Quick Reference Guide

PowerConnect B-Series TI24X Configuration Guide 583
53-1002269-02
Configuring IP parameters – Layer 3 Switches
21
Configuration considerations for increasing the MTU
• 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.
• Use the same 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 MTU. For example,
if a port has an MTU setting of 8000 and receives an 8000-byte frame, then must forward the
frame onto a port with an 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 Maximum Transmission Unit (MTU)
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 9198 bytes in a
PowerConnect B-Series TI24X device.
To globally enable jumbo support on all ports, enter commands such as the following.
PowerConnect(config)#jumbo
PowerConnect(config)#write memory
PowerConnect(config)#end
PowerConnect#reload
Syntax: [no] jumbo
NOTE
You must save the configuration change and then reload the software to enable jumbo support.
Changing the Maximum Transmission Unit 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:
• 9198 bytes – The maximum for Ethernet II encapsulation
• 9190 bytes – The maximum for SNAP encapsulation
To change the MTU for interface 5 to 1000, enter the following commands.
PowerConnect(config)#int e 5
PowerConnect(config-if-5)#ip-port-mtu 1000
PowerConnect(config-if-5)#write memory
PowerConnect(config-if-5)#end
Syntax: [no] ip-port-mtu <num>