Dell PowerConnect B-FCXs Configuration Manual page 849

Changing the Maximum Transmission Unit (MTU)
The Maximum Transmission Unit (MTU) is the maximum length of IP packet that a Layer 2 packet
can contain. IP packets that are longer than the MTU are fragmented and sent in multiple Layer 2
packets. You can change the MTU globally or on individual ports.
The default MTU is 1500 bytes for Ethernet II packets and 1492 for Ethernet SNAP packets.
MTU enhancements
Dell PowerConnect devices contain the following enhancements to jumbo packet support:
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NOTE
These enhancements apply only to transit traffic forwarded through the Dell PowerConnect device.
Configuration considerations for increasing the MTU
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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. MTU-exceed hard-drop is the default state of
the router.
Syntax:mtu-exceed [ forward | hard-drop ]
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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.
PowerConnect B-Series FCX Configuration Guide
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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 Dell PowerConnect device sends an ICMP unreachable message to the device that
sent the packet.
The MTU command is applicable to VEs and physical IP interfaces. It applies to traffic routed
between networks.
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.
forward - forwards a packet from a port with a larger MTU to a port with a smaller MTU
hard-drop - resets to default, removes the forward function.
Configuring IP parameters – Layer 3 Switches
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