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looking for some clarification on the jumbo frame config for a cisco 3560 switch.

I understand that by enabling jumbo frames, it enables it on all ports for entire switch, what happens to devices that are not utilizing jumbo frames like (workstations, printers, scanners, etc...) on the same switch? Will it have any issues?

Its a 48 port switch that has about 20 end user devices. We also have 2 SANS and 2 ESXI Hosts that I want to utilize jumbo frames on, but not sure of the effect on the regular devices, if the jumbo frames is enabled on all ports.

Thank you in advance!

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 25 '18 at 9:11
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Jumbos frames need to be enabled by port and by VLAN. Generally, you can't mix devices using only standard size frames and devices using jumbo frames in a single segment/VLAN. All devices in a segment must use the same maximum frame size. There is no arbitration and mixing sizes would cause devices to drop oversized frames (from their POV).

SAN segments run on their own VLANs or on even physically separate segments. You can enable jumbos there if all devices can handle the frame size well. Enabling jumbos on the switch doesn't actually turn them on. It's the first step before you set them on the end nodes - and don't forget the ESXi's vSwitches and VMkernel NICs.

  • Thank you Zac67. Do you have any reference on how to configure/enable jumbo frames on specific ports or VLANs? The only documentation I found was to enable it system wide. – Igor P. Aug 12 '18 at 17:50
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    You're correct - on the 3560 there's only a global jumbo frame setting, no need to configure the VLANs. As long as the end nodes don't actually use jumbo frame for non-jumbo VLANs that won't hurt. – Zac67 Aug 12 '18 at 18:37
  • ... and very probably, the 3560 won't be able to handle jumbo frames at Layer 3 (none of the 2960, 3560, 3750 could, if I remember correctly), so there won't be any inter vlan routing for jumbo frames on the given 3560. – Marc 'netztier' Luethi Aug 12 '18 at 20:06
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    Mixing jumbo-enabled and non-jumbo-enabled end hosts on a given subnet: It can work as long as everything is TCP. TCP MSS negotiation during the 3way handshake helps. Jumbo frame enabled host A (MTU 9000) will announce a TCP MSS of 8960, while non jumbo frame enabled host B (MTU 1500) will announce 1460bytes. A will respect the lower value and never send more than 1460 bytes at once, B won't be able to send anything larger than 1460 anyway. Yes, this is an invalid setup, and it will break HORRIBLY with UDP, and you *really* have to know which and how data is exchanged in that mixed subnet. – Marc 'netztier' Luethi Aug 12 '18 at 20:17

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