Brocade BigIron RX Series Configuration Guide - Manual (Page 262)

184 BigIron RX Series Configuration Guide
53-1002484-04
Configuring packet parameters
7
• Use the same IP MTU size on all ports that will be supporting jumbo frames. If the device
needs to fragment a jumbo frame (and the frame does not have the DF bit set), the device
fragments the frame into 1500-byte fragments, even if the outbound port has a larger IP MTU.
For example, if a port has an IP MTU setting of 8000 and receives an 8000-byte frame, then
must forward the frame onto a port with an IP MTU of 4000, the device does not fragment the
8000-byte frame into two 4000-byte frames. Instead, the device fragments the 8000-byte
frame into six fragments (five 1500-byte fragments and a final, smaller fragment.)
Globally changing the IP MTU
To globally enable jumbo support on all ports, enter commands such as the following.
BigIron RX(config)# ip mtu 5000
BigIron RX(config)# write memory
Syntax: [no] ip mtu <bytes>
The <bytes> parameter specifies the maximum number of bytes an Ethernet frame can have in
order to be forwarded on a port. The global maximum frame size can be configured to a value from
64 through 9212, but the <bytes> parameter value must be 18 bytes less than the configured
global maximum frame size.
NOTE
The BigIron RX will always use 22 Bytes less than the configured MTU in order to compensate for the
4Bytes required for VLAN tags. This is so if a packet is forwarded on both a tagged and untagged
link within a VLAN, it will get through.
Changing the maximum transmission unit on an individual interface
By default, the maximum IP MTU sizes are as follows:
• 1500 bytes – The maximum for Ethernet II encapsulation
• 1492 bytes – The maximum for SNAP encapsulation
NOTE
The IP MTU configured at the physical interface level takes precedence over the IP MTU configured
at the global level for that physical interface.
To change the IP MTU for interface 1/5 to 1000, enter the following commands.
BigIron RX(config)# interface ethernet 1/5
BigIron RX(config-if-e10000-5)# ip mtu 1000
Syntax: [no] ip mtu <bytes>
The <bytes> parameter specifies the IP MTU. Ethernet II packets can hold IP packets from 572 –
1500 bytes long. Ethernet SNAP packets can hold IP packets from 572 – 1492 bytes long.
However, the value of IP MTU on an interface cannot exceed the configured value of IP MTU for an
interface, minus 18 bytes. The default IP MTU for Ethernet II packets is 1500. The default IP MTU
for SNAP packets is 1492.