Jumbo Frame (MTU) and iSCSI Hosts

Answered Question
May 27th, 2010

Quick question:

Assumptions:

iSCSI SAN Adapters are using 9000 MTU

iSCSI SAN Ports (on 6500) are configured for 9216 MTU

Host Server Adapters are configured using 9000 MTU to match iSCSI SAN Adapters

Host Server Ports (on 6500) are configured for 9216 MTU

If documentation says to set port MTU at 9216 but the iSCSI SAN communicates at 9000 MTU, I am assuming that setting the MTU at the max does not hinder or cause any problems with traffic?

Meaning, the servers can communicate at a lower MTU than 9216 with no loss in performance?  Or should the ports be configured to match an MTU of 9000? The documentation should already account for packet/frame overhead right?

Equipment:

Dell Equallogic PS6000 (iSCSI)

Catalyst 6500-E with (WS-X6748-GE-TX) modules

Apple XServe with 3 dual port gig adapters (1 built-in (intel),1 PCIx (Intel), 1 PCIx (Broadcom))

I have this problem too.
0 votes
Correct Answer by Rick Arps about 6 years 6 months ago

You are correct, the 9216 MTU just means that the port will accept anything up to and including 9216 bytes long.  You should be fine using 9000 byte mtus on your adaptors so long as the entire path supports at least 9000 byte mtus

Hope this helps

Rick

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Correct Answer
Rick Arps Thu, 05/27/2010 - 09:54

You are correct, the 9216 MTU just means that the port will accept anything up to and including 9216 bytes long.  You should be fine using 9000 byte mtus on your adaptors so long as the entire path supports at least 9000 byte mtus

Hope this helps

Rick

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