Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 772)

742 Multi-Service IronWare Switching Configuration Guide
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Configuring IP parameters
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Changing the MTU
The IP MTU is the maximum length of an IP packet that a Layer 2 packet can contain. If an IP
packet is larger than the IP MTU allowed by the Layer 2 packet, the Brocade device fragments the
IP packet into multiple parts that will fit into 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. The default IP MTU
is 1500 bytes for Ethernet II packets. You can change the IP MTU globally or for individual IP
interfaces. You can increase the IP MTU size to accommodate large packet sizes, such as jumbo
packets, globally or on individual IP interfaces. However, IP MTU cannot be set higher than the
maximum frame size, minus 18.
NOTE
For multicast data traffic, frames are not fragmented and the IP MTU setting is ignored.
For jumbo packets, the Brocade device supports hardware forwarding of Layer 3 jumbo packets.
Layer 3 IP unicast jumbo packets received on a port that supports the frame's IP MTU size and
forwarded to another port that also supports the frame's IP MTU size are forwarded in hardware.
NOTE
Policy Based Routing (PBR) currently does not support this IP MTU feature.
Configuration considerations for increasing the IP MTU:
• The maximum value of an IP MTU cannot exceed the configured maximum frame size, minus
18. For example, global IP MTU cannot exceed the value of default-max-frame-size, minus 18
bytes. IP MTU for an interface cannot exceed the value of the maximum frame size configured,
minus 18 bytes. The 18 bytes are used for Ethernet header and CRC.
• When you increase the IP MTU size of for an IP interface, the increase uses system resources.
Increase the IP MTU size only on the IP interfaces that need it. For example, if you have one IP
interface connected to a server that uses jumbo frames and two other IP interfaces connected
to clients that can support the jumbo frames, increase the IP MTU only on those three IP
interfaces. Leave the IP MTU size on the other IP interfaces at the default value (1500 bytes).
Globally increase the IP MTU size only if needed.
• The difference between IP MTU and default-max-frame size should be as follows.
• 18 bytes for untagged packets
• 22 bytes for single-tagged packets and
• 26 bytes for dual-tagged packets
How To determine the actual MTU value
An IPv4 interface can obtain it’s MTU value from any of the following sources:
• Default IP MTU setting
• Global MTU Setting
• Interface MTU Setting
An interface determines its actual MTU value through the process described below.
1. If an IPv4 Interface MTU value is configured, that value will be used.
2. If an IPv4 Interface MTU value is not configured and an IPv4 Global MTU value is configured,
the configured global MTU value will be used.