Configuration Guide User guide

FastIron Configuration Guide 975
53-1002494-02
Configuring IP parameters – Layer 3 Switches
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 up to 10,232 bytes in an
IronStack. Devices that are not part of an IronStack support up to 10,240 bytes.
To globally enable jumbo support on all ports of a FastIron device, enter commands such as the
following.
Brocade(config)# jumbo
Brocade(config)# write memory
Brocade(config)# end
Brocade# 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:
• 10,240 bytes– The maximum for Ethernet II encapsulation
• 10,240 bytes – The maximum for SNAP encapsulation
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.
NOTE
The ip mtu command is not supported on FWS devices.
NOTE
You must save the configuration change and then reload the software to enable jumbo support.
To change the MTU for interface 1/5 to 1000, enter the following commands.
Brocade(config)# interface ethernet 1/5
Brocade(config-if-1/5)# ip mtu 1000
Brocade(config-if-1/5)# write memory
Brocade(config-if-1/5)# end
Brocade# reload
Syntax: [no] ip mtu <num>