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

Support for Layer 3 Maximum Transmission Unit (MTU) on 7280R3/7500R3/7800R3 switches includes the following:
  • Enforces the MTU for Layer 3 packets on 7280R3/7500R3/7800R3 switches. The MTU can be set on any SVI and the MTU of that specific SVI is enforced when the packets egress out of a trunk port. This behavior is not supported on 7280E/R/R2 and 7500E/R/R2 line cards.
  • MTU enforcement is done after adding all of the encapsulation headers in the Egress packet. For example, if an interface is configured with an MTU value of 1500 and the packet size is larger than 1500 with all encapsulation headers added, the packet is considered MTU violated. In 7280E/R/R2 and 7500E/R/R2 line cards, MTU is enforced before adding encapsulation headers.
  • Actual MTU configured in hardware is MTU configured in the CLI + 18 bytes. 18 bytes is added to accommodate the L2 header. If the outgoing packet is untagged, the actual MTU configured in hardware is the MTU configured in the CLI + four (4) bytes.

Layer 3 MTU Configuration

You can configure Layer 3 MTU on Layer 3 interfaces and SVI’s 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. In this example, ethernet 1/1 interface is selected to display. IP MTU represents the Layer 3 MTU of the interface.

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 Limitations

The following limitations apply to support of Layer 3 MTU:
  • Only three (3) unique MTU values can be configured on a switch. If the number of unique MTU values exceeds three (3), additional unique MTU definitions will not be enforced on an interface.
    • When the number of unique MTU values becomes less than three (3) (after unconfiguring MTU on some interfaces), MTU on interfaces that are previously unprogrammed, are 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 interfaces of the switch, there is no guarantee which unique MTU values will be active when the switch is rebooted.
  • L3 MTU is not enforced on Multicast packets for Jericho2 based platforms.