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I have brocade icx 7450. I already enabled jumbo frames to increase MTU packets on switch. But unfortunately I am not able to change MTU size of port which I want. When typing after choosing interface ethernet e.g. 1/4/1 and then ip mtu, this command is not recognized.

How I can set up for ports which I wabt MTU packets to 9000. Do I miss something?

EDIT: Interface info:

10GigabitEthernet1/4/1 is up, line protocol is up
  Port up for 3 day(s) 19 hour(s) 19 minute(s) 5 second(s)
  Hardware is 10GigabitEthernet, address is xxxx (bia xxxxx)
  Configured speed 10Gbit, actual 10Gbit, configured duplex fdx, actual fdx
  Configured mdi mode AUTO, actual MDI
  Tagged member of 2 L2 VLANs, port state is FORWARDING
  BPDU guard is Disabled, ROOT protect is Disabled, Designated protect is Disabl                                                                                                                                                             ed
  Link Error Dampening is Disabled
  STP configured to ON, priority is level0, mac-learning is enabled
  Openflow is Disabled, Openflow Hybrid mode is Disabled,  Flow Control is confi                                                                                                                                                             g enabled, oper enabled, negotiation disabled
  Mirror disabled, Monitor disabled
  Mac-notification is disabled
  VLAN-Mapping is disabled
  Not member of any active trunks
  Not member of any configured trunks
  No port name
  IPG XGMII 96 bits-time
  MTU 10200 bytes, encapsulation ethernet
  MMU Mode is Store-and-forward
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  • IP MTU is, at least in the Cisco world, different than the layer-2 MTU. I think you may be confusing layer-2 and layer-3.
    – Ron Maupin
    Commented Jul 1, 2023 at 13:23

2 Answers 2

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Both this document from Ruckus (bought Brocade) and this video from Brocade/Ruckus indicate that you must set the entire device to Jumbo mode (which seems to enable a jumbo frame size MTU of 10200 bytes on all interfaces by default) and then save and reload the operating system.

Once that is done, you can then reconfigure any ports desired back to 1500 byte MTU as needed and set common Jumbo MTU (9000 or 9216, etc.) on other ports as needed.

You may also need to set the conditional action to handle when the MTU is exceeded on a Layer 3 (routing) interface using the 'mtu-exceed [hard-drop|forward]' configuration. The default seems to be hard-drop but you can set it to forward which will enable fragmentation instead. The mtu-exceed command is a global configuration and cannot be changed per interface.

The guide indicates that using jumbo frame support requires additional resources on the switch/router so you should only set the MTU larger than 1500 on interfaces where it is actually needed.

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  • You should point out that forwarding with fragmentation is for L3 forwarding/routing only.
    – Zac67
    Commented Jun 30, 2023 at 20:51
  • Yes that was confusing since the documentation doesn't make it clear from what I saw but I would assume it is the case that fragmentation is only possible on Layer 3 routing/forwarding. The mtu-exceed configuration is a global configuration but should only affect Layer 3 interface traffic forwarding. Commented Jun 30, 2023 at 23:18
  • Jumbo frames are enabled and I can see maximum MTU on each port now 10200.. But when I go to interface ethernet 1/4/1 I can not set MTU to 9000 by command IP mtu 9000 because command is not recognizable..
    – Frihu
    Commented Jul 1, 2023 at 6:00
  • I was able to assign IP MTU to 9000 when I assigned UNTAGGED port to VLAN... Is there any chance to change IP MTU for only tagged ports? @FrameHowitzer
    – Frihu
    Commented Jul 1, 2023 at 8:56
  • I don't know why that would matter. MTU is effective on the interface level but perhaps they implement it per VLAN due to hardware restrictions. Try removing any interface configuration. Then apply the MTU and then re-apply the desired VLAN configuration. Commented Jul 1, 2023 at 18:02
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On Brocade switches, you can only set the (L3) MTU on L3 routed ports. The maximum frame size for L2 switching is configured using the global jumbo command.

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