Link to home
Start Free TrialLog in
Avatar of rookie_b
rookie_bFlag for United Kingdom of Great Britain and Northern Ireland

asked on

MTU sizes for ISCSI tagged vlans

Hello,

We have standard virtual switch that has 2x40G fibre links that are a  portchannel and are tagged with  the ISCSI vlan on both the switch and the hypervisor ends. The San is connected to untagged ports on the same ISCSI vlan.  The San and the hypervisor internall settings have MTU of 9000. However, the physical switches between them  have MTUsettings of 9024 and 12000
 Is this likely to cause issues?

Are there any implications for a frame that starts at MTU 9000 untagged at the San end, and then has a vlan ID added to it?



Thank you!
SOLUTION
Avatar of Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)
Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)
Flag of United Kingdom of Great Britain and Northern Ireland image

Link to home
membership
This solution is only available to members.
To access this solution, you must be a member of Experts Exchange.
Start Free Trial
ASKER CERTIFIED SOLUTION
Link to home
membership
This solution is only available to members.
To access this solution, you must be a member of Experts Exchange.
Start Free Trial
SOLUTION
Link to home
membership
This solution is only available to members.
To access this solution, you must be a member of Experts Exchange.
Start Free Trial
Avatar of Member_2_231077
Member_2_231077

MTU should be same for hosts and storage but it can be bigger on the switch, what matters it that it does not get fragmented.
Avatar of rookie_b

ASKER

Thanks everyone very helpful advice. I am confused about having the same MTU. Doesn't that mean that a a fram at max MTU sent by the untagged device, could end up being max MTU + vlan tag by the time it gets to the tagged hypervisor? Doesn't this suggest that tagged interfaces should have a little bit extra to account for the untagged at max MTU + vlan tag? Or I'm I getting this wrong?
MTU is set on the TCPIP stack.   The frame size will expand wth.1Q (or .1QQ)  with a few bytes.
on the switches you should enable jumbo frames. The tagging is the format on the wire.

in memory there allways is a tag attached to the buffer.

The TCP MTU size should be the same everywhere.
Ok, I have tried and failed.

 Vmkping -s 9000 from hosts fails. I set the SAN settings to 9000, the untagged port on the switch  I tried 9000, 9216 and 12000(the global switch setting is 12000 but allows for individual port settings ).

 The next link is a portchannel uplink(9216)to another switch where there is a global 9216 setting. That second switch has a similar SAN on the same VLAN attached to it - it is set to 9000 on the San and the global 9216 setting on the untagged ports and works fine. It vmkpings -s 9000 fine too.


At the moment we have set the MTU on the new SAN to 1500, which works, but obviously we want to sort this out. It looks like an issue with the first switch, but no idea what.


Luckily we have 2 controllers on the SAN, so we can out one in maintenance and change and troubleshoot any settings without disruption, by I need to figure out where the problem is.

Thanks!