Support for Layer 3 MTU on 7280R3/7500R3/7800R3

7280R3/7500R3/7800R3 switches support Layer 3 Maximum Transmission Unit (MTU) with the following features:
  • These switches enforce the MTU for Layer 3 packets. You can set the MTU on any SVI. The switch enforces the MTU of that specific SVI when packets egress out of a trunk port. 7280E/R/R2 and 7500E/R/R2 line cards do not support this behavior.

  • The switch enforces the MTU after adding all encapsulation headers to the egress packet. For example, if you configure an interface with an MTU value of 1500 and the packet size exceeds 1500 after the switch adds all encapsulation headers, the switch considers the packet to have violated the MTU. The 7280E/R/R2 and 7500E/R/R2 line cards enforce the MTU before adding encapsulation headers.

  • The system configures the actual MTU in hardware as the MTU configured in the CLI plus 18 bytes. It adds 18 bytes to accommodate the L2 header. If the outgoing packet is untagged, the system configures the actual MTU in hardware as the MTU configured in the CLI plus four (4) bytes.

Layer 3 MTU Configuration

You configure Layer 3 MTU on Layer 3 interfaces and SVIs using the mtu command within interface configuration mode.

This command sets the MTU size of 1492 bytes on interface ethernet 1/1.

(config)# interface ethernet 1/1
(config-if-Et1/1)# mtu 1492

MTU is independently configurable on all routable interfaces. The switch supports MTU sizes ranging from 68 to 9214 bytes. The default MTU size is 1500 bytes.

The no mtu and default mtu commands restore the interface’s MTU to the default value by removing the corresponding mtu command from running-config.

Layer 3 MTU Show Commands

The show interfaces command displays the MTU configured on the L3 interface (ethernet 1/1), as shown in the following example. The IP MTU represents the interface's Layer 3 MTU value.

switch(config-if-Et9/1)# show interfaces ethernet 1/1
...
  IP MTU 1600 bytes , BW 100000000 kbit
  Full-duplex, 100Gb/s over 4 lanes, auto negotiation: off, uni-link: n/a
...

L3 MTU Considerations

The following considerations apply to support of Layer 3 MTU:
  • A switch supports only three (3) unique MTU values. If you configure more than three (3) unique MTU values, the switch will not enforce additional unique MTU definitions on an interface.
    • When the number of unique MTU values becomes less than three (3) (after unconfiguring MTU on some interfaces), MTU on previously unprogrammed interfaces is not reprogrammed automatically. CLI commands no mtu followed by mtu must be called to program MTU on an interface.

  • When the startup configuration has more than three (3) unique MTU values defined on different switch interfaces, it is not guaranteed which unique MTU values will be active when the switch is rebooted.

  • Jericho2-based platforms do not enforce L3 MTU on Multicast packets.