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Nexus 3064 Jumbo Frames Configuration

Avery Spates

I have a Cisco Nexus 3064 that I am using as part of a flat network for the Lab. I have 30 Virtualization Servers(MS HyperV and VMware vSphere) connected to this switch and I want to enable jumbo frames. The Virtualization Servers are able to ping the local VM's using 8K bytes. However I am unable to ping from server to server using 8K bytes. I have configuration (in abbreviation). All the servers are in the same network which I configured as L2 ports with the "switchport" command. However, the interface "MTU" command is unavailable in L2 mode. I am only able to get the interface "MTU" command only in L3 mode with the "no switchport" command on the interface.

     # int eth1/2-45

     # no switchport

     # mtu 9216

     # no shut

Is there anything that I am missing? I can ping the servers with less than 1500 bytes, but anything larger fails.

Thank You for any response.




The Cisco Nexus 3000 Series  switch does not fragment frames. As a result, the switch cannot have  two ports in the same Layer 2 domain with different maximum             transmission units (MTUs). A  per-physical Ethernet interface MTU is  not supported because switch does cut thru switching i.e switch only see the destination mac address and begins to forwarding frame without knowing it has large frame coming in. When you show the interface settings, a default  MTU of                 1500 is displayed for physical Ethernet interfaces.

Instead, the MTU is set according to the             QoS classes. You modify the MTU by setting Class and Policy maps.

You need to define MTU with policy-map type network-qos.

When configuring MTU, follow these guidelines:

  • For  the Cisco Nexus 3000 Series switch , the MTU is controlled by the value  configured on the class-default. The same MTU must be configured on all  classes.
  • Enter the                      system                       jumbomtu command to define the upper bound of any MTU in the system.                     The system jumbo MTU has a default value of 9216 bytes. The minimum MTU is 1500 bytes and the maximum MTU is 9216 bytes.
  • The system class MTU sets the MTU for all                     packets in the class. The system class MTU cannot be configured larger than the                     global jumbo MTU.
  • The default system class has a default MTU                     of 1500 bytes. You can configure this value.
  • The switch sends the MTU configuration to                     network adapters that support DCBX.

Here is the sample.

Hi Krun_Shah,

  So can I create multiple Policy Maps with different MTU's? If so, can I apply different policy maps to the interface?

Thank You


No. Policy map type network-qos can only go under system-qos. you cannot apply it under any interface.

NX3K01(config-if)# service-policy type ?

  qos      Qos policy

  queuing  Queuing policy

NX3K01(config-sys-qos)# service-policy type ?

  network-qos  Network Qos policy

  qos          System-level QoS policy

  queuing      DCE Queuing policy

Any advice on how to configure the class map for this?

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