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I have a Cisco 3750X switch stack (2 switches stacked) and have a Compellent and three Equalogics as well as three VMware hosts directly connected but separated via different vLANs on this switch. The SAN appliances need Jumbo packets turned on (MTU 9000) and the hosts will have the ISCSI interfaces set to use MTU 9000 as well but some of the other interfaces need to remain on MTU 1500.

Since this particular switch is unable to be configured per interface to have different MTU settings if I change the System MTU to 9000 will this have any adverse effects on those devices still needing their MTU setting set to 1500? Fo the the SAN appliances will I also need to change SYSTEM MTU ROUTING to match the MTU setting of 9000 in order for those to fully utilize Jumbo frames?

Unsure and trying to follow best practices for these Dell storage appliances.

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Setting the switch to use MTU of 9k globally will not be an issue for host device interfaces that are programmed to use 1500 as a maximum. The host will compile frames that have an MTU size of 1500 maximum and while the switch accepts a maximum of 9000, there will be no fragmentation and it will accept the frames without discarding them.

An issue would only arrise if you were to do it the other way round. Passing 9K frames on a switch accepting only a maximum of 1500 Bytes.

Just make sure that any 9K frames are passed along switches that support 9K frames.

Normally your ISCSI network would remain isolated from the rest of the network and routing would not be enabled due to latency concerns.

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  • Thanks SleepyMan. It is the answer I suspected but really needed validation. Thanks again. – Ronnie May 10 '17 at 14:04

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