Brocade ICX 6650 Layer 3 Routing Configuration Guide 29
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Configuring IP parameters – Layer 3 Switches
The default MTU is 1500 bytes for Ethernet II packets and 1492 for Ethernet SNAP packets.
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. .
•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.
•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-d rop command. MTU-exceed hard-drop is the default state of
the router.
Syntax:mtu-exceed [ forward | hard-drop ]
•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.
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 10,240 bytes.
To globally enable jumbo support on all ports of a Brocade ICX 6650 device, enter commands such
as the following.