2. vmnic (mtu1500) -> vswitch (mtu9000) -> storage nic (mtu9000)

[Mattallford]

‌Of course. I stand corrected (and show my lack of network knowledge!)

[isaps]

Great!

Then i should be fine with that. Thank you too!