Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 46)

The entire IP packet, including the source and destination address and other control information and
the data, is placed in the data portion of the Layer 2 packet. Typically, an Ethernet network uses one
of two different formats of Layer 2 packet:
• Ethernet II
• Ethernet SNAP (also called IEEE 802.3)
The control portions of these packets differ slightly. All IP devices on an Ethernet network must use
the same format. BrocadeLayer 3 switches use Ethernet II by default. You can change the IP
encapsulation to Ethernet SNAP on individual ports if needed.
NOTE
All devices connected to the Layer 3 switch port must use the same encapsulation type.
To change the IP encapsulation type on interface 5 to Ethernet SNAP, enter the following commands.
device(config)# interface ethernet 5
device(config-if-e1000-5)# ip encapsulation snap
Syntax: ip encapsulation{ snap | ethernet_ii }
Changing the 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
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 7750, the IPv4 and IPv6 MTU values are the same. Modifying one also changes the value
of the other.
• For ICX 7750, the minimum IPv4 and IPv6 MTU values for both physical and virtual interfaces are
1280.
Changing the MTU
46 FastIron Ethernet Switch Layer 3 Routing Configuration Guide
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