Changing the MTU

The Maximum Transmission Unit (MTU) is the maximum length of IP packet that a Layer 2 packet can contain. IP packets that are longer than the MTU are fragmented and sent in multiple Layer 2 packets. You can change the MTU globally or on individual ports.

The default MTU is 1500 bytes for Ethernet II packets and 1492 for Ethernet SNAP packets.

MTU enhancements

Brocade devices contain the following enhancements to jumbo packet support:

  • Hardware forwarding of Layer 3 jumbo packets - Layer 3 IP unicast jumbo packets received on a port that supports the frame MTU size and forwarded to another port that also supports the frame MTU size are forwarded in hardware. Previous releases support hardware forwarding of Layer 2 jumbo frames only.
  • ICMP unreachable message if a frame is too large to be forwarded - If a jumbo packet has the Do not Fragment (DF) bit set, and the outbound interface does not support the packet MTU size, the Brocade device sends an ICMP unreachable message to the device that sent the packet.
NOTE
These enhancements apply only to transit traffic forwarded through the Brocade device.

Configuration considerations for increasing the MTU

  • The MTU command is applicable to VEs and physical IP interfaces. It applies to traffic routed between networks.
  • For ICX 7150, ICX 7250, ICX 7450, and ICX 7750 devices, the IPv4 and IPv6 MTU values are the same. Modifying one also changes the value of the other.
  • For ICX 7150, ICX 7250, ICX 7450, and ICX 7750 devices, the minimum IPv4 and IPv6 MTU values for both physical and virtual interfaces are 1280.
  • You cannot use this command to set Layer 2 maximum frame sizes per interface. The global jumbo command causes all interfaces to accept Layer 2 frames.
  • When you increase the MTU size of a port, the increase uses system resources. Increase the MTU size only on the ports that need it. For example, if you have one port connected to a server that uses jumbo frames and two other ports connected to clients that can support the jumbo frames, increase the MTU only on those three ports. Leave the MTU size on the other ports at the default value (1500 bytes). Globally increase the MTU size only if needed.

Forwarding traffic to a port with a smaller MTU size

In order to forward traffic from a port with 1500 MTU configured to a port that has a smaller MTU (for example, 750) size, you must apply the mtu-exceed forward global command. To remove this setting, enter the mtu-exceed hard-drop command. The hard-drop option is enabled by default on the router.

Syntax: mtu-exceed { forward | hard-drop }

  • forward—Fragments and forwards a packet from a port with a larger MTU to a port with a smaller MTU.
  • hard-drop—Resets to default and removes the forward function.

Globally changing the Maximum Transmission Unit

The Maximum Transmission Unit (MTU) is the maximum size an IP packet can be when encapsulated in a Layer 2 packet. If an IP packet is larger than the MTU allowed by the Layer 2 packet, the Layer 3 switch fragments the IP packet into multiple parts that will fit into the Layer 2 packets, and sends the parts of the fragmented IP packet separately, in different Layer 2 packets. The device that receives the multiple fragments of the IP packet reassembles the fragments into the original packet.

You can increase the MTU size to accommodate jumbo packet sizes up to 10,200 bytes.

To globally enable jumbo support on all ports of a FastIron device, enter commands such as the following. 

device(config)# jumbo
device(config)# write memory
device(config)# end
device# reload
NOTE
You must save the configuration change and then reload the software to enable jumbo support.

Changing the MTU on an individual port

By default, the maximum Ethernet MTU sizes are as follows:

  • 1500 bytes - The maximum for Ethernet II encapsulation
  • 1492 bytes - The maximum for SNAP encapsulation

When jumbo mode is enabled, the maximum Ethernet MTU sizes are as follows:

  • 10,218 bytes - The maximum for Ethernet II encapsulation (Default MTU: 9216)
  • 10,214 bytes - The maximum for SNAP encapsulation (Default MTU: 9216)
NOTE
If you set the MTU of a port to a value lower than the global MTU and from 576 through 1499, the port fragments the packets. However, if the port MTU is exactly 1500 and this is larger than the global MTU, the port drops the packets. For ICX 7150, ICX 7250, ICX 7450, and ICX 7750 devices, the minimum IPv4 and IPv6 MTU values for both physical and virtual interfaces are 1280.
NOTE
You must save the configuration change and then reload the software to enable jumbo support.

To change the MTU for interface 1/1/5 to 1000, enter the following commands. 

device(config)# interface ethernet 1/1/5
device(config-if-1/1/5)# ip mtu 1000
device(config-if-1/1/5)# write memory
device(config-if-1/1/5)# end
device# reload

The num variable specifies the MTU. Ethernet II packets can hold IP packets from 576 through 1500 bytes long. If jumbo mode is enabled, Ethernet II packets can hold IP packets up to 10,218 bytes long. Ethernet SNAP packets can hold IP packets from 576 through 1492 bytes long. If jumbo mode is enabled, SNAP packets can hold IP packets up to 10,214 bytes long. The default MTU for Ethernet II packets is 1500. The default MTU for SNAP packets is 1492.

Path MTU discovery (RFC 1191) support

ICX 7250, ICX 7450, and ICX 7750 devices support the path MTU discovery method described in RFC 1191. When the Brocade device receives an IP packet that has its Do not Fragment (DF) bit set, and the packet size is greater than the MTU value of the outbound interface, then the Brocade device returns an ICMP Destination Unreachable message to the source of the packet, with the Code indicating "fragmentation needed and DF set". The ICMP Destination Unreachable message includes the MTU of the outbound interface. The source host can use this information to help determine the maximum MTU of a path to a destination.

RFC 1191 is supported on all interfaces.

Parent topic: Configuring packet parameters