Brocade Communications Systems ICX 7250 series, Configuration Manual

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MTU enhancements
Brocade devices contain the following enhancements to jumbo packet support:
• Hardware forwarding of Layer 3 jumbo packets - Layer 3 IP unicast jumbo packets received on a port that supports the frame
MTU size and forwarded to another port that also supports the frame MTU size are forwarded in hardware. Previous releases
support hardware forwarding of Layer 2 jumbo frames only.
• ICMP unreachable message if a frame is too large to be forwarded - If a jumbo packet has the Do not Fragment (DF) bit set,
and the outbound interface does not support the packet MTU size, the Brocade device sends an ICMP unreachable message to
the device that sent the packet.
NOTE
These enhancements apply only to transit traffic forwarded through the Brocade
device.
Configuration considerations for increasing the MTU
• The MTU command is applicable to VEs and physical IP interfaces. It applies to traffic routed between networks.
• For ICX 7250, ICX 7450, and ICX 7750 devices, the IPv4 and IPv6 MTU values are the same. Modifying one also changes
the value of the other.
• For ICX 7250, ICX 7450, and ICX 7750 devices, the minimum IPv4 and IPv6 MTU values for both physical and virtual
interfaces are 1280.
• You cannot use this command to set Layer 2 maximum frame sizes per interface. The global jumbo command causes all
interfaces to accept Layer 2 frames.
• 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.
Forwarding traffic to a port with a smaller MTU size
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
Configuring IP parameters - Layer 3 switches
Brocade FastIron Layer 3 Routing Configuration Guide
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