Jumbo Frames

The jumbo frames feature supports Ethernet frames greater than 1500 bytes of payload on a port. By default, jumbo frame support is disabled on all ports and path MTU discovery is enabled. When jumbo frame support is enabled, path MTU discovery should also be enabled. Path MTU discovery is set using the set mtu command.

It is possible for jumbo administrative status to be enabled and jumbo operational status to be deferred. Jumbo frame support is supported on all module ports.

Some S-Series modules can only handle 12 jumbo enabled ports at one time. If on a module that only supports 12 jumbo frame ports, you enable jumbo frames, without specifying a port, by default, the first 12 ports are enabled. Should you then enable a port number higher than 12, it will immediately take back the jumbo frame resources for ports 1 - 12 and initially show an operational status of deferred. Resetting the module will enable deferred ports.

When enabling jumbo frame support on any device, the maximum frame size defaults to 10239 bytes for untagged packets and supports 10243 bytes for tagged packets. Jumbo frame size can be set to any value between 1000 – 10239 bytes. The standard Ethernet frame MTU for untagged packets is 1518 bytes including the 18 Ethernet header bytes. The standard Ethernet frame MTU for tagged packets is 1522 bytes.

When setting the jumbo frame size, keep in mind that large frames require more packet buffers than standard frames, reducing the total available packet buffers. This limitation can be improved by selecting a smaller MTU jumbo frame size.

Some applications require extra header bytes beyond the standard tagged packet size. For example, provider bridging (Q-in-Q) requires an extra header of 4 additional bytes. The S- K- and 7100-Series support the dynamic MTU frame which when enabled will automatically add the extra 4 bytes to the currently configured jumbo MTU frame size, allowing you to set the jumbo MTU frame size to the standard 1518 bytes and the 4 additional tagged header bytes will automatically be added for Q-in-Q frames. The dynamic MTU frame feature currently only supports Q-in-Q. Other applications requiring additional header bytes will be added in future releases. Dynamic MTU frame is disabled by default.

Use the show port jumbo command to verify the operational status of a jumbo enabled port including display of supported jumbo MTU frame size and the state of the dynamic MTU frame feature. A jumbo administratively disabled port will always have a jumbo operational status of disabled.

If on the S- or K-Series you have manually enabled jumbo frames support on the maximum number of ports allowed on the module, and you attempt to enable additional ports, the additional jumbo frame configurations will fail. You must free up resources by disabling jumbo frames on a port for each additional port you are trying to add before continuing.

The 7100-Series platform supports the forwarding of unicast, multicast, and broadcast jumbo frames.

Use the set port jumbo command in any command mode to enable or disable jumbo frame support on the specified ports.

Use the set port jumbo mtu command in any command mode to change the size of the jumbo MTU frame on the port. Jumbo MTU frame must be enabled for the frame size change to take affect.

Use the set port jumbo mtu dynamic command to enable or disable dynamic MTU frame on the port.

Use the show port jumbo command to verify the operational status of a jumbo enabled port.

The following example enables the port jumbo frame feature, sets the jumbo MTU frame size to 1522 and enables dynamic MTU frame on port ge.1.1 (Q-in-Q must be enabled for the MTU delta and Application name fields to display as presented here):

System(rw)->set port jumbo enable ge.1.1
System(rw)->set port jumbo mtu 1522 ge.1.1
System(rw)->set port jumbo mtu dynamic enable ge.1.1
System(rw)->show port jumbo ge.1.1
* Applicable only if port jumbo is enabled
    Port     Oper     Admin     MTU    MTU   MTU   MTU   MTU   MTU   Application
            Status   Status  Dynamic   Min   Max  Oper* Admin Delta   Name(size)
---------- -------- -------- -------- ----- ----- ----- ----- ----- ------------
        ge.1.1 Enabled  Enabled  Enabled  1000 10239  1522  1522      4  Q-in-Q
System(rw)->