Brocade Communications Systems FastIron SX 800 Configuration Manual page 47

Forwarding traffic to a port with a smaller MTU size
NOTE
Forwarding traffic to a port with a smaller MTU size is not supported on the FastIron X
Series.
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
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
– 10,200 bytes - The maximum for Ethernet II encapsulation (Default MTU: 9216)
– 10,174 bytes - The maximum for SNAP encapsulation (Default MTU: 9216)
• For ICX 6430, ICX 6430-C12, and ICX 6450 devices
– 10,178 bytes - The maximum for Ethernet II encapsulation (Default MTU: 9216)
– 10,174 bytes - The maximum for SNAP encapsulation (Default MTU: 9216)
• For other devices
– 10,218 bytes - The maximum for Ethernet II encapsulation (Default MTU: 9216)
– 10,214 bytes - The maximum for SNAP encapsulation (Default MTU: 9216)
FastIron Ethernet Switch Layer 3 Routing
53-1003627-04
Configuring IP parameters - Layer 3 switches
47