Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 47)

• 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
NOTE
This feature is not supported on FastIron X Seriess.
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. MTU-exceed hard-drop is the default state of 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, 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
Syntax: [no] jumbo
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:
• For ICX 6610 devices
IP Configuration
FastIron Ethernet Switch Layer 3 Routing Configuration Guide 47
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